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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.
+