cnsy@vax5.CIT.CORNELL.EDU (06/01/89)
_ __ ______ _ __
' ) ) / ' ) )
/--' __. __ , --/ __ __. _. o ____ _, / / _ , , , _
/ \_(_/|_/ (_/_ (_/ / (_(_/|_(__<_/ / <_(_)_ / (_</_(_(_/_/_)_
/ /|
' |/
"Light Makes Right"
January 6, 1989
Volume 2, Number 1
Compiled by Eric Haines, 3D/Eye Inc, 2359 Triphammer Rd, Ithaca, NY 14850
607-257-1381, hpfcla!hpfcrs!eye!erich@hplabs.hp.com
All contents are US copyright (c) 1988,1989 by the individual authors
Contents:
Introduction - Eric Haines
New Members - David Jevans, Subrata Dasgupta, Darwin Thielman,
Steven Stadnicki, Mark Reichert
Multiprocessor Visualization of Parametric Surfaces - Markku Tamminen,
comments from many others
Miscellany - K.R.Subramanian, David F. Rogers, Steven Stadnicki,
Joe Smith, Mark Reichert, Tracey Bernath
Supersampling Discussion - David Jevans, Alan Paeth, Andrew Woo,
Loren Carpenter
Distributed Ray Tracer Available - George Kyriazis
Ray Tracing Program for 3b1 - Sid Grange
Map Archive - Gary L. Crum
Index of Back Issues - Eric Haines
-----------------------------------------------------------------------------
Introduction
------------
Well, this has been a busy time around here. First of all, note my
change of address (it's in the header). We've moved to a larger building with
much better environmental controls (i.e. we don't have to cool the machines in
the winter by opening the windows). In the meantime I've been trying to
actually finish a product and maybe even make some money from it. There's
also those SIGGRAPH deadlines on January 10th.... Busy times, so excuse the
long delay in getting this issue out.
The other great struggle has been to try to get my new Cornell account
to talk with the outside world. DEC's EUNICE operating system has foiled me so
far, so this issue is being distributed by Michael Cohen, who's now at the
University of Utah. Many thanks, Michael.
Due to the length between issues, my cullings from USENET have
accumulated into an enormous amount of material. As such, the condensed
version of these will be split between this and the next issue. This issue
contains what I felt was the best of the supersampling discussion. If this
material is old hat, please write and tell us of your experiences with
supersampling: what algorithm do you use? are you satisfied with it? what kind
of filtering is used, and what are your subdivision criteria?
This issue is the first one that has a "Volume X, Number Y" in the
header. This has been added partly for ease of reference, but also (more
importantly) for avoiding dropouts. If you get "Number 1", then "Number 3",
you know you've missed something (probably due to email failure). At the end
of this issue is a list of all the past issues. If you are missing any, please
write and I'll send them on.
-------------------------------------------------------------------------------
New Members
-----------
From: David Jevans <hpfcla!jevans@cpsc.UCalgary.CA>
I can be reached at the U of Calgary. I work days at Jade Simulations
International, ph # 403-282-5711.
My interests in ray tracing are in multi-process (networks of SUNS, BBN
Butterfly, and Transputers) ray tracing, space subdivision, and ray tracing
functionally defined iso-surfaces.
I am working on optimistic multi-processor ray tracing and combining
adaptive and voxel spatial subdivision techniques. I have implemented a
parallel ray tracer on the University of Calgary's BBN Butterfly. My ray
tracers handle a variety of object types including polygons, spline surfaces,
and functionally defined iso-surfaces. My latest projects are using TimeWarp
to speed up multi-processor ray tracing, adding a texture language, frame
coherence for ray tracing animation, and developing the ray tracing answer to
radiosity.
David Jevans, U of Calgary Computer Science, Calgary AB T2N 1N4 Canada
uucp: ...{ubc-cs,utai,alberta}!calgary!jevans
--------
# Subrata Dasgupta - raycasting of free-form surfaces, surface representations
# Duke University
# Dept. of Computer Science
# Durham, NC 27706
# (919)-684-5110
alias subrata_dasgupta sdg@cs.duke.edu
I am relatively new the field of ray tracing. I am involved in the design of a
raycasting system based on the original design by Gershon Kedem and John Ellis
[Proc. 1984 Int'l conference on Computer Design]. The original design uses
Constructive Solid Geometry for building up a complex object out of simple
primitives like cones, cylinders, and spheres. The main drawback of such a
system is that representing an object with cubic or higher order surfaces
require numerous quadratic primitives and even then is at best an approximation
to the original surface.
The raycasting machine uses an array of parallel rays and intersects them with
primitives. The applications of such a machine are potentially large like:
modeling surfaces for NC machines, calculating volume and moment of inertia,
finding fast surface intersections, to name just a few. At present there are 2
working models of the raycasting machine, one of which is in the Dept. of
Mechanical Engg. at Cornell. The other one is our experimental m/c which is
located at the U. of N. Carolina at Chapel Hill (The operative word in this
sentence is "located" :-) ). Although my input may not be very frequent at the
beginning I will be an avid reader of the raycasting news. Thanks for inviting
me in.
--------
From: Darwin G. Thielman <hpfcla!thielman@cs.duke.edu>
Subject: Duke ray casting group
# Darwin Thielman - Writing software to control the raycasting machine at Duke
# Duke University
# Computer Science Dept.
# Durham, NC. 27706
# (919) 684-3048 x246
alias darwin_thielman thielman@duke.cs.duke.edu
At Duke we have designed a system that does ray casting on many primitives in
parallel. This is achieved with 2 types of processors, a primitive classifier
(PC) and a combine classifier (CC). The PC's solve systems of quadratic
equations and the CC's combine these results.
I am the system software person, I am writing micro code for an Adage 3000
machine. The micro code is responsible for controlling the ray casting board
and creating images from the output of the board.
In addition the following people also work on the ray casting system at Duke.
Gershon Kedem
John Ellis
Subrata Dasgupta
Jack Briner
Ricardo Pantazis
Sanjay Vishin
Also at UNC Chapel Hill there is a eng. who is working on the hardware design
of the system he is Tom Lyerly.
Since I do not want to attempt to explain what each one of these people is
doing (with the possibility of getting it wrong) I will not try. All of the
above people will have access to the RTN and if any of them are interested they
may respond. Also If anyone want to get a hold of any of them just send me a
message and I will forward it to the proper person.
We also have one of our boards at Cornell, there is a group there that is
working on solid modeling, and hopes to use our hardware. If you want you can
contact Rich Marisa (607) 255-7636 for more information on what they are doing.
His mail address is marisa@oak.cadif.cornell.edu. Also I have talked to him
and if you want to see a demo of our system he would be glad to show it to you.
If you have any questions or comments please feel free to contact me.
Darwin Thielman
--------
From: Steven Stadnicki <hpfcla!stadnism@clutx.clarkson.edu>
Steven Stadnicki - shadowing from reflected light sources, tracing atomic
orbitals, massively parallel ray tracing
212 Reynolds Dormitory
Clarkson University
Potsdam, NY 13676
(315) 268-4079
stadnism@clutx.clarkson.edu
Right now, I'm working on writing a simple ray tracer to implement my reflected
shadowing model (see the E-mail version of RTNews, Nov. 4), and then I'll be
trying a few texture models. (Texture mapping on to atoms... the marble
P-orbital!)
--------
From: hpfcla!sunrock!kodak!supra!reichert@Sun.COM (Mark Reichert x25948)
Subject: RT News intro
#
# Mark Reichert - diffuse interreflections
# Work:
# Eastman Kodak Company
# Advanced Systems Group
# Building 69
# Rochester, NY 14650
# 716-722-5948
#
# Home:
# 45 Clay Ave.
# Rochester, NY 14613
# 716-647-6025
#
I am currently interested in global illumination simulation using ray
tracing with auxiliary data structures for holding illuminance values.
I am also interested in ray tracing from the lights into the environment -
maybe just a few bounces, then storing illuminance as above.
What I would really like is a ray tracer (or whatever) that would do a nice
job of modeling triangular glass prisms.
alias mark_reichert hpfcrs!hpfcla!hplabs!sun!sunrock!kodak!supra!reichert
-------------------------------------------------------------------------------
Multiprocessor Visualization of Parametric Surfaces
From: Markku Tamminen <hpfcla!mit%hutcs.hut.fi@CUNYVM.CUNY.EDU>
I obtained the Ray-Tracing News from Panu Rekola, and sent the message below to
some people on your distribution list interested in related matters.
I have done research in geometric data structures and algorithms in 1981-84. A
practical result was the EXCELL spatial index (like an octree with binary
subdivision, together with a directory allowing access by address computation).
My present interests are described below.
Charles Woodward has developed a new and efficient method for ray-tracing
parametric surfaces using subdivision for finding the ray/patch intersection.
============================================================================
I am putting together a project proposal with the abstract below. I would be
interested in obtaining references to any new work you have done in ray-tracing
and hardware, and later in an exchange of ideas. I will send an
acknowledgement to any message I get, so if you don't see one something has
gone wrong. (Email has often been very unreliable.)
Looking forward to hearing something from you,
Markku Tamminen
Helsinki University of Technology
Laboratory of Information Processing Science
02150 ESPOO 15, FINLAND
Tel: 358-0-4513248 (messages: 4513229, home: 710317)
Telex: 125161 HTKK SF
Telefax: 358-0-465077
ARPANET: mit%hutcs.uucp%fingate.bitnet@cunyvm.cuny.edu
INTERNET: mit@hutcs.hut.fi
BITNET: mit%hutcs.uucp@fingate
UUCP: mcvax!hutcs!mit
Multiprocessor visualization of parametric surfaces
Project proposal
Markku Tamminen (mit@hutcs.hut.fi)
Charles Woodward (cwd@hutcs.hut.fi)
Helsinki University of Technology
Laboratory of Information Processing Science
Otakaari 1 A, SF-02150 Espoo, Finland
ABSTRACT
The proposed research aims at an efficient system architecture
and improved algorithms for realistic visualization of complex
scenes described by parametric surfaces.
