jef@well.sf.ca.us (Jef Poskanzer) (07/22/90)
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---
Jef
Jef Poskanzer jef@well.sf.ca.us {ucbvax, apple, hplabs}!well!jef
"...Is this a trick question?"
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Contents:
1) General references for graphics questions.
2) Drawing three-dimensional objects on a two-dimensional screen.
3) Quantizing 24 bit images down to 8 bits.
4) Converting color into grayscale.
5) Quantizing grayscale to black&white.
6) Rotating a raster image by an arbitrary angle.
7) Free image manipulation software.
8) Format documents for GIF, TIFF, IFF, BIFF, WHIFF, etc.
9) Converting between vector formats.
10) How to get Pixar films.
11) How do I draw a circle as a Bezier (or B-spline) curve?
12) How to order standards documents.
13) How to FTP by email.
14) How to tell whether a point is within a planar polygon.
1) General references for graphics questions:
Computer Graphics: Principles and Practice (2nd Ed.), J.D. Foley,
A. van Dam, S.K. Feiner, J.F. Hughes, Addison-Wesley 1990, ISBN
0-201-12110-7
Principles of Interactive Computer Graphics (2nd Ed.), Newman and Sproull,
McGraw Hill, ISBN 0-07-046338-7
Procedural Elements for Computer Graphics, David F. Rogers, McGraw Hill,
ISBN 0-07-053534-5
Mathematical Elements for Computer Graphics 2nd Ed., David F. Rogers and
J. Alan Adams, McGraw Hill, ISBN 0-07-053530-2
Applied Concepts in Microcomputer Graphics, Bruce Artwick, Prentice-Hall,
ISBN 0-13-039322-3
Digital Picture Processing, vols. 1&2, Azriel Rosenfeld and Avi Kak,
Academic Press 1976
Three Dimensional Computer Graphics, Alan Watt, Addison-Wesley, ISBN
0-201-15442-0
An Introduction to Ray Tracing, Andrew Glassner (ed.), Academic Press,
1989, ISBN 0-12-286160-4
2) Drawing three-dimensional objects on a two-dimensional screen. The
simple answer is, you divide by the depth. For a more verbose explanation,
see any of the above references.
3) Quantizing 24 bit images down to 8 bits. Find a copy of "Color
Image Quantization for Frame Buffer Display" by Paul Heckbert, SIGGRAPH
'82 Proceedings, page 297. There are other algorithms, but this one
works well and is fairly simple. Implementations are included in
most raster toolkits (see item 7 below).
4) Converting color into grayscale. The NTSC formula is:
luminosity = .299 red + .587 green + .114 blue
5) Quantizing grayscale to black&white. The only reference you need
for this stuff is:
Digital Halftoning, Robert Ulichney, MIT Press, ISBN 0-262-21009-6
But before you go off and start coding, check out the image manipulation
software mentioned in item 7 below. All of the packages mentioned can do
some form of gray to b&w conversion.
6) Rotating a raster image by an arbitrary angle. The obvious but
wrong method is to loop over the pixels in the source image, transform
each coordinate, and copy the pixel to the destination. This is wrong
because it leaves holes in the destination. Instead, loop over the
pixels in the destination image, apply the *reverse* transformation to
the coordinates, and copy that pixel from the source. This method is
quite general, and can be used for any one-to-one 2-D mapping, not just
rotation. You can add anti-aliasing by doing sub-pixel sampling.
However, there is a much faster method, with antialising included,
which involves doing three shear operations. The method was originally
created for the IM Raster Toolkit (see below); an implementation is
also present in PBMPLUS. Reference: "A Fast Algorithm for Raster
Rotation", by Alan Paeth (awpaeth@watcgl.waterloo.edu) Graphics
Interface '86 (Vancouver). An article on the IM toolkit appears in
the same journal.
7) Free image manipulation software. There are a number of toolkits
for converting from one image format to another, doing simple image
manipulations such as size scaling, plus the above-mentioned 24 -> 8,
color -> gray, gray -> b&w conversions. Here are pointers to some of
them:
PBMPLUS, by Jef Poskanzer. Comprehensive format conversion and image
manipulation package. The latest version is always available via
anonymous FTP as expo.lcs.mit.edu:contrib/pbmplus.tar.Z and
ftp.ee.lbl.gov:pbmplus.tar.Z. The version of 22nov89 (which currently
is still the latest version, except for the one official patch so far)
was posted to comp.sources.misc, and is therefore accessible via mail
to one of the archive servers. This version is also available in the
X.V11R4 release tape.
IM Raster Toolkit, by Alan Paeth (awpaeth@watcgl.uwaterloo.ca).
