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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 the
colormap. 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 come
directly from the info->cmap.red that was listed above. The prgb is the rgb
that the app wants to match to. The above code is doing what looks like a least
squares matching function. That's why the cmap entries can't be set to the left
hand side boundaries of a color range.