Documentation/fb/cmap_xfbdev.txt

   1 Understanding fbdev's cmap
   2 --------------------------
   3
   4 These 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
   7 struct 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 }
  14 struct fb_fix_screeninfo {
  15         .visual =       FB_VISUAL_STATIC_PSEUDOCOLOR,
  16 }
  17 for (i = 0; i < 8; i++)
  18         info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;
  19 memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);
  20 memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);
  21
  22 *. X11 apps do something like the following when trying to use grayscale.
  23 for (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 }
  32 There's also named equivalents like gray1..x provided you have an rgb.txt.
  33
  34 Somewhere in X's callchain, this results in a call to X code that handles the
  35 colormap. For example, Xfbdev hits the following:
  36
  37 xc-011010/programs/Xserver/dix/colormap.c:
  38
  39 FindBestPixel(pentFirst, size, prgb, channel)
  40
  41 dr = (long) pent->co.local.red - prgb->red;
  42 dg = (long) pent->co.local.green - prgb->green;
  43 db = (long) pent->co.local.blue - prgb->blue;
  44 sq = dr * dr;
  45 UnsignedToBigNum (sq, &sum);
  46 BigNumAdd (&sum, &temp, &sum);
  47
  48 co.local.red are entries that were brought in through FBIOGETCMAP which come
  49 directly from the info->cmap.red that was listed above. The prgb is the rgb
  50 that the app wants to match to. The above code is doing what looks like a least
  51 squares matching function. That's why the cmap entries can't be set to the left
  52 hand side boundaries of a color range.
  53