The key components of such a system are a spatial index and
a surface patch intersector. For both very efficient uniproces-
sor solutions have been developed by the authors at the Hel-
sinki University of Technology. However, to obtain sufficient
speed, at least the latter should be based on a specialized ar-
chitecture.
We propose obtaining a balanced complete system by gradually as-
cending what we call a specialization hierarchy. At its bottom
are solutions based on multiprocessors or networks of independent
computing units (transputers). In this case an important research
problem is how to avoid duplicating the data base in the proses-
sors. At the top of the hierarchy are specialized processors im-
plemented in VLSI.
The research will produce general insight into the possibilities
of utilizing concurrency and specialized processors in
geometric search and computation.
PREVIOUS WORK
M. Mantyla and M. Tamminen , ``Localized Set Operations for Solid
Modeling ,'' Computer Graphics , vol. 17, no. 3, pp. 279-289 ,
1983.
M. Tamminen , The EXCELL Method for Efficient Geometric Access to
Data , Acta Polytechnica Scandinavica, Ma 34 , 1981.
Markku Tamminen, Olli Karonen , and Martti Mantyla, ``Ray-
Casting and Block Model Conversion Using a Spatial Index,''
Computer Aided Design, vol. 16, pp. 203 - 208, 1984.
C. Woodward, ``Skinning Techniques for Interactive B-Spline
Surface Interpolation,'' Computer-Aided Design, vol. 20, no. 8,
pp. 441-451, 1988.
C. Woodward, ``Ray Tracing Parametric Surfaces By Subdivision in
Viewing Plane,'' to Appear in Proc. Theory and Practice of
Geometric Modelling, ed. W. Strasser, Springer-Verlag, 1989.
--------
[a later message from Markku Tamminen:]
I thought I'd send this summary of responses as feedback, because some answers
may not have found their way through the network. I think mit@hutcs.hut.fi
might be the safest address to use for me.
Our project has not yet started - I have just applied for funding with the
proposal whose abstract I sent to you. Also, I am a software person, but hope
to get somebody hardware-oriented involved in the project. We will be using
transputers to start with, but so far I have just made some small experiments
with them.
Our ray/patch intersection method is based on subdivision. It is a new method
developed by Charles Woodward, and quite a bit more efficient than Whitted's
original one. However, it takes 3 ms for a complete ray/patch intersection on a
SUN4. Thus, we'd like to develop a specialized processor for this task - the
algorithm is well suited for that.
Our spatial index is EXCELL, which I originally published in 1981, and whose 3D
application was described in SIGGRAPH'83 by Martti Mantyla and me. I have
lately tuned it quite a bit for ray-tracing, and we are very satisfied with its
performance. (EXCELL uses octree-like, but binary, subdivision. It has a
directory, which is an array providing direct access by address computation,
and a data part, which corresponds to the leaf cells of an octree. Binary
subdivision leads to fewer leaf-cells than 8-way. There is an overflow
criterion that decides when subdivision will be discontinued.)
We have obtained best results when we store in the spatial index for each patch
the bounding box of its control points, and further cut it with a "slab"
defined by two planes parallel to a "mean normal" of the patch. Using this
method we have to perform, on the average, less than 2 complete patch/ray
intersection tests.
Our method has not been as efficient as that of subdividing the patches to all
the way to triangles. However, as much less storage is required, we consider
our technique more suited for distributed architectures.
In the proposed project we want to look both into developing a specialized
(co)processor for the ray/patch intersection task and into distributing the
whole computation on several processors. I think that the most difficult
research problem is the partitioning of the data base in a loosely coupled
system. In our case the ray/patch intersection task is so time consuming that
it would help (to begin with) to keep the ray/database traversal on the
workstation and jost distribute the intersection task to other processors.
Some questions:
Does anybody know of HW work in the ray/patch intersection area; e.g.,
as a continuation of Pulleyblank & Kapenga's article in CG&A?
Does anybody know of somebody working with transputers in ray-tracing? (We
do know of the INMOS ray-tracing demo.)
How enthusiastic are you about the approach of using digital signal
processors? What other off-the shelf processors would be specially suited
as a basis for a ray-tracing coprocessor?
What would be the specific computation to offload to a coprocessor?
I don't know what more to write now about our project. Below is a short summary
of the responses I got:
From: kyriazis@yy.cicg.rpi.edu (George Kyriazis)
Works with pixel machine, and will transport RPI's ray-tracer to it. Main
problem: duplication of code and data.
From: jeff@CitIago.Bitnet (Jeff Goldsmith)
Has implemented ray-tracer with distributed database on hypercube.
Communication between parts of database by RPC. With 128 processors 50%
utilization. (Ref: 3rd Hypercube Concurrent Computation Proc.) A proposal to
build custom silicon for intersection testing. In this case the rest of the
system could reside on a ordinary PC or workstation.
From: gray@rhea.cray.com (Gray Lorig)
"Your abstract sounds interesting."
From: Frederik Jansen <JANSEN@ibm.com>
Mike Henderson at Yorktown Heights is working on ray/patch intersection problem
(software approach).
From: Mark VandeWettering <markv@drizzle.cs.uoregon.edu>
"As to HW implementation, my advice is NOT to take the path that I took, but to
implement one simple primitive: a bilinear interpolated triangular patch."
"Take a look at AT&T's 12 DSP chip raytracing board." Would like to experiment
with implementing an accelerator based on Motorola DSP56001.
From: tim%ducat.caltech.edu@Hamlet.Bitnet (Tim Kay)
"I am interested how fast a *single* general purpose processor can raytrace
when it is assisted by a raytracing coprocessor of some sort. there seems to be
tremendous potential for adding a small amount of raytracing HW to graphics
work stations." "I am quite capable of ray tracing over our TCP/IP network."
From: priol@tokaido.irisa.fr
"I work on ray-tracing on a MIMD computer like the Hypercube." Partitions scene
boundary as suggested by Cleary. To do it well sub-samples image before
parallel execution. With 30-40 processors 50% efficiency. Part of work
published in Eurographics'88.
From: toc@wisdom.TN.CORNELL.EDU (Timothy F. O'Connor)
"Abstract sounds interesting. My main interest is in radiosity approach."
From: Russ Tuck <tuck@cs.unc.edu>
"My work is summarized in hardcopy RTnews vol.2, no. 2, June '88, "Interactive
SIMD Ray Tracing."
That's it for now. I hope there has been some interest in this multi-feedback.
I'll get more meat of my own in the messages when our new work starts.
Thanks to you all! / Markku
-------------------------------------------------------------------------------
Miscellany
----------
From: hpfcla!subramn@cs.utexas.edu
Subject: Ray Tracing articles.
I would like you to bring this to the attention of the RT news group (if you
consider it appropriate).
There are lots of conference proceedings and journals other than SIGGRAPH &
CG&A which contain ray tracing articles. At least, here, at our university we
don't get all those journals (for example, the Visual Computer) due to budget
constraints. It would be nice for someone to post relevant articles on ray
tracing so that all of us will be aware of the work going on in ray tracing
every where. For instance, I have found several articles in the Visual Computer
that were relevant to what I was doing after being pointed at by someone else.
If these can be listed by someone who gets these journals, then it would make
it easier to get access to these articles.
K.R.Subramanian
Dept. of Computer Sciences
The University of Texas at Austin
Austin, Tx-78712.
subramn@cs.utexas.edu
{uunet}!cs.utexas.edu!subramn
--------
[My two cents: we don't get "Visual Computer" around here, either. I've been
helping to keep Paul Heckbert's "Ray Tracing Bibliography" up to date and would
like to obtain relevant references (preferably in REFER format) for next year's
copy for use in SIGGRAPH course notes. See SIGGRAPH '88 notes from the
"Introduction to Ray Tracing" course for the current version to see the state
of our reference list. - Eric]
----------------------------------------
From: "David F. Rogers" <hpfcla!dfr@USNA.MIL>
Subject: Transforming normals
Was skimming the back issues of the RT News and your memo on transforming
normals caught my eye. Another way of looking at the problem is to recall that
a polygonal volume is made up of planes that divide space into two parts. The
columns of the volume matrix are formed from the coefficients of the plane
equations. Transforming a volume matrix requires that you premultiply by the
inverse of the manipulation matrix. The components of a normal are the first
three coefficients of the plane equation. Hence the same idea should apply.
(see PECG Sec. 4-3 on Robert's algorithm pp 211-213). Surprising what you can
learn from Roberts' algorithm yet most people discount it.
----------------------------------------
>From: stadnism@clutx.clarkson.edu (Steven Stadnicki,212 Reynolds,2684079,5186432664)
Subject: Some new thoughts on how to do caustics, mirrored reflection, etc.
[source: comp.graphics]
Here's a new idea I came up with to do caustics, etc. by ray tracing: from your
point on some surface, shoot out some number (say ~100) in "random" directions
(I would probably use a jittered uniform distribution on the sphere). For each
light source, keep track of all rays that come within some "distance" (some
angle) from the light source. Then, for each of these rays, try getting closer
to the light source using some sort of Newton-type iteration method... for
example, to do mirrored reflection:
\|/
-o- Light source
/|\
|
+---+ | M
| O | | i
| b | | r
| j | | r
| e | | o
| c | | r
| t | |
----------------+---+--X-------+
>From point X, shoot out rays in the ~100 "random" directions mentioned above;
say one of them comes within 0.05 radians of the light source. Do some sort of
update procedure on the ray to see if it keeps getting closer to the light
source; if it does, then you have a solution to the "mirrored reflection"
problem, and you can shade X properly. This procedure will work for curved
mirrors as well as planar ones (unlike the previous idea I mentioned), and will
also handle caustics well. It seems obvious to me that there will be bad cases
for the method, and it is certainly computationally expensive, but it looks
like a useful method. Any comments?
