Very Computer

Anyone knows how to convert Red/Green/Blue (i.e. prime color of light)
to Red/Yellow/Blue (.e. prime color for paint materials and printer's
ink) ?

I want to write a painter program that allow user to mix color of Red,
Yellow, Blue.  The conversion routine is necessary.
--
Peter Liu

       Hong Kong

convert from the screen display (additive system : Red-Green-Blue) to printing
(substractive system: should really be Magenta-Yellow-Cyan)

Well, try this (or a similar digital version involving multiplying by 256 etc)

Magenta = 1 - fraction of Green and Blue
Yellow = 1 - fraction of red +blue
Cyan = 1- fraction of green + red

You will find that the contrast/shade would be a bit strange and you will have to
adjust the gamma for your specific medium (reflectance of printing paper,
satuation of paints and their actual shades, the logorithmic sensitivity of
the eye etc). To adjust the gamma ( it is called "gamma" in most display/image
processing situations, but I think it refers to "gain"  or something)

Try this algorithm; 0 < values < 1

New value = (old value) to the power some number.

The simple algorthm keeps the end points unchanged but shifts the middle up and
down depending on the power ( less than one or more than one). A value of either
0.5 or 2 should be sensible - don't know which way.

Good luck.  

|> > Anyone knows how to convert Red/Green/Blue (i.e. prime color of light)
|> > to Red/Yellow/Blue (.e. prime color for paint materials and printer's
|> > ink) ?
|> >
|> > I want to write a painter program that allow user to mix color of Red,
|> > Yellow, Blue.  The conversion routine is necessary.
|> You mean CMYK ????
|>
|> Regds
|> LAM Chi-fung
|> ------------------------------------------------------------------------------
|> PC Game Cheat & Info list maintainer

|> (current version is 6.0,wait for your comments)
|> ------------------------------------------------------------------------------

Actually, I mean Red/Yellow/Blue, not CMYK.

Conversion from RGB to CMYK is nothing new.

What I want is conversion from RGB to Red/Yellow/Blue, the primary
colors for paint materials used by artists.

       Hong Kong

Quote:> Anyone knows how to convert Red/Green/Blue (i.e. prime color of light)
> to Red/Yellow/Blue (.e. prime color for paint materials and printer's
> ink) ?

> I want to write a painter program that allow user to mix color of Red,
> Yellow, Blue.  The conversion routine is necessary.

Regds
LAM Chi-fung
------------------------------------------------------------------------------
PC Game Cheat & Info list maintainer

(current version is 6.0,wait for your comments)
------------------------------------------------------------------------------

R:255
G:255
B:255

has no R/Y/B equiv, you'll have to add white paint, and to get black
from R/Y/B is nearly impossible.

--
The Cookie Monster (TCM)
 - As far as anybody knows, I have a nice, normal mind...

RGB -> RYB

Red -> Red
Red+Green -> Yellow
Blue -> Blue

and converting back: RYB -> RGB

Red -> Red
Yellow-Red -> Green
Blue -> Blue

Sounds fairly logical to me.
Eric van Orsouw

Cookie Monster [Glenn Fisher]) writes:

Quote:>> What I want is conversion from RGB to Red/Yellow/Blue, the primary
>> colors for paint materials used by artists.

>It's not possible:

>R:255
>G:255
>B:255

>has no R/Y/B equiv, you'll have to add white paint, and to get black
>from R/Y/B is nearly impossible.

What you could do is work in CMY (which is 255-R, 255-G, 255-B) and
then use M+Y as your red paint and C+M as your blue paint. Then add
the amounts selected to get the CMY colour.

love Panther
---



http://www.pinkboard.com.au/People/Panther.html
"Do you like boys or girls? It's so confusing these days."

If I was attempting to do this, I would use the HIS (Hue, Intensity,
Saturation) conversions to/from RGB.  Once you are in the HIS color space,
you can determine the color tint from the Hue component.  A good book to
use for a reference (equations, explanations, etc...) is called:

Title: Raster Graphics Handbook
Author: Conrac Corporation
Publisher: Van Nostrand Reinhold Company

Although I'm certainly not an expert in this area, its seems as though
all your paint mixing and assignment of stock and custom colors could
be made in HIS color space.  There might also be a standard assignment
of colors that you could use.  I've seen references to Pantone color
charts in some programs.  They might already have specific HIS color
specifications.

John Anderson
Algorithm Engineering

: Cookie Monster [Glenn Fisher]) writes:
: >

: [...]
: >> What I want is conversion from RGB to Red/Yellow/Blue, the primary
: >> colors for paint materials used by artists.
: >
: >It's not possible:
: >
: >R:255
: >G:255
: >B:255
: >
: >has no R/Y/B equiv, you'll have to add white paint, and to get black
: >from R/Y/B is nearly impossible.

: If you assume white paper (the usual), then
:     RGB(255,255,255) ==> RYB(0,0,0)
: And black is all colors subtracted, so it's RYB(255,255,255) (or the
: equivalent largest values in whatever system you're using).-Wm

The impossible colours to get are things such as CYAN and MAGENTA. You
can't get those with RYB as your paints. [eg Mixing R and B will give you
a murkey dark magenta, not pure magenta.]

Now what has this to do with comp.unix.programer? Don't ask me.

--

Philips Semiconductors Ltd
Southampton                                 My views are my own.
United Kingdom
 Are you using ISO8859-1? Do you see ? as copyright, as division and ? as 1/2?

Quote:>>It's not possible:

Quote:>If you assume white paper (the usual), then
>    RGB(255,255,255) ==> RYB(0,0,0)
>And black is all colors subtracted, so it's RYB(255,255,255) (or the
>equivalent largest values in whatever system you're using).-Wm

white = Red=255,Green=255,Blue=255              (RGB)
white = Cyan=0,Magenta=0,Yellow=0,Black=0       (CMYK)

These are used because computer monitors and colour filmwork use a light source.

The three primary colours for artists is a subtractive color system.
Paint only reflects light so the way each color mix is created is totally different.

Although CMYK is a subtractive system, it is merely an inversion of the RGB system and is
still used for light.e.g.

Red = {R=255,G=0,B=0} = {C=0,M=255,Y=255,B=0}

When a light source has a magenta filter placed in front of it, red & blue light passes
through. When a yellow filter is used, red and green light is passes through. When both
filters are used, blue light is blocked by the yellow filter and green light is blocked
by the magenta filter, only red light passes through.

Magenta paint reflects red & blue light, yellow paint reflects red & green light. When
magenta and yellow paint is mixed, some blue and some green is still reflected along with
the red light and the color is not true.

Are you attempting to duplicate milticolor artwork, for the printing industry, in a
screenprinting(or other paint based) system?

The inks printers use are unlike paint as they are a FILTER for the white light reflected
from the page.

You will need to work out your own conversion system to convert palettes.

hope this helps.

Jeff

Stealthy Kiwi

"My opinion is my own (unless someone else thought of it first)"
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