linux/Documentation/fb/cmap_xfbdev.txt
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   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
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