0001 ==========================
0002 Understanding fbdev's cmap
0003 ==========================
0004
0005 These notes explain how X's dix layer uses fbdev's cmap structures.
0006
0007 - example of relevant structures in fbdev as used for a 3-bit grayscale cmap::
0008
0009 struct fb_var_screeninfo {
0010 .bits_per_pixel = 8,
0011 .grayscale = 1,
0012 .red = { 4, 3, 0 },
0013 .green = { 0, 0, 0 },
0014 .blue = { 0, 0, 0 },
0015 }
0016 struct fb_fix_screeninfo {
0017 .visual = FB_VISUAL_STATIC_PSEUDOCOLOR,
0018 }
0019 for (i = 0; i < 8; i++)
0020 info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;
0021 memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);
0022 memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);
0023
0024 - X11 apps do something like the following when trying to use grayscale::
0025
0026 for (i=0; i < 8; i++) {
0027 char colorspec[64];
0028 memset(colorspec,0,64);
0029 sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);
0030 if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))
0031 printf("Can't get color %s\n",colorspec);
0032 XAllocColor(outputDisplay, testColormap, &wantedColor);
0033 grays[i] = wantedColor;
0034 }
0035
0036 There's also named equivalents like gray1..x provided you have an rgb.txt.
0037
0038 Somewhere in X's callchain, this results in a call to X code that handles the
0039 colormap. For example, Xfbdev hits the following:
0040
0041 xc-011010/programs/Xserver/dix/colormap.c::
0042
0043 FindBestPixel(pentFirst, size, prgb, channel)
0044
0045 dr = (long) pent->co.local.red - prgb->red;
0046 dg = (long) pent->co.local.green - prgb->green;
0047 db = (long) pent->co.local.blue - prgb->blue;
0048 sq = dr * dr;
0049 UnsignedToBigNum (sq, &sum);
0050 BigNumAdd (&sum, &temp, &sum);
0051
0052 co.local.red are entries that were brought in through FBIOGETCMAP which come
0053 directly from the info->cmap.red that was listed above. The prgb is the rgb
0054 that the app wants to match to. The above code is doing what looks like a least
0055 squares matching function. That's why the cmap entries can't be set to the left
0056 hand side boundaries of a color range.