Steven Stadnicki
Clarkson University
stadnism@clutx.clarkson.edu
stadnism@clutx.bitnet
----------------------------------------
>From: jms@antares.UUCP (joe smith)
Organization: Tymnet QSATS, San Jose CA
Subject: Re: Ray Tracing Novice Needs Your Help
[source: comp.graphics]
In article <2399@ssc-vax.UUCP> dmg@ssc-vax.UUCP (David Geary) writes:
> What I'd really like is to have the source in C to a *simple*
>ray-tracer - one that I could port to my Amiga without too much
>difficulty.
>~ David Geary, Boeing Aerospace, ~
My standard answer to this question when it comes up is to locate the May/June
1987 issue of Amiga World. It's the one that has the ray-traced robot juggler
on the cover. The article "Graphic Scene Simulations" is a great overview of
the subject, and it includes the program listing in C. (Well, most of the
program. Details such as inputting the coordinates of all the objects are
omitted.)
----------------------------------------
From: hpfcla!sunrock!kodak!supra!reichert@Sun.COM (Mark Reichert x25948)
Subject: A call for vectors.....
Imagine a unit sphere centered at the origin.
Imagine a vector, the "reference vector", from the origin to any point on the
surface of this sphere.
I would like to create n vectors which will evenly sample the surface of our
sphere, within some given angle about that "reference vector".
I need to be able to jitter these vectors in such a way that no two vectors in
a given bunch could be the same.
This appears to be a job for spherical coordinates, but I can't seem to find a
formula that can treat the surface of a sphere as a "uniform" 2D surface (ie.
no bunching up at the poles).
I desire these vectors for generating soft shadows from spherical light
sources, and for diffuse illumination guessing.
I have something now which is empirical and slow - neither of which trait I
find very desirable.
I will have a need for these vectors often, and seldom will either the
angle or the number of vectors needed be the same across consecutive requests.
Can anyone help me?
----------------------------------------
>From: hoops@watsnew.waterloo.edu (HOOPS Workshop)
Subject: Needing Ray Tracing Research Topic
[source: comp.graphics]
As a System Design Engineeering undergrad at University of Waterloo, I am
responsible for preparing a 'workshop' paper each term. I am fascinated with
ray tracing graphics, but what I really need is a good application that I can
work into a workable research topic that can be completed in 1 or 2 terms.
If anyone in netland can offer any information on an implementation of ray
tracing graphics for my workshop please email me.
Thanks in advance folks,
Tracey Bernath
System Design Engineering
University of Waterloo
Waterloo, Ontario, Canada
hoops@watsnew.uwaterloo.ca
Bitnet: hoops@water.bitnet
CSNet: hoops@watsnew.waterloo.edu
uucp: {utai,uunet}!watmath!watsnew!hoops
-------------------------------------------------------------------------------
Supersampling Discussion
------------- ----------
[A flurry of activity arose when someone asked about doing supersampling in a
ray tracer. Below are some of the more interesting and useful replies. - Eric]
----------------------------------------
>From: jevans@cpsc.ucalgary.ca (David Jevans)
[source: comp.graphics]
Summary: blech
In article <5548@thorin.cs.unc.edu>, brown@tyler.cs.unc.edu (Lurch) writes:
> In article <5263@cbmvax.UUCP> steveb@cbmvax.UUCP (Steve Beats) writes:
> >In article <1351@umbc3.UMD.EDU> bodarky@umbc3.UMD.EDU (Scott Bodarky) writes:
> >If you sample the scene using one pixel per ray, you will get
> >pretty severe aliasing at high contrast boundaries. One trick is to sample
> >at twice the vertical and horizontal resolution (yielding 4 rays per pixel)
> >and average the resultant intensities. This is a pretty effective method
> >of anti-aliasing.
> From what I understand, the way to achieve 4 rays per pixel is to sample at
> vertical resolution +1, horizontal resolution +1, and treat each ray as a
> 'corner' of each pixel, and average those values. This is super cheap compared
> to sampling at twice vertical and horizontal.
Blech! Super-sampling, as suggested in the first article, works ok but is very
slow and 4 rays/pixel is not enough for high quality images. Simply rendering
vres+1 by hres+1 doesn't gain you anything. All you end up doing is blurring
the image. This is VERY unpleasant and makes an image look out of focus.
Aliasing is an artifact of regular under-sampling. Most people adaptively
super-sample in areas where it is needed (edges, textures, small objects).
Super-sampling in a regular pattern often requires more than 16 rays per
anti-aliased pixel to get acceptable results. A great improvement comes from
filtering your rays instead of simply averaging them. Even better is to fire
super-sample rays according to some distribution (eg. Poisson) and then filter
them.
Check SIGGRAPH proceedings from about 84 - 87 for relevant articles and
pointers to articles. Changing a ray tracer from simple super-sampling to
adaptive super-sampling can be done in less time than it takes to render an
image, and will save you HUGE amounts of time in the future. Filtering and
distributing rays takes more work, but the results are good.
David Jevans, U of Calgary Computer Science, Calgary AB T2N 1N4 Canada
uucp: ...{ubc-cs,utai,alberta}!calgary!jevans
----------------------------------------
>From: awpaeth@watcgl.waterloo.edu (Alan Wm Paeth)
Subject: Re: raytracing in || (supersampling speedup)
[source: comp.graphics]
In article <5548@thorin.cs.unc.edu> brown@tyler.UUCP (Lurch) writes:
>
>From what I understand, the way to achieve 4 rays per pixel is to sample at
>vertical resolution +1, horizontal resolution +1, and treat each ray as a
>'corner' of each pixel, and average those values. This is super cheap compared
>to sampling at twice vertical and horizontal.
This reuses rays, but since the number of parent rays and number of output
pixels match, this has to be the same as low-pass filtering the output produced
by a raytracer which casts the same number of rays (one per pixel).
The technique used by Sweeney in 1984 (while here at Waterloo) compares the
four pixel-corner rays and if they are not in close agreement subdivides the
pixel. The recursion terminates either when the rays from the subpixel's
corners are in close agreement or when some max depth is reached. The subpixel
values are averaged to form the parent pixel intensity (though a more general
convolution could be used in gathering up the subpieces).
This approach means that the subpixel averaging takes place adaptively in
regions of pixel complexity, as opposed to globally filtering the entire output
raster (which the poster's approach does implicitly).
The addition can be quite useful. For instance, a scene of flat shaded polygons
renders in virtually the same time as a "one ray per pixel" implementation,
with some slight overhead well spent in properly anti-aliasing the polygon
edges -- no time is wasted on the solid areas.
/Alan Paeth
Computer Graphics Laboratory
University of Waterloo
----------------------------------------
>From: andreww@dgp.toronto.edu (Andrew Chung How Woo)
Subject: anti-aliasing
[source: comp.graphics]
With all these discussions about anti-aliasing for ray tracing, I thought I
would get into the fun also.
As suggested by many people, adaptive sampling is a good way to start dealing
with anti-aliasing (suggested by Whitted). For another quick hack on top of
adaptive sampling, you can add jitter (suggested by Cook). The jitter factor
can be controlled by the recursive depth of the adaptive sampling. This
combination tends to achieve decent quality.
Another method which nobody has mentioned is "stratified sampling". This is
also a rather simple method. Basically, the pixel is divided into a N-size
grid. You have a random number generator to sample a ray at (x,y) of the grid.
Then shoot another ray, making sure that the row x and column y are discarded
from further sampling, etc. Repeat this for N rays. Note, however, no sharing
of point sampling information is available here.
Andrew Woo
----------------------------------------
>From: loren@pixar.UUCP (Loren Carpenter)
Subject: Re: anti-aliasing
[source: comp.graphics]
[This is in response to Andrew Woo's article - Eric]
Rob Cook did this too. He didn't call it "stratified sampling", though. The
idea is suggested by the solutions to the "8 queens problem". You want N
sample points, no 2 of which are in the same column, and no 2 of which are in
the same row. Then you jitter on top of that....
p.s. You better not use the same pattern for each pixel...
Loren Carpenter
...!{ucbvax,sun}!pixar!loren
----------------------------------------
[By the way, what kind of adaptive supersampling have people been using? We've
had fairly good results with the simple "check four corners" algorithm and box
filtering the values generated. We find that for complex scenes an adaptive
algorithm (which shoots 5 more rays if the subdivision criteria is met) shoots
about 25% more rays overall (a result that Linda Roy also obtained with her
ray-tracer). What the subdivision criteria are affects this, of course. We've
been using 0.1 as the maximum ratio of R,G, or B channels of the four pixel
corners. How have you fared, and what kinds of filtering have you tried?
- Eric]
-------------------------------------------------------------------------------
Distributed Ray Tracer Available
>From: kyriazis@rpics (George Kyriazis)
[source: comp.graphics]
During the last week I put myself together and managed to pull out a
second version of my ray tracer (the old one was under the subject: "Another
simple ray tracer available"). Tis one includes most of the options described
in R. Cook's paper on Distributed ray tracing.
Capabilities of the ray tracer are:
Gloss (blurred reflection)
Translucency (blurred refraction)
Penumbras (area light sources)
Motion Blur
Phong illumination model with one light source
Spheres and squares
Field of view and arbitrary position of the camera
The ray tracer has been tested on a SUN 3 and SUN 4. I have it available under
anonymous ftp on life.pawl.rpi.edu (128.113.10.2) in the directory pub/ray
under the name ray.2.0.shar. There are some older version there if you want to
take a look at them. If you can't ftp there send me mail and I'll send you a
copy. I also have a version for the AT&T Pixel Machine (for the few that have
access to one!).