Provides a portable and efficient format and related toolkit. The
format is versatile in supporting pixels of arbitrary channels,
components, and bit precisions while allowing compression and machine
byte-order independence. The kit contains more than 50 tools with
extensive support of image manipulation, digital halftoning and format
conversion. Previously distributed on tape c/o the University of
Waterloo, an FTP version will appear someday.
Utah RLE Toolkit. Conversion and manipulation package, similar to
PBMPLUS. Available via FTP as cs.utah.edu:pub/toolkit-2.0.tar.Z and
ucsd.edu:graphics/utah-raster-toolkit.tar.Z.
Fuzzy Pixmap Manipulation, by Michael Mauldin <mlm@nl.cs.cmu.edu>.
Conversion and manipulation package, similar to PBMPLUS. Version 1.0
available via FTP as nl.cs.cmu.edu:/usr/mlm/ftp/fbm.tar.Z,
uunet.uu.net:pub/fbm.tar.Z, and ucsd.edu:graphics/fbm.tar.Z.
Img Software Set, by Paul Raveling <raveling@venera.isi.edu>. Reads and
writes its own image format, displays on an X.V11 screen, and does some
image manipulations. Version 1.3 is available via FTP as expo.lcs.mit.edu:
contrib/img_1.3.tar.Z, and venera.isi.edu:pub/img_1.3.tar.Z along with
a large collection of color images.
Xim, by Philip R. Thompson. Reads and writes its own image format,
displays on an X.V11 screen, and does some image manipulations.
Available in your nearest X.V11R4 source tree as contrib/clients/xim.
xloadimage, by Jim Frost <madd@std.com>. Reads in images in various
formats and displays them on an X.V11 screen. Available via FTP as
expo.lcs.mit.edu:contrib/xloadimage*, and in your nearest comp.sources.x
archive.
TIFF Software, by Sam Leffler <sam@okeeffe.berkeley.edu>. Nice
portable library for reading and writing TIFF files, plus a few tools
for manipulating them and reading other formats. Available via FTP as
ucbvax.berkeley.edu:pub/tiff/*.tar.Z or uunet.uu.net:graphics/tiff.tar.Z
ALV, a Sun-specific image toolkit. Version 2.0.6 posted to
comp.sources.sun on 11dec89. Also available via email to
alv-users-request@cs.bris.ac.uk.
popi, an image manipulation language. Version 2.1 posted to
comp.sources.misc on 12dec89.
Don't forget to set binary mode when you FTP tar files. For you MILNET
folks who still don't have name servers, the IP addresses are:
expo.lcs.mit.edu 18.30.0.212
ftp.ee.lbl.gov 128.3.254.68
cs.utah.edu 128.110.4.21
NL.CS.CMU.EDU 128.2.222.56
venera.isi.edu 128.9.0.32
okeeffe.berkeley.edu 128.32.130.3
Please do *not* post or mail messages saying "I can't FTP, could
someone mail this to me?" There are a number of automated mail servers
that will send you things like this in response to a message. See
item 13 below for details on some.
Also, the newsgroup alt.graphics.pixutils is specifically for discussion
of software like this. You may find useful information there.
8) Format documents for GIF, TIFF, IFF, BIFF, WHIFF, etc. You almost
certainly don't need these. Read the above item 7 on free image
manipulation software. Get one or more of these packages and look
through them. Chances are excellent that the image converter you were
going to write is already there. But if you still want one of the
format documents, the following files are available by anonymous ftp
from titan.rice.edu (128.42.1.30) in directory public/graphics.formats:
2498 Apr 26 11:07 cgm.info
1637 Apr 26 11:07 facesaver.doc
31426 Apr 26 11:07 gif.doc
1579 Apr 26 11:07 gks.ptr
1193 Apr 26 11:07 hpgl.ptr
63217 Apr 26 11:07 iff.doc
501 Apr 26 11:08 iges.ptr
45172 Apr 26 11:08 lzw.and.gif.doc
3068 Apr 26 11:08 sun.rasterfile.doc
54516 Apr 26 11:08 tiff.doc
The ".doc" files are a description of the file formats. The ".ptr"
files describe how to order a description. cgm.info describes what cgm
is. These files were collected off the net and are believed to be
correct. Other file format descriptions are welcome, send to Mark
Hall <foo@rice.edu>.
9) Converting between vector formats. A lot of people ask about
converting from HPGL to PostScript, or MacDraw to CGM, or whatever. It
is important to understand that this is a very different problem from
the image format conversions in item 7. Converting one image format to
another is a fairly easy problem, since once you get past all the file
header junk, a pixel is a pixel -- the basic objects are the same for
all image formats. This is not so for vector formats. The basic
objects -- circles, ellipses, drop-shadowed patttern-filled
round-cornered rectangles, etc. -- vary from one format to another.
