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From: ranger@ecsvax.UUCP (Rick N. Fincher)
Newsgroups: comp.graphics
Subject: Re: Amiga World Ray-tracing article...
Message-ID: <2928@ecsvax.UUCP>
Date: Fri, 17-Apr-87 17:49:05 EST
Article-I.D.: ecsvax.2928
Posted: Fri Apr 17 17:49:05 1987
Date-Received: Sun, 19-Apr-87 13:08:21 EST
References: <239@rocky.STANFORD.EDU> <662@aurora.UUCP>
Distribution: comp
Organization: UNC Educational Computing Service
Lines: 18
Summary: Hold and Modify...


The Amiga can get 4096 color on the screen at once in the Hold And 
Modify mode (HAM) by using the number associated with a pixel to represent
a change in color from the pixel to its left.  One of the six bits is 
used to flag whether the pixel is to use an offset into the color table
or a relative change in color from that of the pixel on its immediaterO  
left.  This allows a gradual shift of colors for shading, yet still
allows abrupt changes from one color to another.  This rather clever
method of rendering colors allows smooth shading by taking advantage of
the fact that in a shaded image, a pixel is very similar to the ones around
it.  As long as a color is either in the color table or within 5 bits of
the color to the left there is no problem.  Shading in the vertical 
direction can be done by using different color tables for each scan line.
The programming gets complicated for these images and there are some
limitations, but it allows some interesting effects.

Rick Fincher
ranger@ecsvax