tada@athena.mit.edu (Ivan Tadayoff) (01/31/88)
Does anyone know what values of RGB to use to make a nice looking silver? (as in a metallic object?) I've tried a variety of combinations and none of them seem very pleasing. While on the subject, does anyone know a good book on how to color ray- tracing views? For example, how much does a light ray change when it reflects off a metallic sphere? Thanks in advance for any help.
atc@ut-sally.UUCP (Alvin T. Campbell III) (02/02/88)
In article <2679@bloom-beacon.MIT.EDU> tada@athena.mit.edu (Ivan Tadayoff) writes: > >Does anyone know what values of RGB to use to make a nice looking silver? >(as in a metallic object?) I've tried a variety of combinations and none >of them seem very pleasing. > >While on the subject, does anyone know a good book on how to color ray- >tracing views? For example, how much does a light ray change when it >reflects off a metallic sphere? > >Thanks in advance for any help. You are attempting to do something very difficult. Metallic reflection can not be done very accurately using only an RGB triple to model reflective properties. The most common shading model in graphics, developed at the University of Utah, assumes that the diffuse color is dependent on the light source color and the object color, but the specular color is dependent only upon the light color. This is a reasonable approximation for plastic, but not for metal. Most of the perceived color of metal is from a specular component dependent on the light and the surface properties. A reference which gives a starting point on how to do metallic shading is the following: Cook, Robert L., and Kenneth E. Torrance, "A Reflectance Model for Computer Graphics", Computer Graphics (Proceedings of SIGGRAPH '81) v. 15, no. 3, (August 1981), pp.307-316. After digesting the material in the above reference, which will probably take a while, you can get reflectance spectra from Purdue University, Thermophysical Properties of Matter, v. 7: Thermal Radiative Properties of Metals, 1970. If all of this seems like too much work, and you just want to settle for an RGB triple for silver, the best you can do is probably the following: R = .95 G = .95 B = .95 Diffuse Coefficient = .25 Specular Coefficient = .75 Specular Power = 6.0 I hope this helps. --A. T. Campbell-- Computer Graphics Lab Department of Computer Sciences University of Texas Austin, Texas atc@sally.CS.UTEXAS.EDU
cfchiesa@bsu-cs.UUCP (Sir Xetwnk) (02/04/88)
In article <2679@bloom-beacon.MIT.EDU>, tada@athena.mit.edu (Ivan Tadayoff) writes: > > Does anyone know what values of RGB to use to make a nice looking silver? > (as in a metallic object?) I've tried a variety of combinations and none > of them seem very pleasing. > > While on the subject, does anyone know a good book on how to color ray- > tracing views? For example, how much does a light ray change when it > reflects off a metallic sphere? > > Thanks in advance for any help. Without going into the math, but perhaps filling in a few general concepts, your question isn't so much one of finding the right COLOR for "silver," but rather of how to vary that color properly so that it gives the impression of a reflective, rather than dull or "matte," surface. A silver object is actually colorless, or perhaps "gray" (as the previous respondent's "0.95-for- all-three-primary-colors indicates), but usually reflects its surroundings. In general, the more clearly (less fuzzy or vague) the reflections appear, the shinier (silverier) your object appears to be. If I'm not mistaken, the answer to "how much does a light ray change...etc" has to do with defining a cone whose tip is at the reflection point and whose axis is the normal to the sphere surface at that point. I *think* you then determine whether your light source is within that cone, how far (angle) it is from the cone axis, and use that distance to compute how much "specular" contribution to consider at that point, to be added to the overall illumina- tion of the surface. The wider (larger angle-at-the-tip) the cone, the more diffuse the specular reflection, and the softer "finish" your surface appears to have. As for reflecting other objects in the environment, I haven't seen the specifics anywhere, but would think that they should be handled similarly.
hansen@mips.COM (Craig Hansen) (02/04/88)
In article <2679@bloom-beacon.MIT.EDU>, tada@athena.mit.edu (Ivan Tadayoff) writes: > Does anyone know what values of RGB to use to make a nice looking silver? > (as in a metallic object?) I've tried a variety of combinations and none > of them seem very pleasing. > > While on the subject, does anyone know a good book on how to color ray- > tracing views? For example, how much does a light ray change when it > reflects off a metallic sphere? > > Thanks in advance for any help. The important feature to note is that a flat shade isn't going to look silver. If you add specular reflections, it will make a dramatic difference between a silver-colored plastic-appearing object and a silver-matallic object. The color of the specular reflection, unlike the diffuse reflection is generally the color of the light source, not the color of the object. [This is a simplification, there are much more sophisticated metallic color models. I think there was a SigGraph paper by Cook and Torrance that covered this well.] -- Craig Hansen Manager, Architecture Development MIPS Computer Systems, Inc. ...{ames,decwrl,prls}!mips!hansen or hansen@mips.com 408-991-0234
jackm@devvax.JPL.NASA.GOV (Jack Morrison) (02/05/88)
In article <1487@mips.mips.COM> hansen@mips.COM (Craig Hansen) writes: >> Does anyone know what values of RGB to use to make a nice looking silver? >> (as in a metallic object?) I've tried a variety of combinations and none >The important feature to note is that a flat shade isn't going to look >silver. If you add specular reflections, it will make a dramatic difference >The color of the specular reflection, unlike the diffuse reflection is >generally the color of the light source, not the color of the object. But if you really want the shiny silver look (like Abel's "Sexy Robot"), what you need to do is map reflections from a made-up environment. Check your Siggraph proceedings - I believe it was Dr. Blinn's paper, "Texture and Reflection Mapping" or some such. You know, the ol' teapot. -- Jack C. Morrison Jet Propulsion Laboratory (818)354-1431 jackm@jpl-devvax.jpl.nasa.gov "The paycheck is part government property, but the opinions are all mine."
