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Understanding fbdev's cmap
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==========================
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These notes explain how X's dix layer uses fbdev's cmap structures.
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- example of relevant structures in fbdev as used for a 3-bit grayscale cmap::
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struct fb_var_screeninfo {
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.bits_per_pixel = 8,
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.grayscale = 1,
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.red = { 4, 3, 0 },
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.green = { 0, 0, 0 },
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.blue = { 0, 0, 0 },
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}
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struct fb_fix_screeninfo {
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.visual = FB_VISUAL_STATIC_PSEUDOCOLOR,
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}
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for (i = 0; i < 8; i++)
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info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;
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memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);
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memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);
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- X11 apps do something like the following when trying to use grayscale::
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for (i=0; i < 8; i++) {
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char colorspec[64];
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memset(colorspec,0,64);
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sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);
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if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))
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printf("Can't get color %s\n",colorspec);
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XAllocColor(outputDisplay, testColormap, &wantedColor);
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grays[i] = wantedColor;
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}
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There's also named equivalents like gray1..x provided you have an rgb.txt.
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Somewhere in X's callchain, this results in a call to X code that handles the
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colormap. For example, Xfbdev hits the following:
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xc-011010/programs/Xserver/dix/colormap.c::
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FindBestPixel(pentFirst, size, prgb, channel)
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dr = (long) pent->co.local.red - prgb->red;
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dg = (long) pent->co.local.green - prgb->green;
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db = (long) pent->co.local.blue - prgb->blue;
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sq = dr * dr;
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UnsignedToBigNum (sq, &sum);
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BigNumAdd (&sum, &temp, &sum);
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co.local.red are entries that were brought in through FBIOGETCMAP which come
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directly from the info->cmap.red that was listed above. The prgb is the rgb
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that the app wants to match to. The above code is doing what looks like a least
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squares matching function. That's why the cmap entries can't be set to the left
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hand side boundaries of a color range.
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