1========================== 2Understanding fbdev's cmap 3========================== 4 5These notes explain how X's dix layer uses fbdev's cmap structures. 6 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++) 20 info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16; 21 memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8); 22 memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8); 23 24- X11 apps do something like the following when trying to use grayscale:: 25 26 for (i=0; i < 8; i++) { 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; 34 } 35 36There's also named equivalents like gray1..x provided you have an rgb.txt. 37 38Somewhere in X's callchain, this results in a call to X code that handles the 39colormap. For example, Xfbdev hits the following: 40 41xc-011010/programs/Xserver/dix/colormap.c:: 42 43 FindBestPixel(pentFirst, size, prgb, channel) 44 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); 51 52co.local.red are entries that were brought in through FBIOGETCMAP which come 53directly from the info->cmap.red that was listed above. The prgb is the rgb 54that the app wants to match to. The above code is doing what looks like a least 55squares matching function. That's why the cmap entries can't be set to the left 56hand side boundaries of a color range. 57