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From: doyle@b11.ingr.com (Doyle Davidson)
Newsgroups: comp.windows.x
Subject: Color to Grayscale mapping
Message-ID: <5631@b11.ingr.com>
Date: 4 Aug 89 21:25:21 GMT
Organization: Intergraph Corp. Huntsville, AL
Lines: 67

I have a question about how color allocation is done on a StaticGray
color display.  I have a client (twm for example) that has its color 
parameters specified by names like "red", "blue", "thistle" etc...  
Upon running this on a StaticGray display, I discovered that the 
following occurs:

    Color	 R   G   B		Gray level
    --------    --- --- ---		-------------
    "red"	255   0   0		    33%
    "blue"	  0   0 255		    33%
    "green"	  0 255   0		    33%
    "black"	  0   0   0		    0%
    "white"     255 255 255		    100%

Now, I have read several interpretations and none really match this
except for the code itself!

1) O'Reilly & Assoc:
	p.207   intensity = (0.30 * red) + (0.59 * green) + (0.11 * blue)
    really happens? WRONG!  (no flame intended at O'Reilly & Assoc)

2) O'Reilly & Assoc:
	p.207   next sentence:
	"If only a GrayScale visual is available, the red value ONLY will be
	used for gray scale intensity"
    really happens? WRONG!  I thought that #1 just said...  owell.
    If #2 is right then "green" and "blue" should map to black.

3) I read the protocal on the distribution tape: nothing that I found 
    said anything specific and is not part of the DDX layer :-(
    I remember reading in something that R G and B should all be set to
    the same value by the client because it cannot be sure which one of these
    three that the server will use as an intensity.  I cannot find it
    now to quote from it unfortunately.

4) I looked at the code: it searches through the RGB values for the Grayscale
    color table (which are all the same) and finds the color that minimizes
    the following (in pseudocode):
    in dix/colormap.c:  FindBestPixel()

    for (all RGB pixel values in table)
	value = (red_asked - red_table)^2 + (greed_asked - green_table)^2 +
			(blue_asked - blue_table)^2
		/* like a 3D distance formula but no square root */
	if (value < best_so_far)
	{
	    pixel = current_pixel_being_testing_from_table;
	    best_so_far = value;
	}
    really happens? Yes!! Hey this is it but not documented ANYWHERE

Hey you guys at MIT, got the answer?

Thanks in advance,

Doyle (I RTFMs and am still confused :-O ) Davidson

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Doyle C. Davidson              |
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