Except in extremely restricted cases, it is simply not possible to do a
one-to-one conversion between vector formats.
On the other hand, it is quite possible to do a close approximation,
rendering an image from one format using the primatives from another.
But as far as I know, no one has put together a toolkit of converters.
So, the current answer to this frequently asked question is "Uhhhhh..."
Stay tuned for further developments.
A related frequent question is how to convert from some vector format
to a bitmapped image - from PostScript to Sun raster format, or HPGL to
X11 bitmap. For example, some of the commercial PostScript clones for
PC's allow you to render to a disk file as well as a printer. But in
general, the answer is again "Uhhhhh..." However, if someone were to
put together a vector to vector conversion toolkit, adding a vector to
raster converter would be trivial.
10) How to get Pixar films.
The various John Lasseter / Pixar computer animated shorts are
available on video tape. You can order them from Direct Cinema
Limited:
Film Price
Luxo, Jr. $14.95
Red's Dream $19.95
Tin Toy $24.95
Knickknack $24.95
Luxo, Jr./Red's Dream/Tin Toy $49.95
All tapes are on 1/2" VHS NTSC. Add $10/tape for PAL format. Also
available:
Tin Toy T-shirt $15.00
Knickknack 3D T-shirt $15.00 (includes glasses)
Add $5 S&H for the first tape or shirt, $2 for each additional tape or
shirt. Foreign shipping, add $3/tape or shirt. Call 213-652-8000 to
charge to your credit card. I would suggest calling first to verify
prices and availability.
11) How do I draw a circle as a Bezier (or B-spline) curve?
The short answer is, "You can't." Unless you use a rational spline you
can only approximate a circle. The approximation may look acceptable,
but it is sensitive to scale. Magnify the scale and the error of
approximation magnifies. Deviations from circularity that were not
visible in the small can become glaring in the large. If you want to
do the job right, consult the article:
"A Menagerie of Rational B-Spline Circles"
by Leslie Piegl and Wayne Tiller
in IEEE Computer Graphics and Applications, volume 9, number 9,
September, 1989, pages 48-56.
For rough, non-rational approximations, consult the book:
Computational Geometry for Design and Manufacture
by I. D. Faux and M. J. Pratt,
Ellis Horwood Publishers, Halsted Press, John Wiley.
For the best known non-rational approximations, consult the article:
"Good Approximation of Circles by Curvature-continuous Bezier Curves"
by Tor Dokken, Morten Daehlen, Tom Lyche, and Knut Morken
in Computer Aided Geometric Design, volume 7, numbers 1-4 (combined),
June, 1990, pages 33-41 [Elsevier Science Publishers (North-Holland)]
12) How to order standards documents.
The American National Standards Institute sells ANSI standards, and also
ISO (international) standards. Their sales office is at (212) 642-4900,
mailing address is 1430 Broadway, NY NY 10018. It helps if you have the
complete name and number.
Some useful numbers to know:
CGM (Computer Graphics Metafile) is ANSI X3.122-1986. GKS (Graphical
Kernel System) is ANSI X3.124-1985. PHIGS (Programmer's Hierarchical
Interactive Graphics System) is ANSI X3.144-1988. Language bindings
are often separate but related numbers; for example, the GKS FORTRAN
binding is X3.124.1-1985.
Standards-in-progress are made available at key milestones to solicit
comments from the graphical public (this includes you!). ANSI can let
you know where to order them; most are available from Global Engineering
at (800) 854-7179. The earliest public review is called "draft
proposed" (dpANS or dpIS), the last one before publication is "draft"
(dIS). For example, the CGI standard is currently dIS X3.161.
13) How to FTP by email.
There are a number of sites that archive the Usenet sources newsgroups
and make them available via an email query system. You send a message
to an automated server saying something like "send comp.sources.unix/fbm",
and a few hours or days later you get the file in the mail.
There are also a couple of sites that will perform general FTP retrievals
for you in response to a similar mail query. For information on using
one of them, send a message like this:
To: info-server@cs.net
request: info
topic: help-ftp
request: end
(NOTE: this server is currently "down for repairs". No estimate on when
or if it will return.)
And for info on another one, send this:
To: bitftp@pucc.bitnet
help
Please be considerate, and don't over-use these services. If people
start using them to retrieve hundreds of megabytes of GIF files, they
will probably disappear.
14) How to tell whether a point is within a planar polygon.
Consider a ray originating at the point of interest and continuing to
infinity. If it crosses an odd number of polygon edges along the way,
the point is within the polygon. If the ray crosses an even number of
edges, the point is either outside the polygon, or within an interior
hole formed from intersectiong polygon edges. This idea is known in
the trade as the Jordan curve theorem; see Eric Haines' article in
Glassner's ray tracing book (above) for more information, including
treatment of special cases.