COLOUR(6) COLOUR(6)
NAME
colour - representation of pixels and colours
DESCRIPTION
An image manipulated by draw(2) (via the device draw(3)),
including the image corresponding to a physical display,
contains a set of pixels. Each pixel has one or more compo-
nents: 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 components determines 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
channels of that image (see image(6)).
The values of the red, green and blue components are chosen
so 0 represents no 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
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
the maximum value (eg, 255 for an 8-bit alpha component)
represents opaque colour. The alpha is premultiplied into
the other channels, as described in the paper by Porter and
Duff cited in draw-image(2). The function Draw->setalpha
(see draw-intro(2)) aids the initialisation of colour values
with non-trivial alpha.
Because images are stored in memory managed by draw(3) and
operated through draw-image(2), the details of pixel repre-
sentation internally can be ignored by many applications.
The representation is visible, however, when using the oper-
ations Image.readpixels and Image.writepixels (see draw-
image(2)). The bits representing a pixel's channel compo-
nents are packed contiguously, and pixels are stored in con-
tiguous bytes. 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. This results in
unintuitive pixel formats. For example, for the RGB24 for-
mat, the byte ordering is blue, green, red.
To maintain a constant external representation, the draw(3)
interface as well as the various graphics libraries
Page 1 Plan 9 (printed 11/22/24)
COLOUR(6) COLOUR(6)
represent colours by 32-bit integers, containing red, blue,
green and alpha components as 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).
On displays with 8 bits per pixel or less, to address prob-
lems of consistency and portability amongst Inferno applica-
tions, Inferno uses a fixed colour map, called rgbv.
Although this avoids problems caused by multiplexing colour
maps between applications, it requires that the colour map
chosen be suitable for most purposes and usable for all.
Other systems that use fixed colour maps tend to sample the
colour cube uniformly, which has advantages-mapping from a
(red, green, blue) triple to the colour 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 colour
cube uniformly gives a colour map with many different hues,
but only a few shades of each. Continuous tone images con-
verted into such maps demonstrate conspicuous artifacts.
Rather than dice the colour cube into subregions of size
6x6x6 (as in Netscape Navigator) or 8x8x4 picking 1 colour
in each, the rgbv colour map uses a 4x4x4 subdivision, with
4 shades in each subcube. The idea is to reduce the colour
resolution by dicing the colour 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 sam-
ples in each), 13 shades of each primary and secondary
colour (3 subcubes with 4 samples plus black) and a reason-
able selection of colours covering the rest of the colour
cube. The advantage is better representation of continuous
tones.
The following function computes the 256 3-byte entries in
the colour 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;
Page 2 Plan 9 (printed 11/22/24)
COLOUR(6) COLOUR(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 colour 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
Image.draw (see draw-image(2)) 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 many 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
colour 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
colour cube with colours whose differences are almost imper-
ceptible. Either of these reasons suggests concentrating
the available intensities at the low end of the range.
Third, the compositing computations underlying the graphics
operations in draw-image(2) assume a linear colour space.
Finally, the right value for gamma correction is determined
in part by the characteristics of the physical display
device, and correction should be done on output.
SEE ALSO
draw-intro(2), draw-image(2)
Page 3 Plan 9 (printed 11/22/24)