color.7 (5186B)
1 .TH COLOR 7 2 .SH NAME 3 color \- representation of pixels and colors 4 .SH DESCRIPTION 5 To address problems of consistency and portability among applications, 6 Plan 9 uses a fixed color map, called 7 .BR rgbv , 8 on 8-bit-per-pixel displays. 9 Although this avoids problems caused by multiplexing color maps between 10 applications, it requires that the color map chosen be suitable for most purposes 11 and usable for all. 12 Other systems that use fixed color maps tend to sample the color cube 13 uniformly, which has advantages\(emmapping from a (red, green, blue) triple 14 to the color map and back again is easy\(embut ignores an important property 15 of the human visual system: eyes are 16 much more sensitive to small changes in intensity than 17 to changes in hue. 18 Sampling the color cube uniformly gives a color map with many different 19 hues, but only a few shades of each. 20 Continuous tone images converted into such maps demonstrate conspicuous 21 artifacts. 22 .PP 23 Rather than dice the color cube into subregions of 24 size 6\(mu6\(mu6 (as in Netscape Navigator) or 8\(mu8\(mu4 25 (as in previous releases of Plan 9), picking 1 color in each, 26 the 27 .B rgbv 28 color map uses a 4\(mu4\(mu4 subdivision, with 29 4 shades in each subcube. 30 The idea is to reduce the color resolution by dicing 31 the color cube into fewer cells, and to use the extra space to increase the intensity 32 resolution. 33 This results in 16 grey shades (4 grey subcubes with 34 4 samples in each), 13 shades of each primary and secondary color (3 subcubes 35 with 4 samples plus black) and a reasonable selection of colors covering the 36 rest of the color cube. 37 The advantage is better representation of 38 continuous tones. 39 .PP 40 The following function computes the 256 3-byte entries in the color map: 41 .IP 42 .EX 43 .ta 6n +6n +6n +6n 44 void 45 setmaprgbv(uchar cmap[256][3]) 46 { 47 uchar *c; 48 int r, g, b, v; 49 int num, den; 50 int i, j; 51 52 for(r=0,i=0; r!=4; r++) 53 for(v=0; v!=4; v++,i+=16) 54 for(g=0,j=v-r; g!=4; g++) 55 for(b=0; b!=4; b++,j++){ 56 c = cmap[i+(j&15)]; 57 den = r; 58 if(g > den) 59 den = g; 60 if(b > den) 61 den = b; 62 if(den == 0) /* would divide check; pick grey shades */ 63 c[0] = c[1] = c[2] = 17*v; 64 else{ 65 num = 17*(4*den+v); 66 c[0] = r*num/den; 67 c[1] = g*num/den; 68 c[2] = b*num/den; 69 } 70 } 71 } 72 .EE 73 .PP 74 There are 4 nested loops to pick the (red,green,blue) coordinates of the subcube, 75 and the value (intensity) within the subcube, indexed by 76 .BR r , 77 .BR g , 78 .BR b , 79 and 80 .BR v , 81 whence 82 the name 83 .IR rgbv . 84 The peculiar order in which the color map is indexed is designed to distribute the 85 grey shades uniformly through the map\(emthe 86 .IR i 'th 87 grey shade, 88 .RI 0<= i <=15 89 has index 90 .IR i ×17, 91 with black going to 0 and white to 255. 92 Therefore, when a call to 93 .B draw 94 converts a 1, 2 or 4 bit-per-pixel picture to 8 bits per pixel (which it does 95 by replicating the pixels' bits), the converted pixel values are the appropriate 96 grey shades. 97 .PP 98 The 99 .B rgbv 100 map is not gamma-corrected, for two reasons. First, photographic 101 film and television are both normally under-corrected, the former by an 102 accident of physics and the latter by NTSC's design. 103 Second, we require extra color resolution at low intensities because of the 104 non-linear response and adaptation of the human visual system. 105 Properly 106 gamma-corrected displays with adequate low-intensity resolution pack the 107 high-intensity parts of the color cube with colors whose differences are 108 almost imperceptible. 109 Either reason suggests concentrating 110 the available intensities at the low end of the range. 111 .PP 112 On `true-color' displays with separate values for the red, green, and blue 113 components of a pixel, the values are chosen so 0 represents no intensity (black) and the 114 maximum value (255 for an 8-bit-per-color display) represents full intensity (e.g., full red). 115 Common display depths are 24 bits per pixel, with 8 bits per color in order 116 red, green, blue, and 16 bits per pixel, with 5 bits of red, 6 bits of green, and 5 bits of blue. 117 .PP 118 Colors may also be created with an opacity factor called 119 .BR alpha , 120 which is scaled so 0 represents fully transparent and 255 represents opaque color. 121 The alpha is 122 .I premultiplied 123 into the other channels, as described in the paper by Porter and Duff cited in 124 .MR draw (3) . 125 The function 126 .B setalpha 127 (see 128 .MR allocimage (3) ) 129 aids the initialization of color values with non-trivial alpha. 130 .PP 131 The packing of pixels into bytes and words is odd. 132 For compatibility with VGA frame buffers, the bits within a 133 pixel byte are in big-endian order (leftmost pixel is most 134 significant bits in byte), while bytes within a pixel are packed in little-endian 135 order. Pixels are stored in contiguous bytes. This results 136 in unintuitive pixel formats. For example, for the RGB24 format, 137 the byte ordering is blue, green, red. 138 .PP 139 To maintain a constant external representation, 140 the 141 .MR draw (3) 142 interface 143 as well as the 144 various graphics libraries represent colors 145 by 32-bit numbers, as described in 146 .MR color (3) . 147 .SH "SEE ALSO" 148 .MR color (3) , 149 .MR graphics (3) , 150 .MR draw (3)