Color questions: color systems, "black bodies", EGB palette

Color questions: color systems, "black bodies", EGB palette

Post by n.. » Wed, 02 May 1990 01:24:00



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 "* wavelength, purity,
        reflectance" to RGB?
        How about between the Munsell colors and RGB or XYZ?
        How about software which runs on, say, a Sun lets you navigate around
        the different 3-d color spaces?
2) How about equations to provide the xy or RGB values for various black
        body temperatures (when they say that a "G" star is yellow, I'd like
        to be more specific....)
3) Where would I find specifications on the gamuts of specific color monitors
        or tv sets?  Are they all about the same, or is there significant
        variation?
4) How would I calibrate a monitor?  What do the "hue", "saturation", etc knobs
        really do?
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?
6) What are good reference books for questions like these?

Thanks a lot for any help!

-Neal McBurnett, ihnp4!druhi!neal

 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by Rahul Dhe » Wed, 02 May 1990 12:58:00


Keywords:


Quote:>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

 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by Danny Shar » Wed, 02 May 1990 20:31:00



Quote:>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 "* wavelength, purity,
>    reflectance" to RGB?

        .
        .
        .

Quote:>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

 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by t.. » Wed, 02 May 1990 05:23:00



>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


 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by Eugene Miya » Wed, 02 May 1990 13:23:00


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

  "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

 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by awpa.. » Wed, 02 May 1990 15:47:00


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.

* 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

 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by Chris Tor » Wed, 02 May 1990 11:38:00



>... 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)

 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by j.. » Sat, 02 Jun 1990 09:52:00




> >... 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

 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by Chris Tor » Sat, 02 Jun 1990 06:42:00



>>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.


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)

 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by chi.. » Sat, 02 Jun 1990 11:50:00




>>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.


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

 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by f.. » Sat, 02 Jun 1990 16:54:00



> 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.


 
 
 

Color questions: color systems, "black bodies", EGB palette

Post by hu.. » Sat, 02 Jun 1990 03:00:00



>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.

RGBA --> CYMB seems a pretty natural thing.  Am I wrong?

Is it as simple as:

    R/2 --> YM   G/2 --> CY   B/2 --> CM   A --> B  ???

The human eye is supposedly happy with 100dpi (not printers, just humans :-),
whether this is true or not this leads to another question.

At what dpi does one switch from ordered dithers to halftone dithers?  With
certain varieties of plotters (thermals all) the number of pixels turned on
does not have a linear effect on the amount of ink/crayon smeared on the media.
Halftone tone dithers can be arranged so that the balls are of the proper size
to deal with making hot-charged/cool-synced spots of appropriate sizes.

Any thoughts?

--
    Jim Hutchison               UUCP:   {dcdwest,ucbvax}!sdcsvax!hutch

Disklame'r:
    One greater than the greatest signature representable with 184 symbols.

 
 
 

1. "Meshing Color Palettes?"

Hi there:

        I have 4 GIF files with 4 different palettes.  I need to display them
all on the screen at the same time.  Is there any methods out there that can
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        Thanks in advance.
--
+-----------------------------------------------------------------------------+
 William Y.L. Wong                   http://www.declab.usu.edu:8080/~williamw/

 Utah State University, Logan, Utah

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