Understanding fbdev’s cmap¶

These notes explain how X’s dix layer uses fbdev’s cmap structures.

• example of relevant structures in fbdev as used for a 3-bit grayscale cmap:

  struct fb_var_screeninfo {        .bits_per_pixel = 8,        .grayscale      = 1,        .red =          { 4, 3, 0 },        .green =        { 0, 0, 0 },        .blue =         { 0, 0, 0 },}struct fb_fix_screeninfo {        .visual =       FB_VISUAL_STATIC_PSEUDOCOLOR,}for (i = 0; i < 8; i++)    info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);

• X11 apps do something like the following when trying to use grayscale:

  for (i=0; i < 8; i++) {    char colorspec[64];    memset(colorspec,0,64);    sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);    if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))            printf("Can't get color %s\n",colorspec);    XAllocColor(outputDisplay, testColormap, &wantedColor);    grays[i] = wantedColor;}

There’s also named equivalents like gray1..x provided you have an rgb.txt.

Somewhere in X’s callchain, this results in a call to X code that handles thecolormap. For example, Xfbdev hits the following:

xc-011010/programs/Xserver/dix/colormap.c:

FindBestPixel(pentFirst, size, prgb, channel)dr = (long) pent->co.local.red - prgb->red;dg = (long) pent->co.local.green - prgb->green;db = (long) pent->co.local.blue - prgb->blue;sq = dr * dr;UnsignedToBigNum (sq, &sum);BigNumAdd (&sum, &temp, &sum);

co.local.red are entries that were brought in through FBIOGETCMAP which comedirectly from the info->cmap.red that was listed above. The prgb is the rgbthat the app wants to match to. The above code is doing what looks like a leastsquares matching function. That’s why the cmap entries can’t be set to the lefthand side boundaries of a color range.