No speed improvements have been made yet, but I hope I will have Goldsmith's
algorithm running on it soon. I know that my file format is not standard, but
people can try to write their own routine to read the file. It's pretty easy.
Hope you have fun!
George Kyriazis
kyriazis@turing.cs.rpi.edu
kyriazis@ss0.cicg.rpi.edu
-------------------------------------------------------------------------------
Ray Tracing Program for 3b1
>From: sid@chinet.UUCP (Sid Grange)
Subject: v05i046: ray tracing program for 3b1
[source: comp.sources.misc]
Posting-number: Volume 5, Issue 46
Archive-name: tracer
[This was posted to comp.sources.misc. I include some of the documentation so
you can get a feel for it. Nothing special, but it might be fun if you have a
3b1 (it's supposed to work on other UNIX systems, too). - Eric]
NAME
tracer- run a simple ray tracing procedure
DESCRIPTION
Tracer is a program developed originally to study how
ray tracing works, and was later modified to the present state
to make it more compatible for animated film production.
It is capable of depicting a number of balls (up to 150)
and a plane that is covered with a tiling of any bitmapped picture.
PROGRAM NOTES
This program generates a file containing a header with x and y sizes,
followed by the data in 8-bit greyscale, one pixel to a character, in
scanlines.
There are two necessary input files: ball data, and a pattern bitmap.
The tiling bitmap can be digitized data, it must be in the form of
scan lines no longer than 512 bytes followed by newlines.
-------------------------------------------------------------------------------
Map Archive
>From: crum@lipari.usc.edu (Gary L. Crum)
Subject: DB:ADD SITE panarea.usc.edu (Internet archive for maps)
[source: comp.archive, and ftp]
An Internet archive for maps of geographic-scale maps has been set up, starting
with data from the United States Geological Survey (USGS) National Cartographic
Information Center (NCIC), specifically a map of terrain in USGS Digital
Elevation Model (DEM) format.
The archive is on host panarea.usc.edu [128.125.3.54], in anonymous FTP
directory pub/map. Gary Crum <crum@cse.usc.edu> is maintainer.
The pub/map directory is writable by anonymous ftp. Approximately 50M bytes
are available for the map archive as of this writing.
NOTES:
* Files ending in the .Z extension have been compressed with the "compress"
program available on comp.sources archives such as j.cc.purdue.edu. They
should be transferred using the "binary" mode of ftp to UNIX systems. Send
mail to the maintainer to request format changes (e.g., to uuencoded form split
into small pieces).
* Some maps, e.g., DEM files from USGS, contain long lines which have been
observed to cause problems transferring with some FTP implementations. In
particular, a version of the CMU TCP/IP package for VAX/VMS did not support
these long lines.
* Source code for UNIX tools that manipulate ANSI labeled tapes and VMS tapes
is available as pub/ansitape.tar.Z on the map archive host.
-----------------
Index for Map Archive on Internet Host panarea.usc.edu [128.125.3.54].
version of Mon Nov 14 09:41:10 PST 1988
NOTE: This INDEX is descriptive to only the directory level in many cases.
-rw-r--r-- 1 crum 1090600 May 26 09:33 dem/MAP.N0009338
-rw-r--r-- 1 crum 278140 Nov 11 14:16 dem/MAP.N0009338.Z
Digital Elevation Model 7.5 minute quad (see dem/README).
Ft. Douglas quadrangle, including part of Salt Lake City, UT.
Southeast corner has coordinates 40.75 N 111.75 W
drwxrwxrwx 2 crum 512 Nov 1 19:23 terrain-old-format/MAP.37N112W
drwxrwxrwx 2 crum 512 Nov 1 19:23 terrain-old-format/MAP.37N119W
drwxrwxrwx 2 crum 512 Nov 1 19:24 terrain-old-format/MAP.38N119W
USGS NCIC terrain maps in "old" format before DEM was introduced.
Files in these directories ending in extensions .[a-s] should be
concatenated together after transfer.
-rw-rw-rw- 1 45 777251 Nov 11 11:10 world-digitized/world-digitized.tar.Z
drwxrwxr-x 7 crum 512 Nov 11 10:56 world-digitized/extracted
The "extracted" directory is produced from the tar file.
From world-digitized/expanded/doc/read.me :
The World Digitized is a collection of more than 100,000
points of latitude and longitude. When connected together, these
co-ordinates form outlines of the entire world's coastlands, islands,
lakes, and national boundaries in surprising detail.
drwxrwxrwx 2 crum 1024 Nov 12 19:10 dlg/35N86W
Digital Line Graph of area with top-left coordinates 35 N 86 W.
See dlg/README. From roskos@ida.org (Eric Roskos).
-------------------------------------------------------------------------------
Index of Back Issues, by Eric Haines
This is a list of all back issues of the RT News, email edition. I'm
retroactively giving these issues numbers for quick reference purposes.
Topics are fully listed in the first issue in which they are discussed, and
follow-up material is listed in braces {}. I've tried to summarize the
main topics covered, not the individual articles.
[Volume 0, August-December 1987] - Standard Procedural Databases, Spline
Surface Intersection, Abnormal Normals
[Volume 1, Number 1,] 1/15/88 - Solid Modelling with Faceted Primitives,
What's Wrong [and Right] with Octrees, Top Ten Hit Parade of Computer
Graphics Books, Comparison of Efficiency Schemes, Subspaces and
Simulated Annealing.
[Volume 1, Number 2,] 2/15/88 - Dore'
[Volume 1, Number 3,] 3/1/88 - {comments on Octrees, Simulated Annealing},
Efficiency Tricks, More Book Recommendations, Octree Bug Alert
[Volume 1, Number 4,] 3/8/88 - Surface Acne, Goldsmith/Salmon Hierarchy
Building, {more Efficiency Tricks}, Voxel/Quadric Primitive Overlap
Determination
[Volume 1, Number 5,] 3/26/88 - {more on Efficiency, Voxel/Quadric}, Linear
Time Voxel Walking for Octrees, more Efficiency Tricks, Puzzle,
PECG Correction
[Volume 1, Number 6,] 4/6/88 - {more on Linear Time Voxel Walking}, Thoughts
on the Theory of RT Efficiency, Automatic Creation of Object
Hierarchies (Goldsmith/Salmon), Image Archive, Espresso Database
Archive
[Volume 1, Number 7,] 6/20/88 - RenderMan & Dore', Commercial Ray Tracing
Software, Benchmarks
[Volume 1, Number 8,] 9/5/88 - SIGGRAPH '88 RT Roundtable Summary, {more
Commercial Software}, Typo in "Intro to RT" Course Notes, Vectorizing
Ray-Object Intersections, Teapot Database Archive, Mark
VandeWettering's Ray Tracer Archive, DBW Render, George Kyriazis
Ray Tracer Archive, Hartman/Heckbert PostScript Ray Tracer (source),
[Volume 1, Number 9,] 9/11/88 - {much more on MTV's Ray Tracer and utilities},
Archive for Ray Tracers and databases (SPD, teapot), Sorting on Shadow
Rays for Kay/Kajiya, {more on Vectorizing Ray-Object Intersection},
{more on George Kyriazis' Ray Tracer}, Bitmaps Library
[Volume 1, Number 10,] 10/3/88 - Bitmap Utilities, {more on Kay/Kajiya},
Wood Texture Archive, Screen Bounding Boxes, Efficient Polygon
Intersection, Bug in Heckbert Ray Tracer (?), {more on MTV's Ray
Tracer and utilities}, Neutral File Format (NFF)
[Volume 1, Number 11,] 11/4/88 - Ray/Triangle Intersection with Barycentric
Coordinates, Normal Transformation, {more on Screen Bounding Boxes},
{comments on NFF}, Ray Tracing and Applications, {more on Kay/Kajiya
and eye rays}, {more on Wood Textures}, Virtual Lighting, Parallel
Ray Tracing, RenderMan, On-Line Computer Graphics References Archive,
Current Mailing List
-----------------------------------------------------------------------------
END OF RTNEWS
_ __ ______ _ __
' ) ) / ' ) )
/--' __. __ , --/ __ __. _. o ____ _, / / _ , , , _
/ \_(_/|_/ (_/_ (_/ / (_(_/|_(__<_/ / <_(_)_ / (_</_(_(_/_/_)_
/ /|
' |/
"Light Makes Right"
February 20, 1989
Volume 2, Number 2
Compiled by Eric Haines, 3D/Eye Inc, 2359 Triphammer Rd, Ithaca, NY 14850
607-257-1381, hpfcla!hpfcrs!eye!erich@hplabs.hp.com
All contents are US copyright (c) 1989 by the individual authors
Contents:
Introduction (Eric Haines)
New Subscribers (Turner Whitted, Mike Muuss)
The BRL CAD Package (Mike Muuss)
New Book: _Illumination and Color in Computer Generated Imagery_, Roy Hall
(Eric Haines)
Uniform Distribution of Sample Points on a Surface
Depth of Field Problem (Marinko Laban)
Query on Frequency Dependent Reflectance (Mark Reichert)
"Best of comp.graphics" ftp Site (Raymond Brand)
Notes on Frequency Dependent Refraction [comp.graphics]
Sound Tracing [comp.graphics]
Laser Speckle [comp.graphics]
-----------------------------------------------------------------------------
Introduction, by Eric Haines
Whew, things have piled up! I've culled my comp.graphics findings as best as
I can. I've decided to delete everything on the question of what a stored
ray-trace image should be called ("image", "bytemap", "pixmap", and "bitmap"
were some of the candidates). It's a good question, but the discussion just
got too long to recap. Paul Heckbert's original posting advocated not using
"bitmap" for 24 bit images, since "bitmap" denotes an M x N x 1 bit deep image
in most settings. It would be pleasant to get a consensus on acceptable usage,
but it's also interesting to me from a `word history' standpoint. If you have
an opinion you'd like to share on this topic, pass it on to me and I'll
summarize them (if possible, a 25 word or less summation would be nice). My
own is: "I'm a product of my environment. Cornell used bitmap, Hewlett-Packard
uses bitmap, so I tend to use `bitmap', `24 bit deep bitmap', or `image'".
