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.

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