linux/Documentation/fb/cmap_xfbdev.rst
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   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