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NAME
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color – representation of pixels and colors
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DESCRIPTION
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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 purposes 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 important 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 6×6×6 (as
in Netscape Navigator) or 8×8×4 (as in previous releases of Plan
9), picking 1 color in each, the rgbv color map uses a 4×4×4 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 samples plus black) and
a reasonable selection of colors covering 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:
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void
setmaprgbv(uchar cmap[256][3])
{
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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;
}
}
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}
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There are 4 nested loops to pick the (red,green,blue) coordinates
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 ix17, 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.
Properly
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 intensities 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 intensity (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(3).
The function setalpha (see allocimage(3)) aids the initialization
of color values with non-
trivial alpha.
The packing of pixels into bytes and words is odd. For compatibility
with VGA frame buffers, the bits within a pixel byte are in big-endian
order (leftmost pixel is most significant 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 example, 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 represent colors by
32-bit numbers, as described in color(3).
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SEE ALSO
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