1 ==========================
2 Understanding fbdev's cmap
3 ==========================
4
5 These 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
36 There's also named equivalents like gray1..x provided you have an rgb.txt.
37
38 Somewhere in X's callchain, this results in a call to X code that handles the
39 colormap. For example, Xfbdev hits the following:
40
41 xc-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
52 co.local.red are entries that were brought in through FBIOGETCMAP which come
53 directly from the info->cmap.red that was listed above. The prgb is the rgb
54 that the app wants to match to. The above code is doing what looks like a least
55 squares matching function. That's why the cmap entries can't be set to the left
56 hand side boundaries of a color range.
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