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In article <1937@druhi.ATT.COM> neal@druhi.ATT.COM (Neal D. McBurnett) writes:
>5) As I understand it, the color palatte of the IBM CGA standard is really
>	limited to 8 colors with 2 intensities of each.  On the other hand,
>	the EGA standard allows for a "64 color palatte".  Which 64 colors?
>	2 bits per primary?

This problem of terminology comes up again and again.  People, the human eye 
needs only three distinct hues for full effect.  This business of 8 colors, or 
64 colors, or 4096 colors, is advertising hype.  Your typical printer will 
tell you he or she uses only three colors, with a little black thrown in.  The 
rest is all a question of INTENSITY.

Remember, THREE colors, and a large number (or 16, or 64, or 4096) of
INTENSITIES.
-- 
Rahul Dhesi         UUCP:  {ihnp4,seismo}!{iuvax,pur-ee}!bsu-cs!dhesi

In article <1937@druhi.ATT.COM> neal@druhi.ATT.COM (Neal D. McBurnett) writes:
>I'm looking for information on color science and computer graphics.
>
>1) Could someone point me to software which converts between the various
>	color systems?  For example, from "dominant wavelength, purity,
>	reflectance" to RGB?
	.
	.
	.
>6) What are good reference books for questions like these?

_Color_in_Business,_Science_and_Industry 3rd ed., by Judd and Wyszecki,
John Wiley & Sons, Inc., 1975, ISBN 0-471-45212-2.

This book isn't about software, but it does give some of the zillions of
formulas for specifying color.  It's rather expensive, and even though it's
a book about color it has no color illustrations :-(.


------------------------------------------------------------------------------
"Mistrust enterprises that require new clothes."

Danny Sharpe
School of ICS
Georgia Insitute of Technology, Atlanta Georgia, 30332
...!{akgua,allegra,amd,hplabs,ihnp4,seismo,ut-ngp}!gatech!gitpyr!dts

In article <725@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes:
>This problem of terminology comes up again and again.  People, the human eye 
>needs only three distinct hues for full effect.  This business of 8 colors, or 
>64 colors, or 4096 colors, is advertising hype.  Your typical printer will 
>tell you he or she uses only three colors, with a little black thrown in.  The 
>rest is all a question of INTENSITY.

You are wrong.  Just some of the things color perceptions depends on are:

	1. whether the human eye(s) are "normal" for color vision
	   (i.e., the person is not "color-blind")
	2. the other colors and shadows surrounding the color surface or
	   source (in the case of lights)
	3. the mixture of wavelengths reaching the eye.
	   Subtractive mixture takes place in paints (where Red, Yellow,
	   and Blue are the "primaries").  Additive mixture takes place
	   when using lights (where Red, Green, and Blue are the "primaries")
	4. the amount of light reaching the eye
	5. the current state of the visual system.  If you are dark adapted
	   and are suddenly flooded with light, all you see is a white
	   glare most of the time.  If your opponent cells are fatigued
	   in one direction the complentary color (of whatever is fatigued)
	   is seen (try placing a green lens over one eye for a few mintues
	   and then look around).

In short, the color you see depends not only on the actual wavelengths
reaching your eye, but also interaction of color after being processed
by higher cortical functions, and the current state of various portions
of the visual system.  Further, the while only three "primaries" are needed
to create all the colors we "see," the "normal" human eye is capable
of distinguishing a great many "colors" (some estimates approach or
exceed one billion).  Anyone who wants to find out more about the
processing of color by the human visual system should read a good book
on the subject.  I think Robert Boynton's "Human Color Vision" may be
a good place to start....todd

-- 
Todd Ogasawara, U. of Hawaii Computing Center
UUCP:		{ihnp4,seismo,ucbvax,dcdwest}!sdcsvax!nosc!uhccux!todd
ARPA:		uhccux!todd@nosc.MIL
INTERNET:	todd@uhccux.UHCC.HAWAII.EDU

People have not answered one part of the questions.
Black bodies:  you want to look at the Boltzmann and Planck equations.
Check out a physics text on radiative transfer and you will find the
curves for black bodies.  I learned about them for weather modelling.
Other people have adequately covered the sensation of the perception of colour.
The black body stuff won't help you for color.

From the Rock of Ages Home for Retired Hackers:

--eugene miya
  NASA Ames Research Center
  eugene@ames-aurora.ARPA
  "You trust the `reply' command with all those different mailers out there?"
  "Send mail, avoid follow-ups.  If enough, I'll summarize."
  {hplabs,hao,ihnp4,decwrl,allegra,tektronix,menlo70}!ames!aurora!eugene

The Boltzmann equations define the spectal energy of a perfect black-body
radiator. For use in color, these values can be converted into cie
chromaticity coordinates (they are well tabulated), which in turn can
be mapped into RGB coordinates through a change of basis, if you know the
chromaticity of your monitor. Enclosed are some sample values.