I've put all the comp.graphics postings at the end, and the good news is that
the queue is now empty. The `Sound Tracing' postings to comp.graphics were many
and wordy. I've tried to pare them down to references, interesting questions
that arose, and informed (or at least informed-sounding to my naive ears)
opinions.
-----------------------------------------------------------------------------
New Subscribers
---------------
# Turner Whitted
# Numerical Design Limited
# 133 1/2 E. Franklin Street
# P.O. Box 1316
# Chapel Hill, NC 27514
alias turner_whitted gould!rti!ndl!jtw@sun.com
[this mail path is just a good guess - does gould have an arpa connection?
The uucp path I use is:
turner_whitted hpfcrs!hpfcla!hplabs!sun!gould!rti!ndl!jtw
]
# Michael John Muuss -- ray-tracing for predictive analysis of 3-D CSG models
# Leader, Advanced Computer Systems Team
# Ballistic Research Lab
# APG, MD 21005-5066
# USA
# ARPANET: mike@BRL.MIL
# (301)-278-6678 [telephone is discouraged, use E-mail instead]
alias mike_muuss mike@BRL.MIL
I lead BRL's Advanced Computer Systems Team (ACST) in research projects in
(a) CSG solid modeling, ray-tracing, and analysis, (b) advanced processor
architectures [mostly MIMD of late], (c) high-speed networking, and (d)
operating systems. We are the developers of the BRL-CAD Package, which is a
sophisticated Combinatorial Solid Geometry (CSG) solid modeling system, with
ray-tracing library, several lighting models, a variety of non-optical
"lighting" models (eg, radar) [available on request], a device independent
framebuffer library, a collection of image-processing tools, etc. This
software totals about 150,000 lines of C code, which we make available in
source form under the terms of a "limited distribution agreement" at no charge.
My personal interests wander all over the map, right now I'm fiddling with some
animation software, some D/A converters for digital music processing, and some
improvements to our network-distributed ray-tracer protocol.
Thanks for the invitation to join!
Best,
-mike
-----------------------------------------------------------------------------
The BRL CAD PACKAGE
Short Summary
In FY87 two major releases of the BRL CAD Package software were
made (Feb-87, July-87), along with two editions of the associated
400 page manual. The package includes a powerful solid modeling
capability and a network-distributed image-processing capability.
This software is now running at over 300 sites. It has been
distributed to 42 academic institutions in twenty states and four
countries including Yale, Princeton, Stanford, MIT,USC, and UCLA.
The University of California - San Diego is using the package for
rendering brains in their Brain Mapping Project at the
Quantitative Morphology Laboratory. 75 different businesses have
requested and received the software including 23 Fortune 500
companies including: General Motors, AT&T, Crysler Motors
Corporation, Boeing, McDonnell Douglas, Lockheed, General
Dynamics, LTV Aerospace & Defense Co., and Hewlett Packard. 16
government organizations representing all three services, NSA,
NASA, NBS and the Veterns Administration are running the code.
Three of the four national laboratories have copies of the BRL
CAD package. More than 500 copies of the manual have been
distributed.
BRL-CAD started in 1979 as a task to provide an interactive
graphics editor for the BRL target description data base.
Today it is > 100,00 lines of C source code:
Solid geometric editor
Ray tracing utilities
Lighting model
Many image-handling, data-comparison, and other
supporting utilities
It runs under UNIX and is supported over more than a dozen product
lines from Sun Workstations to the Cray 2.
In terms of geometrical representation of data, BRL-CAD supports:
the original Constructive Solid Geometry (CSG) BRL data
base which has been used to model > 150 target descriptions,
domestic and foreign
extensions to include both a Naval Academy spline
(Uniform B-Spline Surface) as well as a U. of
Utah spline (Non-Uniform Rational B-Spline [NURB] Surface)
developed under NSF and DARPA sponsorship
a facerted data representation, (called PATCH),
developed by Falcon/Denver
Research Institute and used by the Navy and Air Force for
vulnerability and signature calculations (> 200 target
descriptions, domestic and foreign
It supports association of material (and other attribute properties)
with geometry which is critical to subsequent applications codes.
It supports a set of extensible interfaces by means of which geometry
(and attribute data) are passed to applications:
Ray casting
Topological representation
3-D Surface Mesh Generation
3-D Volume Mesh Generation
Analytic (Homogeneous Spline) representation
Applications linked to BRL-CAD:
o Weights and Moments-of-Inertia
o An array of Vulnerability/Lethality Codes
o Neutron Transport Code
o Optical Image Generation (including specular/diffuse reflection,
refraction, and multiple light sources, animation, interference)
o Bistatic laser target designation analysis
o A number of Infrared Signature Codes
o A number of Synthetic Aperture Radar Codes (including codes
due to ERIM and Northrop)
o Acoustic model predictions
o High-Energy Laser Damage
o High-Power Microwave Damage
o Link to PATRAN [TM] and hence to ADINA, EPIC-2, NASTRAN, etc.
for structural/stress analysis
o X-Ray calculation
BRL-CAD source code has been distributed to approximately 300
computer sites, several dozen outside the US.
----------
To obtain a copy of the BRL CAD Package distribution, you must send
enough magnetic tape for 20 Mbytes of data. Standard nine-track
half-inch magtape is the strongly preferred format, and can be written
at either 1600 or 6250 bpi, in TAR format with 10k byte records. For
sites with no half-inch tape drives, Silicon Graphics and SUN tape
cartridges can also be accommodated. With your tape, you must also
enclose a letter indicating
(a) who you are,
(b) what the BRL CAD package is to be used for,
(c) the equipment and operating system(s) you plan on using,
(d) that you agree to the conditions listed below.
This software is an unpublished work that is not generally available to
the public, except through the terms of this limited distribution.
The United States Department of the Army grants a royalty-free,
nonexclusive, nontransferable license and right to use, free of charge,
with the following terms and conditions:
1. The BRL CAD package source files will not be disclosed to third
parties. BRL needs to know who has what, and what it is being used for.
2. BRL will be credited should the software be used in a product or written
about in any publication. BRL will be referenced as the original
source in any advertisements.
3. The software is provided "as is", without warranty by BRL.
In no event shall BRL be liable for any loss or for any indirect,
special, punitive, exemplary, incidental, or consequential damages
arising from use, possession, or performance of the software.
4. When bugs or problems are found, you will make a reasonable effort
to report them to BRL.
5. Before using the software at additional sites, or for permission to
use this work as part of a commercial package, you agree to first obtain
authorization from BRL.
6. You will own full rights to any databases or images you create with this
package.
All requests from US citizens, or from US government agencies should be
sent to:
Mike Muuss
Ballistic Research Lab
Attn: SLCBR-SECAD
APG, MD 21005-5066
If you are not a US citizen (regardless of any affiliation with a
US industry), or if you represent a foreign-owned or foreign-controlled
industry, you must send your letter and tape through your Ambassador to
the United States in Washington DC. Have your Ambassador submit the
request to:
Army Headquarters
Attn: DAMI-FL
Washington, DC 20310
Best Wishes,
-Mike Muuss
Leader, Advanced Computer Systems Team
ArpaNet: <Mike @ BRL.ARPA>
--------
p.s. from David Rogers:
If you have the _Techniques in Computer Graphics_ book from Springer-Verlag the
frontispiece was done with RT the BRL ray tracer. It is also discussed in a
paper by Mike Muuss in that book.
--------
p.s. from Eric Haines:
Mike Muuss was kind enought to send me the documentation (some two inches
thick) for the BRL package. I haven't used the BRL software (sadly, it does
not seem to run on my HP machine yet - I hope someone will do a conversion
someday...), but the package looks pretty impressive. Also, such things as the
Utah RLE package and `Cake' (an advanced form of `make') come as part of the
distribution. There are also interesting papers on the system, the design
philosophy, parallelism, and many other topics included in the documentation.
-----------------------------------------------------------------------------
_Illumination and Color in Computer Generated Imagery_
by Roy Hall, Springer-Verlag, New York, 1989, 282 pages
(article by Eric Haines)
Roy Hall's book is out, and all I'll say about it is that you should have one.
The text (what little I've delved into so far) is well written and complemented
with many explanatory figures and images. There are also many appendices
(about 100 pages worth) filled with concise formulae and "C" code. Below is
the top-level Table of Contents below to give you a sense of what the book
covers.
The "C" code will probably be available publicly somewhere sometime soon. I'll
post the details here when it's ready for distribution.
1.0 Introduction 8 pages
2.0 The Illumination Process 36 pages
3.0 Perceptual Response 18 pages
4.0 Illumination Models 52 pages
5.0 Image Display 40 pages
Appendix I - Terminology 2 pages
Appendix II - Controlling Appearance 10 pages
Appendix III - Example Code 86 pages
Appendix IV - Radiosity Algorithms 14 pages
Appendix V - Equipment Sources 4 pages
References 8 pages
Index 4 pages
-----------------------------------------------------------------------------
Uniform Distribution of Sample Points on a Surface
[Mark Reichert asked last issue how to get a random sampling of a sphere]
How to generate a uniformly distributed set of rays over the unit sphere:
Generate a point inside the bi-unit cube. (Three uniform random numbers in
[-1,1].) Is that point inside the unit sphere (and not at the origin)? If
not, toss it and generate another (not too often.) If so, treat it as a vector
and normalize it. Poof, a vector on the unit sphere. This won't guarantee a
isotropic covering of the unit sphere, but is helpful to generate random
samples.
