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From: tolani@blanche.ics.uci.edu (Deepak Tolani)
Newsgroups: comp.windows.x
Subject: setting colors on TrueColor visual
Message-ID: <2745E0EB.23619@ics.uci.edu>
Date: 18 Nov 90 01:16:27 GMT
Distribution: usa
Organization: UC Irvine Department of ICS
Lines: 73
Nntp-Posting-Host: blanche.ics.uci.edu



Correct me if I have a misunderstanding but I am under the impression
that with a 24 bit TrueVisual device it should be possible to get X
to display an arbitary 24 bit color by passing an appropriately
initialized XColor to XAllocColor and using the returned pixel number.
For example, to get bright red I would use the code below

	Xcolor color;

	color.red   = 255 * 256; /* Multiply by 256 to scale to 16 bits */
	color.blue  = 0;
	color.green = 0;

	if (XAllocColor(display, colormap, &color) == 0)
		XSetForeground(display, theGC, color.pixel);
	else
		error

However, XAllocColor seems to map my color requests to only a
handful of pixel values. For example, if I vary only the blue
component, I only get four shades of blue as shown below.

red      green          blue            color.pixel returned
0        0              0 - 10922       1
0        0              10923-32767     851
0        0              32768-54613     1701
0        0              54614-65535     2551


I have tried to use colormaps other than the default but
for some reason all of the calls below to XGetStandardColormap
fail

	if (!XGetStandardColormap( theDisplay,
		DefaultRootWindow(theDisplay),
		&theColormap,
		XA_RGB_DEFAULT_MAP))
		printf("cannot get default colormap\n");

	if (!XGetStandardColormap( theDisplay,
		DefaultRootWindow(theDisplay),
		&theColormap,
		XA_RGB_BEST_MAP))
		printf("cannot get best colormap\n");

	if (!XGetStandardColormap( theDisplay,
		DefaultRootWindow(theDisplay),
		&theColormap,
		XA_RGB_RED_MAP))
		printf("cannot get red colormap\n");


I thought that X perhaps does not recognize that I have a
24 bit plane device but when I run the program fragment
below, the call to XMatchVisualInfo succeeds and X reports
the pixel depth is 24.

	theDepth  = DefaultDepth( theDisplay, theScreen );
	printf("Pixel depth = %d\n", theDepth);

	if (!XMatchVisualInfo(theDisplay,theScreen,theDepth,TrueColor,&v))
		printf("cannot find visual info\n");


I am running this program on a DECstation 5000/200 with DECWindows
UWS 2.2 version 1.


tolani@blanche.ics.uci.edu

--
Deepak Tolani