    /Alan Paeth

---------------
CIE chromaticity coordinates for a black-body at T in deg K.
 lambda(max) is the peak wavelength in nm of the Stefan-Bolzmann curve.

Dominant wavelength is not available for ~6500 degrees Kelvin, as the curve is
nearly flat through the visible portion of the spectrum here. Standard daylight
is approximated by illuminant D65, but does not lie along the locus of points
for blackbody radiators.

 Temp-K	x'	y'	lambda(max)

  100	.735	.265	695
  300	.734	.266	684
  500	.721	.279	641.5
 1000	.625	.345	606.7
 1500	.586	.393	594.8
 2000	.526	.413	588.9
 2500	.477	.414	585.2
 2854	.4476	.4074	583.5
 3000	.437	.404	582.9
 3500	.405	.391	580.9
 4000	.380	.377	578.9
 4500	.361	.363	577.2
 4800	.351	.356	575.1
 5000	.345	.352	572.6
 5500	.332	.341	0
 6000	.322	.332	0
 6500	.313	.324	485.7
 7000	.306	.371	483.7
 8000	.295	.305	481.5
10000	.281	.288	479.4
24000	.253	.253	477.0
99999	.240	.234	475.7

---------------------------------------

3x3 multiply to convert XYZ (cie chromaticity) into RGB

(1) set X=x', Y=y', Z=(1-x'-y') and then

(2) compute

   |R|       | 1.73 -.48 -.26 |    |X|
   | |       |                |    | |
   |G|   =   | -.81 1.65 -.02 |  * |Y|
   | |       |                |    | |
   |B|       | 0.08 -.17 1.28 |    |Z|

where these RGB are those defined the the NTSC which standardizes
broadcast colors for North America television transmission (this is the
inverse matrix of the definition, rounded to three significant places).

    /Alan Paeth

In article <725@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes:
>... Your typical printer will tell you he or she uses only three
>colors, with a little black thrown in.

Actually, high quality color magazine printing usually uses four
colours plus black.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	seismo!mimsy!chris

In article <6861@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
> In article <725@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes:
> >... Your typical printer will tell you he or she uses only three
> >colors, with a little black thrown in.
> Actually, high quality color magazine printing usually uses four
> colours plus black.

No, Rahul is correct.  What you see in magazines usually begins as
continuous tone art (photographs or transparencies) and are then
scanned with a Hell or Crosfield laser scanner to produce four
color separations (cyan, magenta, yellow and black) which look like
pieces of black and white transparency film with halftone dots on
them.  A printer can then etch printing plates from these color seps.
For the past year I have been sending images back and forth from our
workstations to Scitex and Hell pre-press equipment at various color
separation houses in the Bay Area and making Cromalins (proofs) so
I've got some idea of what's going on here.

In printing packages they often use a different technique.  They use
the color of ink that is actually getting printed, rather they simulate
continuous tone with CMYK halftone dots.  I'm looking at a "Bounce"
fabric softener box and it looks like they used blue, red, and yellow
ink to produce a white, yellow, orange and blue package.


Jonathan Hue	DuPont Design Technologies/Via Visuals		leadsv!eps2!jon

>In article <6861@mimsy.UUCP> I wrote:
>>Actually, high quality color magazine printing usually uses four
>>colours plus black.

What I neglected to mention was that this information was old; I
learnt it perhaps ten years ago.  The extra color (switching
spelling systems will drive me crazy yet!) was brown; apparently
whatever processes were used did not reproduce browns very well.

In article <93@eps2.UUCP> jon@eps2.UUCP (Jonathan Hue) describes
a laser scanning process that produces `four color separations
(cyan, magenta, yellow and black)'.  Clearly this is a newer
process that has taken over now that scanning lasers are cheap.
I stand corrected.

(We now return to your regularly scheduled news :-) .)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	seismo!mimsy!chris

In article <725@bsu-cs.UUCP>, dhesi@bsu-cs.UUCP (Rahul Dhesi) writes:
>In article <1937@druhi.ATT.COM> neal@druhi.ATT.COM (Neal D. McBurnett) writes:
>>5) As I understand it, the color palatte of the IBM CGA standard is really
>>	limited to 8 colors with 2 intensities of each.

>This business of 8 colors, or 64 colors, or 4096 colors, is advertising hype.
>The rest is all a question of INTENSITY.
>Remember, THREE colors, and a large number (or 16, or 64, or 4096) of
>INTENSITIES.
>Rahul Dhesi         UUCP:  {ihnp4,seismo}!{iuvax,pur-ee}!bsu-cs!dhesi

Just what I want Red, Green, Blue; dark Red, dark Green, dark Blue;
light Red, light Green, light Blue; etc...

