.TH COLOUR 6.SH NAMEcolour \- representation of pixels and colours.SH DESCRIPTIONAn image manipulated by.IR draw (2)(via the device.IR draw (3)),including the image corresponding to a physical display,contains a set of pixels.Each pixel has one or more components:colour components (red, green, blue); greyscale value; colour-map index; alpha value; and``don't care'' (for padding).Each component takes a given number of bits; the sum of the sizes of all componentsdetermines the size of the pixel.The implementation supports only pixel sizes that are either divisors or multiples of 8 bits.All pixels in an image have the same structure, corresponding to the.I channelsof that image (see.IR image (6))..PPThe values of the red, green and blue components are chosen so 0 representsno intensity (black) and the maximum value (all ones, 255 for an 8-bit component)represents full intensity (eg, full red).Common colour physical display depths are 24 bits per pixel, with 8 bits per colour in orderred, green, blue, and 16 bits per pixel, with 5 bits of red, 6 bits of green, and 5 bits of blue..PPColors may also be created with an opacity factor called.IR alpha ,which is scaled so 0 represents fully transparent and the maximum value (eg, 255 foran 8-bit alpha component) represents opaque colour.The alpha is.I premultipliedinto the other channels, as described in the paper by Porter and Duff cited in.IR draw-image (2).The function.B Draw->setalpha(see.IR draw-intro (2))aids the initialisation of colour values with non-trivial alpha..PPBecause images are stored in memory managed by.IR draw (3)and operated through.IR draw-image (2),the details of pixel representation internally can be ignored by many applications.The representation is visible, however, when using the operations.B Image.readpixelsand.B Image.writepixels(see.IR draw-image (2)).The bits representing a pixel's channel components are packed contiguously, andpixels are stored in contiguous bytes.The packing of pixels into bytes and words is odd.For compatibility with VGA frame buffers, the bits within apixel byte are in big-endian order (leftmost pixel is mostsignificant bits in byte), while bytes within a pixel are packed in little-endian order.This results in unintuitive pixel formats. For example, for the RGB24 format,the byte ordering is blue, green, red..PPTo maintain a constant external representation, the.IR draw (3)interfaceas well as thevarious graphics libraries represent colours by 32-bit integers, containing red, blue, green and alpha componentsas 8-bit values, in that order from most to least significant byte.The color component values range from 0 (no colour) to 255 (saturated);alpha ranges from 0 (fully transparent) to 255 (fully opaque)..PPOn displays with 8 bits per pixel or less,to address problems of consistency and portability amongst Inferno applications,Inferno uses a fixed colour map, called.BR rgbv .Although this avoids problems caused by multiplexing colour maps betweenapplications, it requires that the colour map chosen be suitable for most purposesand usable for all.Other systems that use fixed colour maps tend to sample the colour cubeuniformly, which has advantages\(emmapping from a (red, green, blue) tripleto the colour map and back again is easy\(embut ignores an important propertyof the human visual system: eyes aremuch more sensitive to small changes in intensity thanto changes in hue.Sampling the colour cube uniformly gives a colour map with many differenthues, but only a few shades of each.Continuous tone images converted into such maps demonstrate conspicuousartifacts..PPRather than dice the colour cube into subregions ofsize 6\(mu6\(mu6 (as in Netscape Navigator) or 8\(mu8\(mu4picking 1 colour in each,the.B rgbvcolour map uses a 4\(mu4\(mu4 subdivision, with4 shades in each subcube.The idea is to reduce the colour resolution by dicingthe colour cube into fewer cells, and to use the extra space to increase the intensityresolution.This results in 16 grey shades (4 grey subcubes with4 samples in each), 13 shades of each primary and secondary colour (3 subcubeswith 4 samples plus black) and a reasonable selection of colours covering therest of the colour cube.The advantage is better representation ofcontinuous tones..PPThe following function computes the 256 3-byte entries in the colour map:.IP.EX.ta 6n +6n +6n +6nvoidsetmaprgbv(uchar cmap[256][3]){    uchar *c;    int r, g, b, v;    int num, den;    int i, j;     for(r=0,i=0; r!=4; r++)      for(v=0; v!=4; v++,i+=16)        for(g=0,j=v-r; g!=4; g++)          for(b=0; b!=4; b++,j++){            c = cmap[i+(j&15)];            den = r;            if(g > den)                den = g;            if(b > den)                den = b;            if(den == 0) /* would divide check; pick grey shades */                c[0] = c[1] = c[2] = 17*v;            else{                num = 17*(4*den+v);                c[0] = r*num/den;                c[1] = g*num/den;                c[2] = b*num/den;            }          }}.EE.PPThere are 4 nested loops to pick the (red,green,blue) coordinates of the subcube,and the value (intensity) within the subcube, indexed by.BR r ,.BR g ,.BR b ,and.BR v ,whencethe name.IR rgbv .The peculiar order in which the colour map is indexed is designed to distribute thegrey shades uniformly through the map\(emthe.IR i 'thgrey shade,.RI 0<= i <=15has index.IR i ×17,with black going to 0 and white to 255.Therefore, when a call to.B Image.draw(see.IR draw-image (2))converts a 1, 2 or 4 bit-per-pixel picture to 8 bits per pixel (which it doesby replicating the pixels' bits), the converted pixel values are the appropriategrey shades..PPThe.B rgbvmap is not gamma-corrected, for many reasons.  First, photographicfilm and television are both normally under-corrected, the former by anaccident of physics and the latter by NTSC's design.Second, we require extra colour resolution at low intensities because of thenon-linear response and adaptation of the human visual system.Properlygamma-corrected displays with adequate low-intensity resolution pack thehigh-intensity parts of the colour cube with colours whose differences arealmost imperceptible.Either of these reasons suggests concentratingthe available intensities at the low end of the range.Third, the compositing computations underlying the graphics operations in.IR draw-image (2)assume a linear colour space.Finally, the right value for gamma correction is determined in part by thecharacteristics of the physical display device, and correction should be done on output..SH "SEE ALSO".IR draw-intro (2),.IR draw-image (2)