--Jeff Goldsmith
--------
One method is simply to do a longitude/latitude split-up of the sphere (and
randomly sampling within each patch), but instead of making the latitude lines
at even degree intervals, put the latitude divisions at even intervals along
the sphere axis (instead of even altitude [a.k.a. theta] angle intervals).
Equal axis divisions give us equal areas on the sphere's surface (amazingly
enough - I didn't believe it was this simple when I saw this in the Standard
Mathematics Tables book, so rederived it just to be sure).
For instance, let's say you'd like 32 samples on a unit sphere. Say we
make 8 longitude lines, so that now we want to make 4 patches per slice, and so
wish to make 4 latitudinal bands of equal area. Splitting up the vertical axis
of the sphere, we want divisions at -0.5, 0, and 0.5. To change these
divisions into altitude angles, we simply take the arcsin of the axis values,
e.g. arcsin(0.5) is 30 degrees. Putting latitude lines at the equator and at
30 and -30 degrees then gives us equal area patches on the sphere. If we
wanted 5 patches per slice, we would divide the axis of the unit sphere (-1 to
1) into 5 pieces, and so get -0.6,-0.2,0.2,0.6 as inputs for arcsin(). This
gives latitude lines on both hemispheres at 36.87 and 11.537 degrees.
The problem with the whole technique is deciding how many longitude vs.
latitude lines to make. Too many longitude lines and you get narrow patches,
too many latitude and you get squat patches. About 2 * long = lat seems pretty
good, but this is just a good guess and not tested.
Another problem is getting an even jitter to each patch. Azimuth is
obvious, but you have to jitter in the domain for the altitude. For example,
in a patch with an altitude from 30 to 90 degrees, you cannot simply select a
random degree value between 30 and 90, but rather must get a random value
between 0.5 and 1 (the original axis domain) and take the arcsin of this to
find the degree value. (If you didn't do it this way, the samples would tend
to be clustered closer to the poles instead of evenly).
Yet another problem with the above is that you get patches whose geometry
and topology can vary widely. Patches at the pole are actually triangular, and
patches near the equator will be much more squat than those closer to the
poles. If you would rather have patches with more of an equal extent than a
perfectly equal area, you could use a cube with a grid on each face cast upon
the sphere (radiosity uses half of this structure for hemi-cubes). The areas
won't be equal, but they'll be pretty close and you can weight the samples
accordingly. There are many other nice features to using this cast cube
configuration, like being able to use scan-line algorithms, being able to vary
grid size per face (or even use quadtrees), being able to access the structure
without having to perform trigonometry, etc. I use it to tessellate spheres in
the SPD package so that I won't get those annoying clusterings at the poles of
the sphere, which can be particularly noticeable when using specular
highlighting.
--Eric Haines
-----------------------------------------------------------------------------
Depth of Field Problem
From: Marinko Laban via Frits Post dutrun!frits@mcvax.cwi.nl
First an introduction. I'm a Computer Graphics student at the Technical
University of Delft, The Netherlands. My assignment was to do some research
about distributed ray tracing. I actually implemented a distributed ray tracer,
but during experiments a very strange problem came up. I implemented
depth-of-field exactly in the way R.L. Cook described in his paper. I decided
to do some experiments with the shape of the f-stop of the simulated camera.
First I simulated a square-shaped f-stop. Now I now this isn't the real thing
in an actual photocamera, but I just tried. I divided the square f-stop in a
regular raster of N x N sub-squares, just in the way you would subdivide a
pixel in subpixels. All the midpoints of the subsquares were jittered in the
usual way. Then I rendered a picture. Now here comes the strange thing. My
depth-of-field effect was pretty accurate, but on some locations some jaggies
were very distinct. There were about 20 pixels in the picture that showed very
clear aliasing of texture and object contours. The funny thing was that the
rest of the picture seemed alright. When I rendered the same picture with a
circle-shaped f-stop, the jaggies suddenly disappeared! I browsed through my
code of the square f-stop, but I couldn't find any bugs. I also couldn't find a
reasonable explanation of the appearance of the jaggies. I figure it might have
something to do with the square being not point-symmetric, but that's as far as
I can get. I would like to know if someone has experience with the same
problem, and does somebody has a good explanation for it ...
Many thanks in advance,
Marinko Laban
-----------------------------------------------------------------------------
Query on Frequency Dependent Reflectance
From: hpfcla!sunrock!kodak!supra!reichert@Sun.COM (Mark Reichert x25948)
Hello.
I'm adding fresnel reflectance to my shader. I'm in need of data for
reflectance as a function of frequency for non-polarized light at normal
incidence. I would like to build a stockpile of this data for a wide variety
of materials. I currently have some graphs of this data, but would much prefer
the actual sample points in place of the curve-fitted stuff I have now. (not to
mention the typing that you might save me).
If you have stuff such as this, and can share it with me, I would be most
appreciative. Also, if there is some Internet place where I might look, that
would be fine too.
Thanks,
Mark
-----------------------------------------------------------------------------
"Best of comp.graphics" ftp Site, by Raymond Brand
A collection of the interesting/useful [in my opinion] articles from
comp.graphics over the last year and a half is available for anonymous ftp.
It contains answers to most of the "most asked" questions from that period
as well as most of the sources posted to comp.graphics.
Now that you know what is there, you can find it in directory pub/graphics
at albanycs.albany.edu.
If you have anything to add to the collection or wish to update something
in it, or have have some ideas on how to organize it, please contact me at
one of the following.
[There's also a subdirectory called "ray-tracers" which has source code for
you-know-whats and other software--EAH]
--------
Raymond S. Brand rsbx@beowulf.uucp
3A Pinehurst Ave. rsb584@leah.albany.edu
Albany NY 12203 FidoNet 1:7729/255 (518-489-8968)
(518)-482-8798 BBS: (518)-489-8986
-----------------------------------------------------------------------------
Notes on Frequency Dependent Refraction
Newsgroups: comp.graphics
In article <3324@uoregon.uoregon.edu<, markv@uoregon.uoregon.edu (Mark VandeWettering) writes:
< }< Finally, has anyone come up with a raytracer whose refraction model
< }< takes into account the varying indices of refraction of different light
< }< frequencies? In other words, can I find a raytracer that, when looking
< }< through a prism obliquely at a light source, will show me a rainbow?
< }
< } This could be tough. The red, green, and blue components of monitors
< }only simulate the full color spectrum. On a computer, yellow is a mixture
< }of red and green. In real life, yellow is yellow. You'd have to cast a
< }large number of rays and use a large amount of computer time to simulate
< }a full color spectrum. (Ranjit pointed this out in his article and went
< }into much greater detail).
<
< Actually, this problem seems the easiest. We merely have to trace rays
< of differing frequency (perhaps randomly sampled) and use Fresnel's
< equation to determine refraction characteristics. If you are trying to
< model phase effects like diffraction, you will probably have a much more
< difficult time.
This has already been done by a number of people. One paper by T. L. Kunii
describes a renderer called "Gemstone Fire" or something. It models refraction
as you suggest to get realistic looking gems. Sorry, but I can't recall where
(or if) it has been published. I have also read several (as yet) unpublished
papers which do the same thing in pretty much the same way.
David Jevans, U of Calgary Computer Science, Calgary AB T2N 1N4 Canada
uucp: ...{ubc-cs,utai,alberta}!calgary!jevans
--------
>From: coifman@yale.UUCP (Ronald Coifman)
>> This could be tough. ...
>
>This is the easy part...
>You fire say 16 rays per pixel anyway to do
>antialiasing, and assign each one a color (frequency). When the ray
>is refracted through an object, take into account the index of
>refraction and apply Snell's law. A student here did that
>and it worked fine. He simulated rainbows and diffraction effects
>through prisms.
>
> (Spencer Thomas (U. Utah, or is it U. Mich. now?) also implemented
>the same sort of thing at about the same time.
Yep, I got a Masters degree for doing that (I was the student Rob is refer-
ring to). The problem in modelling dispersion is to integrate the primary
sample, over the visible frequencies of light. Using the Monte Carlo integra-
tion techniques of Cook on the visible spectrum yields a nice, fairly simple
solution, albeit at the cost of supersampling at ~10-20 rays per pixel, where
dispersive sampling is required.
Thomas used a different approach. He adaptively subdivided the spectrum
based on the angle of spread of the dispersed ray, given the range of frequen-
cies it represents. This can be more efficient, but can also have unlimited
growth in the number of samples. Credit Spencer Thomas; he was first.
As at least one person has pointed out, perhaps the most interesting aspect
of this problem is that of representing the spectrum on an RGB monitor. That's
an open problem; I'd be really interested in hearing about any solutions that
people have come up with. (No, the obvious CIE to RGB conversion doesn't work
worth a damn.)
My solution(s) can be found in "A Realistic Model of Refraction for Computer
Graphics", F. Kenton Musgrave, Modelling and Simulation on Microcomputers 1988
conference proceedings, Soc. for Computer Simulation, Feb. 1988, in my UC Santa
Cruz Masters thesis of the same title, and (hopefully) in an upcoming paper
"Prisms and Rainbows: a Dispersion Model for Computer Graphics" at the Graphics
Interface conference this summer. (I can e-mail troff sources for these papers
to interested parties, but you'll not get the neat-o pictures.)
For a look at an image of a physical model of the rainbow, built on the
dispersion model, see the upcoming Jan. IEEE CG&A "About the Cover" article.
Ken Musgrave
Ken Musgrave arpanet: musgrave@yale.edu
Yale U. Math Dept.
