COLOR(6) COLOR(6)
NAME
color - representation of pixels and colors
DESCRIPTION
To address problems of consistency and portability among
applications, Plan 9 uses a fixed color map, called rgbv, on
8-bit-per-pixel displays. Although this avoids problems
caused by multiplexing color maps between applications, it
requires that the color map chosen be suitable for most pur-
poses and usable for all. Other systems that use fixed
color maps tend to sample the color cube uniformly, which
has advantages-mapping from a (red, green, blue) triple to
the color map and back again is easy-but ignores an impor-
tant property of the human visual system: eyes are much more
sensitive to small changes in intensity than to changes in
hue. Sampling the color cube uniformly gives a color map
with many different hues, but only a few shades of each.
Continuous tone images converted into such maps demonstrate
conspicuous artifacts.
Rather than dice the color cube into subregions of size
6x6x6 (as in Netscape Navigator) or 8x8x4 (as in previous
releases of Plan 9), picking 1 color in each, the rgbv color
map uses a 4x4x4 subdivision, with 4 shades in each subcube.
The idea is to reduce the color resolution by dicing the
color cube into fewer cells, and to use the extra space to
increase the intensity resolution. This results in 16 grey
shades (4 grey subcubes with 4 samples in each), 13 shades
of each primary and secondary color (3 subcubes with 4 sam-
ples plus black) and a reasonable selection of colors cover-
ing the rest of the color cube. The advantage is better
representation of continuous tones.
The following function computes the 256 3-byte entries in
the color map:
void
setmaprgbv(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;
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COLOR(6) COLOR(6)
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;
}
}
}
There are 4 nested loops to pick the (red,green,blue) coor-
dinates of the subcube, and the value (intensity) within the
subcube, indexed by r, g, b, and v, whence the name rgbv.
The peculiar order in which the color map is indexed is
designed to distribute the grey shades uniformly through the
map-the i'th grey shade, 0<=i<=15 has index i×17, with black
going to 0 and white to 255. Therefore, when a call to draw
converts a 1, 2 or 4 bit-per-pixel picture to 8 bits per
pixel (which it does by replicating the pixels' bits), the
converted pixel values are the appropriate grey shades.
The rgbv map is not gamma-corrected, for two reasons.
First, photographic film and television are both normally
under-corrected, the former by an accident of physics and
the latter by NTSC's design. Second, we require extra color
resolution at low intensities because of the non-linear
response and adaptation of the human visual system. Prop-
erly gamma-corrected displays with adequate low-intensity
resolution pack the high-intensity parts of the color cube
with colors whose differences are almost imperceptible.
Either reason suggests concentrating the available intensi-
ties at the low end of the range.
On `true-color' displays with separate values for the red,
green, and blue components of a pixel, the values are chosen
so 0 represents no intensity (black) and the maximum value
(255 for an 8-bit-per-color display) represents full inten-
sity (e.g., full red). Common display depths are 24 bits
per pixel, with 8 bits per color in order red, green, blue,
and 16 bits per pixel, with 5 bits of red, 6 bits of green,
and 5 bits of blue.
Colors may also be created with an opacity factor called
alpha, which is scaled so 0 represents fully transparent and
255 represents opaque color. The alpha is premultiplied
into the other channels, as described in the paper by Porter
and Duff cited in draw(2). The function setalpha (see
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COLOR(6) COLOR(6)
allocimage(2)) aids the initialization of color values with
non-trivial alpha.
The packing of pixels into bytes and words is odd. For com-
patibility with VGA frame buffers, the bits within a pixel
byte are in big-endian order (leftmost pixel is most signif-
icant bits in byte), while bytes within a pixel are packed
in little-endian order. Pixels are stored in contiguous
bytes. This results in unintuitive pixel formats. For exam-
ple, for the RGB24 format, the byte ordering is blue, green,
red.
To maintain a constant external representation, the draw(3)
interface as well as the various graphics libraries repre-
sent colors by 32-bit numbers, as described in color(2).
SEE ALSO
color(2), graphics(2), draw(2)
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