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Diffstat (limited to 'kernel/Documentation/fb/cmap_xfbdev.txt')
| -rw-r--r-- | kernel/Documentation/fb/cmap_xfbdev.txt | 53 |
1 files changed, 53 insertions, 0 deletions
diff --git a/kernel/Documentation/fb/cmap_xfbdev.txt b/kernel/Documentation/fb/cmap_xfbdev.txt new file mode 100644 index 000000000..55e1f0a3d --- /dev/null +++ b/kernel/Documentation/fb/cmap_xfbdev.txt @@ -0,0 +1,53 @@ +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. + |