1==========================
2Understanding fbdev's cmap
3==========================
4
5These notes explain how X's dix layer uses fbdev's cmap structures.
6
7-  example of relevant structures in fbdev as used for a 3-bit grayscale cmap::
8
9    struct fb_var_screeninfo {
10	    .bits_per_pixel = 8,
11	    .grayscale      = 1,
12	    .red =          { 4, 3, 0 },
13	    .green =        { 0, 0, 0 },
14	    .blue =         { 0, 0, 0 },
15    }
16    struct fb_fix_screeninfo {
17	    .visual =       FB_VISUAL_STATIC_PSEUDOCOLOR,
18    }
19    for (i = 0; i < 8; i++)
20	info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;
21    memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);
22    memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);
23
24-  X11 apps do something like the following when trying to use grayscale::
25
26    for (i=0; i < 8; i++) {
27	char colorspec[64];
28	memset(colorspec,0,64);
29	sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);
30	if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))
31		printf("Can't get color %s\n",colorspec);
32	XAllocColor(outputDisplay, testColormap, &wantedColor);
33	grays[i] = wantedColor;
34    }
35
36There's also named equivalents like gray1..x provided you have an rgb.txt.
37
38Somewhere in X's callchain, this results in a call to X code that handles the
39colormap. For example, Xfbdev hits the following:
40
41xc-011010/programs/Xserver/dix/colormap.c::
42
43  FindBestPixel(pentFirst, size, prgb, channel)
44
45  dr = (long) pent->co.local.red - prgb->red;
46  dg = (long) pent->co.local.green - prgb->green;
47  db = (long) pent->co.local.blue - prgb->blue;
48  sq = dr * dr;
49  UnsignedToBigNum (sq, &sum);
50  BigNumAdd (&sum, &temp, &sum);
51
52co.local.red are entries that were brought in through FBIOGETCMAP which come
53directly from the info->cmap.red that was listed above. The prgb is the rgb
54that the app wants to match to. The above code is doing what looks like a least
55squares matching function. That's why the cmap entries can't be set to the left
56hand side boundaries of a color range.
57