| 1 | ==========================
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| 2 | Understanding fbdev's cmap
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| 3 | ==========================
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| 4 |
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| 5 | These notes explain how X's dix layer uses fbdev's cmap structures.
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| 6 |
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| 7 | - example of relevant structures in fbdev as used for a 3-bit grayscale cmap::
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| 8 |
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| 9 | struct fb_var_screeninfo {
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| 10 | .bits_per_pixel = 8,
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| 11 | .grayscale = 1,
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| 12 | .red = { 4, 3, 0 },
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| 13 | .green = { 0, 0, 0 },
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| 14 | .blue = { 0, 0, 0 },
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| 15 | }
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| 16 | struct fb_fix_screeninfo {
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| 17 | .visual = FB_VISUAL_STATIC_PSEUDOCOLOR,
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| 18 | }
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| 19 | for (i = 0; i < 8; i++)
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| 20 | info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;
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| 21 | memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);
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| 22 | memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);
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| 23 |
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| 24 | - X11 apps do something like the following when trying to use grayscale::
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| 25 |
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| 26 | for (i=0; i < 8; i++) {
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| 27 | char colorspec[64];
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| 28 | memset(colorspec,0,64);
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| 29 | sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);
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| 30 | if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))
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| 31 | printf("Can't get color %s\n",colorspec);
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| 32 | XAllocColor(outputDisplay, testColormap, &wantedColor);
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| 33 | grays[i] = wantedColor;
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| 34 | }
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| 35 |
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| 36 | There's also named equivalents like gray1..x provided you have an rgb.txt.
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| 37 |
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| 38 | Somewhere in X's callchain, this results in a call to X code that handles the
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| 39 | colormap. For example, Xfbdev hits the following:
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| 40 |
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| 41 | xc-011010/programs/Xserver/dix/colormap.c::
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| 42 |
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| 43 | FindBestPixel(pentFirst, size, prgb, channel)
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| 44 |
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| 45 | dr = (long) pent->co.local.red - prgb->red;
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| 46 | dg = (long) pent->co.local.green - prgb->green;
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| 47 | db = (long) pent->co.local.blue - prgb->blue;
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| 48 | sq = dr * dr;
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| 49 | UnsignedToBigNum (sq, &sum);
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| 50 | BigNumAdd (&sum, &temp, &sum);
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| 51 |
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| 52 | co.local.red are entries that were brought in through FBIOGETCMAP which come
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| 53 | directly from the info->cmap.red that was listed above. The prgb is the rgb
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| 54 | that the app wants to match to. The above code is doing what looks like a least
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| 55 | squares matching function. That's why the cmap entries can't be set to the left
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| 56 | hand side boundaries of a color range.
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