Don't you really mean THREE colors, each at its own intensity?

The IBM gives you eight different colors, plus a darker or lighter version
of the eight, depending on how you look at it.

David Childs - Cadnetix Corp.  childs@cadnetix.UUCP

Disclaimer - I in no way speak for Cadnetix since Cadnetix machines have
only eight colors.  (All at the same intensity.  Except black.)

In article <93@eps2.UUCP>, jon@eps2.UUCP (Jonathan Hue) writes:
> 
> No, Rahul is correct.  What you see in magazines usually begins as
> continuous tone art (photographs or transparencies) and are then
> scanned with a Hell or Crosfield laser scanner to produce four
> color separations (cyan, magenta, yellow and black) which look like
> pieces of black and white transparency film with halftone dots on
> them.


Here's a related question, does anybody know the conversion factors to
map colors in Red-Green-Blue space to Cyan-Yellow-Magenta-Black?  What
I would like to do is generate four images on a high-res laser printer
that can be used directly for printing; i.e. do the seperation in
software.

		-ed falk, falk@sun.com, sun!falk

In article <20537@sun.uucp>, falk@sun.uucp (Ed Falk) writes:
> Here's a related question, does anybody know the conversion factors to
> map colors in Red-Green-Blue space to Cyan-Yellow-Magenta-Black?  What
> I would like to do is generate four images on a high-res laser printer
> that can be used directly for printing; i.e. do the seperation in
> software.
> 
> 		-ed falk, falk@sun.com, sun!falk

It's trivial.

where R=G=B,
	then Black = 1-R (or G or B)
	C=M=Y=0
else

	black = 0
	+- -+	+- -+   +- -+
	| R |   | 1 |   | C |
	| G | = | 1 | - | M |
	| B |   | 1 |   | Y |
	+- -+   +- -+   +- -+
-- 
uucp:	 ...decvax!elrond!amamaral		I would rather be a
phone:	 (603) 885-8075				fool than a king...
us mail: Calcomp/Sanders DPD (PTP2-2D01)
	 Hudson NH 	03051-0908

In article <943@elrond.CalComp.COM>, amamaral@elrond.CalComp.COM (Alan Amaral) writes:
> It's trivial.

Oh come on.  Everyone who reads this newsgroup knows how to translate between
the RGB and CMY color spaces.  And I'm sure everyone could figure out how to
do the crude undercolor removal...

The question Ed asks is not the simple one that you think he is asking.  From
his posting it is obvious that he knows about how real inks used by printers
work, and that they do not absorb light like ideal cyan, magenta, and yellow
inks.  None of the simple solutions I've posted to the net are adequate.

I have a Pantone color simulator in front of me.  Just looking at the front
page, I can see that there is no way you can print a bright orange color on
paper without using ink that is actually bright orange.

I asked this same question about a year ago.  One useful piece of information
I received was from someone at the University of Utah.  He told me that
Maureen Stone at Xerox PARC has done work in this area.  I found out she
has published papers and she recently gave a talk out here.

Ed, one thing you may want to do is buy a Pantone color simulator.  You can
get a feel for what the different inks look like when mixed together.  The
book consists of about 500 of the Pantone process colors, and then a color
chip next to them showing a close approximation using CMYK ink.  There are
three more CMYK color chips which can be torn out next to the big chip.  You
can find this at most any art store.  The price is a bit steep, $200.  The
other four Pantone books which make up the rest of the set are only $250 total.
Of course the colors printed are dependent on the ink, paper, screen angles,
etc, but you can get an idea of how things work from this book.

I know that Eikonix has a lot of people who are experts in this area, but
they're not going to divulge a lot of the information because it is
proprietary.  My personal opinion is that if Eikonix ever gets around to
announcing a Sun-3 prepress machine (if they haven't already), one of their
big features will be their expertise in color fidelity.  Our systems are
targeted at design, not production, so our "big features" are our tools for
graphic designers.

I made an error in a previous posting about halftone dot sizes.  Although
prepress machines store their pixels as 8-bit quantities, typically dot
size is controllable on an output scanner only to 2% increments, so there
are only 50 different dot sizes.  The top of the line Hell scanner generates
dots electronically at a maximum of 146 dots/mm.  If you are doing 12 halftone
dots/mm, that gives you about a 12x12 grid of dots to simulate your halftone
dot, so I guess you could assume 144 dot sizes.


Jonathan Hue	DuPont Design Technologies/Via Visuals		leadsv!eps2!jon