DITHER(3) 3 (2/2/87) DITHER(3)
NAME
dithermap, bwdithermap, make_square, dithergb, ditherbw -
functions for dithering color or black and white
images.
SYNOPSIS
dithermap( levels, gamma, rgbmap, divN, modN, magic )
int levels;
double gamma;
int rgbmap[][3], divN[256], modN[256], magic[16][16];
bwdithermap( levels, gamma, bwmap, divN, modN, magic )
int levels;
double gamma;
int bwmap[], int divN[256], modN[256], magic[16][16];
make_square( N, divN, modN, magic )
double N;
int divN[256], modN[256], magic[16][16];
dithergb( x, y, r, g, b, levels, divN, modN, magic )
int x, y, r, g, b, levels;
int divN[256], modN[256], magic[16][16];
ditherbw( x, y, val, divN, modN, magic )
int x, y, val, divN[256], modN[256], magic[16][16];
DESCRIPTION
These functions provide a common set of routines for
dithering a full color or gray scale image into a lower
resolution color map.
Dithermap computes a color map and some auxiliary parameters
for dithering a full color (24 bit) image to fewer bits.
The argument levels tells how many different intensity
levels per primary color should be computed. To get maximum
use of a 256 entry color map, use levels=6. The computed
map uses levels^3 entries. The gamma argument provides for
gamma compensation of the generated color map (that is, the
values in the map will be adjusted to give a linear
intensity variation on a display with the given gamma). The
computed color map will be returned in the array rgbmap.
divN and modN are auxiliary arrays for computing the
dithering pattern (see below), and magic is the magic square
dither pattern.
To compute a color map for dithering a black and white image
to fewer intensity levels, use bwdithermap. The arguments
are as for dithermap, but only a single channel color map is
computed. The value of levels can be larger than for
dithermap, as the computed map only has levels entries.
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DITHER(3) 3 (2/2/87) DITHER(3)
To just build the magic square and other parameters, use
make_square. The argument N should be equal to 255.0
divided by the desired number of intensity levels less one
(i.e., N = 255.0 / (levels - 1)). The other arguments are
filled in as above.
The color map index for a dithered full color pixel is
computed by dithergb. Since the pattern depends on the
screen location, the first two arguments x and y, specify
that location. The true color of the pixel at that location
is given by the triple r, g, and b. The number of intensity
levels and the dithering parameter matrices computed by
dithermap are also passed to dithergb.
The color map index for a dithered gray scale pixel is
computed by ditherbw. Again, the screen position is
specified, and the intensity value of the pixel is supplied
in val. The dithering parameters must also be supplied.
Alternatively, the dithering may be done in line instead of
incurring the extra overhead of a function call, which can
be significant when repeated a million times. The
computation is as follows:
row = y % 16;
col = x % 16;
#define DMAP(v,col,row) (divN[v] + (modN[v]>magic[col][row] ? 1 : 0))
pix = DMAP(r,col,row) + DMAP(g,col,row)*levels +
DMAP(b,col,row)*levels*levels;
For a gray scale image, it is a little simpler:
pix = DMAP(val,row,col);
And on a single bit display (assuming a 1 means white):
pix = divN[val] > magic[col][row] ? 1 : 0
SEE ALSO
rgb_to_bw(3), librle(3), RLE(5).
AUTHOR
Spencer W. Thomas
University of Utah
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