Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!deimos.cis.ksu.edu!atanasoff!hascall
From: hascall@atanasoff.cs.iastate.edu (John Hascall)
Newsgroups: comp.graphics
Subject: Re: color dithering?
Message-ID: <1615@atanasoff.cs.iastate.edu>
Date: 11 Oct 89 15:11:02 GMT
References: <6350007@hpcupt1.HP.COM> <216@olive11.UUCP>
Reply-To: hascall@atanasoff.UUCP (John Hascall)
Organization: Iowa State Univ. Computation Center
Lines: 49

In article <216@olive11.UUCP> stephens@cell.mot.COM (Kurt Stephens) writes:
}In article <6350007@hpcupt1.HP.COM>, dclaar@hpcupt1.HP.COM (Doug Claar) writes:
}> 
}> I have a 1024x768 6 plane display, upon which it seems I could display a
}> picture of 512x384 with a greater number of apparent colors by some sort of
}> 2x2 dithering. Can anyone give me a reference on this, or advice, clues,
}> something-already-written-in-c?
}> 

     What I have used is the following. 

	 Decide which colors you are going to use.  I usually use a
	 "color cube".  Since I have an 8-plane system (256 colors)
	 I use a 6x6x6 cube (216 colors), you can use a 4x4x4 cube.

		idx = 0;
		for (r=0; r<cubesize; r++)
		   for (g=0; g<cubesize; g++)
		      for (b=0; b<cubesize; b++) {
			 /* set color "idx" to the
			     color <r/cubesize, g/cubesize, b/cubesize> */
                         idx++;
		      }

	Then, for each color you want to display, determine the 2 colors
	in the cube which are closest to the desired color.

	(a 1-dimension example)

	      ... ---------+-----------+-----------+--------- ...
			   x   ^       y           z
			       |
                            desired color

        (colors x & y are the closest colors)

	Next, determine the distance from the desired color to color x and
	to color y and use this to determine which dithering pattern to use.

	x x   y x   y x   y y   y y   <-- dither patterns
	x x   x x   x y   x y   y y
	    |     |     |     |
       -----+-----+-----+-----+-----
           0.2   0.4   0.6   0.8      <-- ratio of distances


John Hascall
Iowa State U
hascall@atanasoff.cs.iastate.edu