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