Re: Colors

["J. Ali Harlow" <gtk-list optosun7 city ac uk>]

On Sun, 08 Apr 2001, Valery Avaux wrote:
> Hello,
> 
> I use a drawing area with 2 colors red and blue. But I don't know to get
> other colors . For these two I use the following defintion that were
> given me.
> 
> 	blue.red   = 0;
> 	blue.green = 0;
> 	blue.blue  = 0xff * 0x100;
> 	gdk_colormap_alloc_color(colormap, &blue, FALSE, TRUE);
> 
> 	red.red   = 0xff * 0x100;
> 	red.green = 0;
> 	red.blue  = 0;
> 
> Could someone explain how to get other colors (for example white) ?

Monitors are light based. Thus you create colours in exactly the way
you would when mixing colours from spotlights (assuming you were taught
this in physics classes where you are):

Red + Green = yellow
Green + Blue = cyan
Blue + Red = magenta
Red + Green + Blue = white

If you've got a copy of rgb.txt (ie., in /usr/X11R6/lib/X11/rgb.txt) this has
X11's list of colour names and values for red, green and blue. You'll want to
scale these from 0-255 to 0-65535 (as you have done above by multiplying by
0x100).

For example, rgb.txt contains the line:

 46 139  87             SeaGreen

So we might create this using:

seagreen.red = 46 * 0x100;
seagreen.green = 139 * 0x100;
seagreen.blue = 87 * 0x100;

(some people prefer to multiply by 257 rather than 0x100 (256) because it gives
better scaling. ie., 255 * 257 = 65535.)

-- 
Ali Harlow                              Email: ali avrc city ac uk
Research programmer                     Tel:   (020) 7477 8000 X 4348
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