val@terminus.UUCP (Val Kartchner) (02/05/88)
In article <2679@bloom-beacon.MIT.EDU>, tada@athena.mit.edu (Ivan Tadayoff) writes: > > Does anyone know what values of RGB to use to make a nice looking silver? > (as in a metallic object?) I've tried a variety of combinations and none > of them seem very pleasing. > There is no one color that makes silver or any other metallic looking colors. The metallic appearance of an object (or picture) is that the metal is reflecting the light that is shinning on it. This produces deviations from the most predominant color giving a metallic look. If the metal is smoother, you get a more perfect reflection, with the tint of the metal applied to the reflection. (e.g.: silver reflects white or a bright gray, but gold will reflect a yellow (red and green).) -------------------------- HLS to RGB conversion --------------------- Does anyone know how to convert colors from RGB (Red/Green/Blue) specifications to HLS (Hue/Lightness/Saturation) [HIS (Hue/Intensity/Saturation) or HSI]. (We would also be interested in converting from HSI to RGB.) We have VT241/VT340's which require specifications in HSI, and we want to plot images that are specified in RGB values. Mathematical specifications would be OK, but PLEASE include an English translation. What we would like is to have a function, subroutine, or program (written in a common computer language) that does either or both tricks. Thank you in advance, -=:[ VAL ]:=- -- ---- /\ ------------------------------------------------------------------ /\/\ . /\ | Val Kartchner | This space / \/ \/\/ \ | #include <disclaimer.h> | intentionally blank ===/ U i n T e c h \===!ihnp4!utah-cs!utah-gr!uplherc!sp7040!terminus!val===
markv@uoregon.UUCP (Mark VandeWettering) (02/06/88)
In article <2679@bloom-beacon.MIT.EDU> tada@athena.mit.edu (Ivan Tadayoff) writes: >Does anyone know what values of RGB to use to make a nice looking silver? >(as in a metallic object?) I've tried a variety of combinations and none >of them seem very pleasing. It is not merely a matter of selecting the appropriate RGB values, but also building a reflection model that is realistic. Normal surfaces reflect light evenly in every direction in every wavelength (I know, an oversimplification). Metallic surfaces exhibit a more complex behavior. Making a convincing metallic surface has been covered in some of the appropriate literature, but I am at home and my bibliography for such stuff isn't here. >While on the subject, does anyone know a good book on how to color ray- >tracing views? For example, how much does a light ray change when it >reflects off a metallic sphere? Books indeed could be written, but I know of none. Siggraph Course notes are great if you can get 'em. I wish I hadn't lent mine out to some person in need. > >Thanks in advance for any help.
alibaba@ucscb.UCSC.EDU (73539000) (02/09/88)
I would suggest you pick up a copy of "Fundamentals of Interactive Computer Graphics" by J.D. Foley and VanDam. Addison- Wesley publishes it. It tells ya how to convert every system to every other. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~ Alexander M. Rosenberg ~ INTERNET: alibaba@ucscb.ucsc.edu ~ Yoyodyne ~ ~ Crown College, UCSC ~ UUCP:...!ucbvax!ucscc!ucscb!alibaba~ Propulsion ~ ~ Santa Cruz, CA 95064 ~ BITNET:alibaba%ucscb@ucscc.BITNET ~ Systems, Inc~ ~ (408) 426-8869 ~ Disclaimer: Nobody is my employer ~ :-) ~ ~ ~ so nobody cares what I say. ~ ~
dave@onfcanim.UUCP (Dave Martindale) (02/23/88)
In article <2029@bsu-cs.UUCP> cfchiesa@bsu-cs.UUCP (Sir Xetwnk) writes: > >Without going into the math, but perhaps filling in a few general concepts, >your question isn't so much one of finding the right COLOR for "silver," >but rather of how to vary that color properly so that it gives the impression >of a reflective, rather than dull or "matte," surface. A silver object is >actually colorless, or perhaps "gray" (as the previous respondent's "0.95-for- >all-three-primary-colors indicates), but usually reflects its surroundings. Actually, silver is not uniformly reflective. I don't have the data handy, but you will find that it reflects more strongly in read than in blue, with green probably somewhere in between. This gives silver the "warm" quality of its reflection. Aluminum, on the other hand, is somewhat weak in red, giving a "cold" reflection (as well as having less overall reflectivity, but that probably wouldn't be visible except in side-by-side comparison). However, this is a subtle difference, if you really want silver to look different from aluminum or tin or rhodium. First, to make something look metallic at all, you have to get it reflecting its environment properly.