Box 2155 Yale Station Primary Operating Principle:
New Haven, CT 06520 Deus ex machina
-------------------------------------------------------------------------------
Sound Tracing
>From: ph@miro.Berkeley.EDU (Paul Heckbert)
Subject: Re: Sound tracing
[source: comp.graphics]
In article <239@raunvis.UUCP> kjartan@raunvis.UUCP
(Kjartan Pierre Emilsson Jardedlisfraedi) asks:
> Has anyone had any experience with the application of ray-tracing techniques
> to simulate acoustics, i.e the formal equivalent of ray-tracing using sound
> instead of light? ...
Yes, John Walsh, Norm Dadoun, and others at the University of British Columbia
have used ray tracing-like techniques to simulate acoustics. They called their
method of tracing polygonal cones through a scene "beam tracing" (even before
Pat Hanrahan and I independently coined the term for graphics applications).
Walsh et al simulated the reflection and diffraction of sound, and were able to
digitally process an audio recording to simulate room acoustics to aid in
concert hall design. This is my (four year old) bibliography of their papers:
%A Norm Dadoun
%A David G. Kirkpatrick
%A John P. Walsh
%T Hierarchical Approaches to Hidden Surface Intersection Testing
%J Proceedings of Graphics Interface '82
%D May 1982
%P 49-56
%Z hierarchical convex hull or minimal bounding box to optimize intersection
testing between beams and polyhedra, for graphics and acoustical analysis
%K bounding volume, acoustics, intersection testing
%A John P. Walsh
%A Norm Dadoun
%T The Design and Development of Godot:
A System for Room Acoustics Modeling and Simulation
%B 101st meeting of the Acoustical Society of America
%C Ottawa
%D May 1981
%A John P. Walsh
%A Norm Dadoun
%T What Are We Waiting for? The Development of Godot, II
%B 103rd meeting of the Acoustical Society of America
%C Chicago
%D Apr. 1982
%K beam tracing, acoustics
%A John P. Walsh
%T The Simulation of Directional Sound Sources
in Rooms by Means of a Digital Computer
%R M. Mus. Thesis
%I U. of Western Ontario
%C London, Canada
%D Fall 1979
%K acoustics
%A John P. Walsh
%T The Design of Godot:
A System for Room Acoustics Modeling and Simulation, paper E15.3
%B Proc. 10th International Congress on Acoustics
%C Sydney
%D July 1980
%A John P. Walsh
%A Marcel T. Rivard
%T Signal Processing Aspects of Godot:
A System for Computer-Aided Room Acoustics Modeling and Simulation
%B 72nd Convention of the Audio Engineering Society
%C Anaheim, CA
%D Oct. 1982
Paul Heckbert, CS grad student
508-7 Evans Hall, UC Berkeley UUCP: ucbvax!miro.berkeley.edu!ph
Berkeley, CA 94720 ARPA: ph@miro.berkeley.edu
--------
>From: jevans@.ucalgary.ca (David Jevans)
Subject: Re: Sound tracing
[source: comp.graphics]
Three of my friends did a sound tracer for an undergraduate project last year.
The system used directional sound sources and microphones and a
ray-tracing-like algorithm to trace the sound. Sound sources were digitized
and stored in files. Emitters used these sound files. At the end of the 4
month project they could digitize something, like a person speaking, run it
through the system, then pump the results through a speaker. An acoustic
environment was built (just like you build a model for graphics). You could
get effects like echoes and such. Unfortunately this was never published. I
am trying to convince them to work on it next semester...
David Jevans, U of Calgary Computer Science, Calgary AB T2N 1N4 Canada
uucp: ...{ubc-cs,utai,alberta}!calgary!jevans
--------
>From: eugene@eos.UUCP (Eugene Miya)
May I also add that you research all the work on acoustic lasers done at places
like the Applied Physics Lab.
--------
>From: riley@batcomputer.tn.cornell.edu (Daniel S. Riley)
Organization: Cornell Theory Center, Cornell University, Ithaca NY
In article <572@epicb.UUCP> david@epicb.UUCP (David P. Cook) writes:
>>In article <7488@watcgl.waterloo.edu> ksbooth@watcgl.waterloo.edu (Kelly Booth) writes:
>>>[...] It is highly unlikely that a couple of hackers thinking about
>>>the problem for a few minutes will generate startling break throughs
>>>(possible, but not likely).
Ok, I think most of us can agree that this was a reprehensible attempt at
arbitrary censorship of an interesting discussion. Even if some of the
discussion is amateurish and naive.
> The statement made above
[...]
> Is appalling! Sound processing is CENTURIES behind image processing.
> If we were to apply even a few of our common algorithms
> to the audio spectrum, it would revolutionize the
> synthizer world. These people are living in the stone
> age (with the exception of a few such as Kuerdswell [sp]).
On the other hand, I think David is *seriously* underestimating the state of
the art in sound processing and generation. Yes, Ray Kurzweil has done lots of
interesting work, but so have many other people. Of the examples David gives,
most (xor'ing, contrast stretching, fuzzing, antialiasing and quantization) are
as elementary in sound processing as they are in image processing. Sure, your
typical music store synthesizer/sampler doesn't offer these features (though
some come close--especially the E-mu's), but neither does your vcr. And the
work Kurzweil music and Kurzweil applied intelligence have done on instrument
modelling and speech recognition go WAY beyond any of these elementary
techniques.
The one example I really don't know about is ray tracing. Sound tracing is
certainly used in some aspects of reverb design, and perhaps other areas of
acoustics, but I don't know at what level diffraction is handled--and
diffraction is a big effect with sound propagation. You also have to worry
about phases, interference, and lots of other fun effects that you can (to
first order) ignore in ray tracing. References, anyone? (Perhaps I should
resubscribe to comp.music, and try there...)
(off on a tangent: does any one know of work on ray tracers that will do things
like coherent light sources, interference, diffraction, etc? In particular,
anyone have a ray tracer that will do laser speckling right? I'm pretty naive
about the state of the art in image synthesis, so I have no idea if such beasts
exist. It looks like a hard problem to me, but I'm just a physicist...)
>No, this is not a WELL RESEARCHED area as Kelly would have us believe. The
>sound people are generally not attacking sound synthesis as we attack
>vision synthesis. This is wonderful thinking, KEEP IT UP!
Much work in sound synthesis has been along lines similar to image synthesis.
Some of it is proprietary, and the rest I think just receives less attention,
since sound synthesis doesn't have quite the same level of perceived
usefulness, or the "sexiness", of image synthesis. But it is there.
Regardless, I agree with David that this is an interesting discussion, and I
certainly don't mean to discourage any one from thinking or posting about it.
-Dan Riley (dsr@lns61.tn.cornell.edu, cornell!batcomputer!riley)
-Wilson Lab, Cornell U.
--------
>From: kjartan@raunvis.UUCP (Kjartan Pierre Emilsson Jardedlisfraedi)
Newsgroups: comp.graphics
We would like to begin by thanking everybody for their good replies, which
will in no doubt come handy. We intend to try to implement such a sound tracer
soon and we had already made some sort of model for it, but we were checking
whether there was some info lying around about such tracers. It seems that our
idea wasn't far from actual implementations and that is reassuring.
For the sake of Academical Curiosity and overall Renaissance-like
Enlightenment in the beginning of a new year we decided to submit our crude
model to the critics and attention of this newsgroup, hoping that it won't
interfere too much with the actual subject of the group, namely computer
graphics.
The Model:
We have some volume with an arbitrary geometry (usually simple such
as a concert hall or something like that). Squares would work just
fine as primitives. Each primitive has definite reflection
properties in addition to some absorption filter which possibly
filters out some frequencies and attenuates the signal.
In this volume we put a sound emitter which has the following
form:
The sound emitter generates a sound sample in the form
of a time series with a definite mean power P. The emitter
emits the sound with a given power density given as some
spherical distribution. For simplicity we tessellate this
distribution and assign to each patch the corresponding mean
power.
At some other point we place the sound receptor which has the
following form:
We take a sphere and cut it in two equal halves, and then
separate the two by some distance d. We then tessellate the
half-spheres (not including the cut). We have then a crude
model of ears.
Now for the actual sound tracing we do the following:
For each patch of the two half-spheres, we cast a ray
radially from the center, and calculate an intersection
point with the enclosing volume. From that point we
determine which patch of the emitter this corresponds to,
giving us the emitted power. We then pass the corresponding
time series through the filter appropriate to the given
primitives, calculate the reflected fraction, attenuate the
signal by the square of the distance, and eventually
determine the delay of the signal.
When all patches have been traced, we sum up all the time
series and output the whole lot through some stereo device.
A more sophisticated model would include secondary rays and
sound 'shadowing' (The shadowing being a little tricky as it is
frequency dependent)
pros & cons ?
Happy New Year !!
-Kjartan & Dagur
Kjartan Pierre Emilsson
Science Institute - University of Iceland
Dunhaga 3
107 Reykjavik
Iceland Internet: kjartan@raunvis.hi.is
--------
>From: brent@itm.UUCP (Brent)
Organization: In Touch Ministries, Atlanta, GA
Ok, here's some starting points: check out the work of M. Schroeder at the
Gottingen. (Barbarian keyboard has no umlauts!) Also see the recent design work
on the Orange County Civic Auditorium and the concert hall in New Zealand.
These should get you going in the right direction. Dr. Schroeder laid the
theoretical work and others ran with it. As far as sound ray tracing and
computer acoustics being centuries behind, I doubt it. Dr. S. has done things
like record music in stereo in concert halls, digitized it, set up playback
equipment in an anechoic chamber (bldg 15 at Murry Hill), measured the path
from the right speaker to the left ear, and from the left speaker to the right
ear, digitized the music and did FFTs to take out the "crossover paths" he
measured. Then the music played back sounded just like it did in the concert
hall. All this was done over a decade ago.
