Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!newstop!sun!peregrine!falk
From: falk@peregrine.Sun.COM (Ed Falk)
Newsgroups: comp.graphics
Subject: Re: bit masks
Message-ID: <141649@sun.Eng.Sun.COM>
Date: 31 Aug 90 01:53:00 GMT
References: <23900013@sunb4>
Sender: news@sun.Eng.Sun.COM
Organization: Sun Microsystems, Mt. View, Ca.
Lines: 75

In article <23900013@sunb4> mcooper@sunb4.cs.uiuc.edu writes:
>
>I'm looking for an algorithm (or C code, of you have it) to generate
>16 x 16 bit masks.  Ideally, I should be able to specify a percentage and
>the program should spit out the pattern filled appropriately.
>
>ie.
>
>100% yeilds
>
> [all 1's]
>
>This is no problem to write,m but the simple way gives you nasty artifacts such
>as:  25 % yeilds
>
> [25% 1's]
>
>
>Does anyone have code that will properly randomize this stuff to avoid the
>inevitable artifacts and moire patterns?

It seems to me that what you're asking for is a way to generate
dither patterns.  If you don't mind regular patterns, which generally
is the best way to go, use this dither matrix:


static	int	d16[16][16] = {
	{  0,192, 48,240, 12,204, 60,252,  3,195, 51,243, 15,207, 63,255},
	{128, 64,176,112,140, 76,188,124,131, 67,179,115,143, 79,191,127},
	{ 32,224, 16,208, 44,236, 28,220, 35,227, 19,211, 47,239, 31,223},
	{160, 96,144, 80,172,108,156, 92,163, 99,147, 83,175,111,159, 95},
	{  8,200, 56,248,  4,196, 52,244, 11,203, 59,251,  7,199, 55,247},
	{136, 72,184,120,132, 68,180,116,139, 75,187,123,135, 71,183,119},
	{ 40,232, 24,216, 36,228, 20,212, 43,235, 27,219, 39,231, 23,215},
	{168,104,152, 88,164,100,148, 84,171,107,155, 91,167,103,151, 87},
	{  2,194, 50,242, 14,206, 62,254,  1,193, 49,241, 13,205, 61,253},
	{130, 66,178,114,142, 78,190,126,129, 65,177,113,141, 77,189,125},
	{ 34,226, 18,210, 46,238, 30,222, 33,225, 17,209, 45,237, 29,221},
	{162, 98,146, 82,174,110,158, 94,161, 97,145, 81,173,109,157, 93},
	{ 10,202, 58,250,  6,198, 54,246,  9,201, 57,249,  5,197, 53,245},
	{138, 74,186,122,134, 70,182,118,137, 73,185,121,133, 69,181,117},
	{ 42,234, 26,218, 38,230, 22,214, 41,233, 25,217, 37,229, 21,213},
	{170,106,154, 90,166,102,150, 86,169,105,153, 89,165,101,149, 85}} ;

This maps intensities from 0-255 to either 0 or 1 by these rules:
Take the x and y coordinates of the pixel, mod 16, and use them as an
index into this array.  If the intensity is less than or equal to
this value, write a 0, otherwise, write a 1.

If the intensity is 256, always write a 1.

This is called ordered dithering.

So, if you were to write a solid field of value 256 (100%), you would get

	1111
	1111
	1111
	1111

If you were to write a solid field of value 64 (50%), you would get

	1010
	0000
	1010
	0000

and so on.  If you were to take a greyscale image, and map each pixel,
you would get something you could display on a monochrome monitor.


	-ed falk, sun microsystems -- sun!falk, falk@sun.com
	"What are politicians going to tell people when the
	Constitution is gone and we still have a drug problem?"
			-- William Simpson, A.C.L.U.