1Understanding fbdev's cmap
2--------------------------
3
4These notes explain how X's dix layer uses fbdev's cmap structures.
5
6*. example of relevant structures in fbdev as used for a 3-bit grayscale cmap
7struct fb_var_screeninfo {
8        .bits_per_pixel = 8,
9        .grayscale      = 1,
10        .red =          { 4, 3, 0 },
11        .green =        { 0, 0, 0 },
12        .blue =         { 0, 0, 0 },
13}
14struct fb_fix_screeninfo {
15        .visual =       FB_VISUAL_STATIC_PSEUDOCOLOR,
16}
17for (i = 0; i < 8; i++)
18	info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;
19memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);
20memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);
21
22*. X11 apps do something like the following when trying to use grayscale.
23for (i=0; i < 8; i++) {
24	char colorspec[64];
25	memset(colorspec,0,64);
26	sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);
27	if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))
28		printf("Can't get color %s\n",colorspec);
29	XAllocColor(outputDisplay, testColormap, &wantedColor);
30	grays[i] = wantedColor;
31}
32There's also named equivalents like gray1..x provided you have an rgb.txt.
33
34Somewhere in X's callchain, this results in a call to X code that handles the
35colormap. For example, Xfbdev hits the following:
36
37xc-011010/programs/Xserver/dix/colormap.c:
38
39FindBestPixel(pentFirst, size, prgb, channel)
40
41dr = (long) pent->co.local.red - prgb->red;
42dg = (long) pent->co.local.green - prgb->green;
43db = (long) pent->co.local.blue - prgb->blue;
44sq = dr * dr;
45UnsignedToBigNum (sq, &sum);
46BigNumAdd (&sum, &temp, &sum);
47
48co.local.red are entries that were brought in through FBIOGETCMAP which come
49directly from the info->cmap.red that was listed above. The prgb is the rgb
50that the app wants to match to. The above code is doing what looks like a least
51squares matching function. That's why the cmap entries can't be set to the left
52hand side boundaries of a color range.
53
54