Also on acoustic ray tracing: sound is much "nastier" to figure than
pencil-rays of light. One must also consider the phase of the sound, and the
specific acoustic impedance of the reflecting surfaces. Thus each reflection
introduces a phase shift as well as direction and magnitude changes. I haven't
seen too many optical ray-tracers worrying about interference and phase shift
due to reflecting surfaces. Plus you have to enter vast world of
psychoacoustics, or how the ear hears sound. In designing auditoria one must
consider "binaural dissimilarity" (Orange County) and the much-debated
"auditory backward inhibition" (see the Lincoln Center re-designs).
Resonance?? how many optical chambers resonate? (outside lasers?) All in all,
modern acoustic simulations bear much more resemblance to Quantum Mechanic
"particle in the concert hall" type calculations than to simple ray-traced
optics.
Postscript: eye-to-source optical ray tracing is a restatement of
Rayleigh's "reciprocity principle of sound" of about a century ago.
Acoustitions have been using it for at least that long.
happy listening,
brent laminack (gatech!itm!brent)
--------
Reply-To: trantow@csd4.milw.wisc.edu (Jerry J Trantow)
Subject: Geometric Acoustics (Sound Tracing)
Summary: Not so easy, but here are some papers
Organization: University of Wisconsin-Milwaukee
Some of the articles I have found include
Criteria for Quantitative Rating and Optimum Design on Concert Halls
Hulbert, G.M. Baxa, D.E. Seireg, A.
University of Wisconsin - Madison
J Acoust Soc Am v 71 n 3 Mar 83 p 619-629
ISSN 0001-4966, Item Number: 061739
Design of room acoustics and a MCR reverberation system for Bjergsted
Concert hall in Stavanger
Strom, S. Krokstad, A. Sorsdal, S. Stensby, S.
Appl Acoust v19 n6 1986 p 465-475
Norwegian Inst of Technology, Trondheim, Norw
ISSN 0003-682X, Item Number: 000913
I am also looking for an English translation of:
Ein Strahlverfolgungs-Verafahren Zur Berechnung von Schallfelern in Raemem
[ Ray-Tracing Program for the calculation of sound fields of rooms ]
Voralaender, M.
Acoustica v65 n3 Feb 88 p 138-148
ISSN 0001-7884, Item Number: 063350
If anyone is interested in doing a translation I can send the German copy that
I have. It doesn't do an ignorant fool like myself any good and I have a hard
time convincing my wife or friends who know Deutch to do the translation.
A good literature search can discover plenty of articles, quite a few of which
are about architectural design of music halls. With a large concert hall, the
calculations are easier because of the dimensions. (the wavelength is small
compared to the dimensions of the hall)
The cases I am interested in are complicated by the fact that I want to work
with relatively small rooms, large sources, and to top it off low (60hz)
frequencies. I vaguely remember seeing a blurb somewhere about a program done
by BOSE ( the speaker company) that calculated sound fields generated by
speakers in a room. I would appreciate any information on such a beast.
The simple source for geometric acoustics is described in Beranek's Acoustic in
the chapter on Radiation of Sound. To better appreciate the complexity from
diffraction, try the chapter on The Radiation and Scattering of Sound in Philip
Morse's Vibration and Sound ISBN 0-88318-287-4.
I am curious as to the commercial software that is available in this area.
Does anyone have any experience they could comment on???
------
>From: markv@uoregon.uoregon.edu (Mark VandeWettering)
Subject: More Sound Tracing
Organization: University of Oregon, Computer Science, Eugene OR
I would like to present some preliminary ideas about sound tracing, and
critique (hopefully profitably) the simple model presented by Kjartan Pierre
Emilsson Jardedlisfraedi. (Whew! and I thought my name was bad, I will
abbreviate it to KPEJ)
CAVEAT READER: I have no expertise in acoustics or sound engineering. Part of
the reason I am writing this is to test some basic assumptions that I have made
during the course of thinking about sound tracing. I have done little/no
research, and these ideas are my own.
KJEP had a model related below:
> We have some volume with an arbitrary geometry (usually simple such
> as a concert hall or something like that). Squares would work just
> fine as primitives. Each primitive has definite reflection
> properties in addition to some absorption filter which possibly
> filters out some frequencies and attenuates the signal.
One interesting form of sound reflector might be the totally
diffuse reflector (Lambertian reflection). It seems that if
this is the assumption, then the appropriate algorithm to use
might be radiosity, as opposed to raytracing. Several problems
immediately arise:
1. how to handle diffraction and interference?
2. how to handle "relativistic effects" (caused by
the relatively slow speed of sound)
The common solution to 1 in computer graphics is to ignore it.
Is this satisfactory in the audio case? Under what
circumstances or applications is 1 okay?
Point 2 is not often considered in computer graphics, but in
computerized sound generation, it seems critical to accurate
formation of echo and reverberation effects. To properly handle
time delay in radiosity would seem to require a more difficult
treatment, because the influx of "energy" at any given time
from a given patch could depend on the outgoing energy at a
number of previous times. This seems pretty difficult, any
immediate ideas?
> Now for the actual sound tracing we do the following:
>
> For each patch of the two half-spheres, we cast a ray
> radially from the center, and calculate an intersection
> point with the enclosing volume. From that point we
> determine which patch of the emitter this corresponds to,
> giving us the emitted power. We then pass the corresponding
> time series through the filter appropriate to the given
> primitives, calculate the reflected fraction, attenuate the
> signal by the square of the distance, and eventually
> determine the delay of the signal.
>
> When all patches have been traced, we sum up all the time
> series and output the whole lot through some stereo device.
One open question: how much directional information is captured
by your ears? Since you can discern forward/backward sounds as
well as left/right, it would seem that ordinary stereo
headphones are incapable of reproducing sounds as complex as one
would like. Can the ears be fooled in clever ways?
The only thing I think this model lacks is secondary "rays" or
echo/reverb effects. Depending on how important they are,
radiosity algorithms may be more appropriate.
Feel free to comment on any of this, it is an ongoing "thought
experiment", and has made a couple of luncheon conversations
quite interesting.
Mark VandeWettering
--------
>From: ksbooth@watcgl.waterloo.edu (Kelly Booth)
Organization: U. of Waterloo, Ontario
In article <3458@uoregon.uoregon.edu> markv@drizzle.UUCP (Mark VandeWettering) writes:
>
> 1. how to handle diffraction and interference?
> 2. how to handle "relativistic effects" (caused by
> the relatively slow speed of sound)
>
> The common solution to 1 in computer graphics is to ignore it.
Hans P. Moravec,
"3D Graphics and Wave Theory"
Computer Graphics 15:3 (August, 1981) pp. 289-296.
(SIGGRAPH '81 Proceedings)
[Trivia Question: Why does the index for the proceedings list this as starting
on page 269?]
Also, something akin to 2 has been tackled in some ray tracers where dispersion
is taken into account (this is caused by the refractive index depending on the
frequency, which is basically a differential speed of light).
-----------------------------------------------------------------------------
Laser Speckle
>From: jevans@cpsc.ucalgary.ca (David Jevans)
In article <11390016@hpldola.HP.COM>, paul@hpldola.HP.COM (Paul Bame) writes:
> A raytracer which did laser speckling right might also be able
> to display holograms.
A grad student at the U of Calgary a couple of years ago did something like
this. He was using holographic techniques for character recognition, and could
generate synthetic holograms. Also, what about Pixar? See IEEE CG&A 3 issues
ago.
David Jevans, U of Calgary Computer Science, Calgary AB T2N 1N4 Canada
uucp: ...{ubc-cs,utai,alberta}!calgary!jevans
--------
>From: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Laser speckle is a particularly special case of interference, because it
happens in your eye, not on the surface that the laser is hitting.
A ray-tracing system that dealt with interference of light from different
sources would show the interference fringes that occur when a laser light
source is split into two beams and recombined, and the interference of acoustic
waves. But to simulate laser speckle, you'd have to trace the light path all
the way back into the viewer's eye and calculate interference effects on the
retina itself.
If you don't believe me, try this: create a normal two-beam interference fringe
pattern. As you move your eye closer, the fringes remain the same physical
distance apart, becoming wider apart in angular position as viewed by your eye.
The bars will remain in the same place as you move your head from side to side.
Now illuminate a target with a single clean beam of laser light. You will see
a fine speckle pattern. As you move your eye closer, the speckle pattern does
not seem to get any bigger - the spots remain the same angular size as seen by
your eye. As you move your head from side to side, the speckle pattern moves.
As the laser light reflects from a matte surface, path length differences
scramble the phase of light traveling by slightly different paths. When a
certain amount of this light is focused on a single photoreceptor in your eye
(or a camera), the light combines constructively or destructively, giving the
speckle pattern. But the size of the "grains" in the pattern is basically the
same as the spacing of the photoreceptors in your eye - basically each cone in
your eye is receiving a random signal independent of each other cone.
The effect depends on the scattering surface being rougher than 1/4 wavelength
of light, and the scale of the roughness being smaller than the resolution
limit of the eye as seen from the viewing position. This is true for almost
anything except a highly-polished surface, so most objects will produce
speckle.
Since the pattern is due to random variation in the diffusing surface, there is
little point in calculating randomness there, tracing rays back to the eye, and
seeing how they interfere - just add randomness directly to the final image
(although this won't correctly model how the speckle "moves" as you move your
head).
However, to model speckle accurately, the pixel spacing in the image has to be
no larger than the resolution limit of the eye, about half an arc minute. For
a CRT or photograph viewed from 15 inches away, that's 450 pixels/inch, far
higher than most graphics displays are capable of. So, unless you have that
sort of system resolution, you can't show speckle at the correct size.
-----------------------------------------------------------------------------
END OF RTNEWS