Commit | Line | Data |
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ab42b818 | 1 | ========================== |
de7c6d15 | 2 | Understanding fbdev's cmap |
ab42b818 | 3 | ========================== |
de7c6d15 JK |
4 | |
5 | These notes explain how X's dix layer uses fbdev's cmap structures. | |
6 | ||
ab42b818 MCC |
7 | - example of relevant structures in fbdev as used for a 3-bit grayscale cmap:: |
8 | ||
9 | struct fb_var_screeninfo { | |
10 | .bits_per_pixel = 8, | |
11 | .grayscale = 1, | |
12 | .red = { 4, 3, 0 }, | |
13 | .green = { 0, 0, 0 }, | |
14 | .blue = { 0, 0, 0 }, | |
15 | } | |
16 | struct fb_fix_screeninfo { | |
17 | .visual = FB_VISUAL_STATIC_PSEUDOCOLOR, | |
18 | } | |
19 | for (i = 0; i < 8; i++) | |
de7c6d15 | 20 | info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16; |
ab42b818 MCC |
21 | memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8); |
22 | memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8); | |
de7c6d15 | 23 | |
ab42b818 MCC |
24 | - X11 apps do something like the following when trying to use grayscale:: |
25 | ||
26 | for (i=0; i < 8; i++) { | |
de7c6d15 JK |
27 | char colorspec[64]; |
28 | memset(colorspec,0,64); | |
29 | sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36); | |
30 | if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor)) | |
31 | printf("Can't get color %s\n",colorspec); | |
32 | XAllocColor(outputDisplay, testColormap, &wantedColor); | |
33 | grays[i] = wantedColor; | |
ab42b818 MCC |
34 | } |
35 | ||
de7c6d15 JK |
36 | There's also named equivalents like gray1..x provided you have an rgb.txt. |
37 | ||
38 | Somewhere in X's callchain, this results in a call to X code that handles the | |
39 | colormap. For example, Xfbdev hits the following: | |
40 | ||
ab42b818 | 41 | xc-011010/programs/Xserver/dix/colormap.c:: |
de7c6d15 | 42 | |
ab42b818 | 43 | FindBestPixel(pentFirst, size, prgb, channel) |
de7c6d15 | 44 | |
ab42b818 MCC |
45 | dr = (long) pent->co.local.red - prgb->red; |
46 | dg = (long) pent->co.local.green - prgb->green; | |
47 | db = (long) pent->co.local.blue - prgb->blue; | |
48 | sq = dr * dr; | |
49 | UnsignedToBigNum (sq, &sum); | |
50 | BigNumAdd (&sum, &temp, &sum); | |
de7c6d15 JK |
51 | |
52 | co.local.red are entries that were brought in through FBIOGETCMAP which come | |
53 | directly from the info->cmap.red that was listed above. The prgb is the rgb | |
54 | that the app wants to match to. The above code is doing what looks like a least | |
55 | squares matching function. That's why the cmap entries can't be set to the left | |
56 | hand side boundaries of a color range. |