==========================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.