johnm@trsvax.UUCP (04/26/89)
Last Friday I was about to post this in response to an article by a fellow who
wanted to dither 256 colors down to black/white for his Sun. Then our Vax
crashed... Here is the belated posting.
******************************************************************************
One respondent suggested Floyd-Steinberg for dithering 8bits/pixel down to
1bit/pixel.
I would suggest instead that you try an Ordered dither. Ordered dithering
is extremely fast (just a table lookup) and does very well considering its
simplicity. Straight Floyd-Steinberg can come out looking strange unless to
process each row in alternating directions (i.e. serpentine), which further
complicates and slows down the process.
In an ordered dither you have a table of values that is applied over and over
the entirety of the picture. The intensity value that the picture has in
each location is compared to the value in the table and a binary value is the
result. Here's kind of a pseudo code version of what you do with the tables.
for (i = 0; i < picture_width; i++) {
for (j = 0; j < picture_height; j++) {
/* Note: The mod values used are governed by the width of the
* table used.
*/
temp = dither256[j % 16][i % 16];
if (intensity[j][i] < temp) {
intensity[j][i] = 1;
} else {
intensity[j][i] = 0;
}
}
}
If the original pixels have actual colors rather than a simple 256 level
gray-scale ramp you can always use the RGB to Y conversion to convert the
color to an intensity value.
John Munsch
/* Here's a set of tables that I generated for something of my own.
*
* They will allow you to convert from 16, 64, or 256 levels of intensity
* down to just one.
*/
int dither16[4][4] = {
0, 8, 2, 10,
12, 4, 14, 6,
3, 11, 1, 9,
15, 7, 13, 5,
};
int dither64[8][8] = {
0, 32, 8, 40, 2, 34, 10, 42,
48, 16, 56, 24, 50, 18, 58, 26,
12, 44, 4, 36, 14, 46, 6, 38,
60, 28, 52, 20, 62, 30, 54, 22,
3, 35, 11, 43, 1, 33, 9, 41,
51, 19, 59, 27, 49, 17, 57, 25,
15, 47, 7, 39, 13, 45, 5, 37,
63, 31, 55, 23, 61, 29, 53, 21,
};
int dither256[16][16] = {
0, 128, 32, 160, 8, 136, 40, 168, 2, 130, 34, 162, 10, 138, 42, 170,
192, 64, 224, 96, 200, 72, 232, 104, 194, 66, 226, 98, 202, 74, 234, 106,
48, 176, 16, 144, 56, 184, 24, 152, 50, 178, 18, 146, 58, 186, 26, 154,
240, 112, 208, 80, 248, 120, 216, 88, 242, 114, 210, 82, 250, 122, 218, 90,
12, 140, 44, 172, 4, 132, 36, 164, 14, 142, 46, 174, 6, 134, 38, 166,
204, 76, 236, 108, 196, 68, 228, 100, 206, 78, 238, 110, 198, 70, 230, 102,
60, 188, 28, 156, 52, 180, 20, 148, 62, 190, 30, 158, 54, 182, 22, 150,
252, 124, 220, 92, 244, 116, 212, 84, 254, 126, 222, 94, 246, 118, 214, 86,
3, 131, 35, 163, 11, 139, 43, 171, 1, 129, 33, 161, 9, 137, 41, 169,
195, 67, 227, 99, 203, 75, 235, 107, 193, 65, 225, 97, 201, 73, 233, 105,
51, 179, 19, 147, 59, 187, 27, 155, 49, 177, 17, 145, 57, 185, 25, 153,
243, 115, 211, 83, 251, 123, 219, 91, 241, 113, 209, 81, 249, 121, 217, 89,
15, 143, 47, 175, 7, 135, 39, 167, 13, 141, 45, 173, 5, 133, 37, 165,
207, 79, 239, 111, 199, 71, 231, 103, 205, 77, 237, 109, 197, 69, 229, 101,
63, 191, 31, 159, 55, 183, 23, 151, 61, 189, 29, 157, 53, 181, 21, 149,
255, 127, 223, 95, 247, 119, 215, 87, 253, 125, 221, 93, 245, 117, 213, 85,
};