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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.
-