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From: upl@puff.UUCP
Newsgroups: comp.graphics
Subject: Re: Amiga World Ray-tracing article...
Message-ID: <660@puff.WISC.EDU>
Date: Fri, 17-Apr-87 15:57:25 EST
Article-I.D.: puff.660
Posted: Fri Apr 17 15:57:25 1987
Date-Received: Sat, 18-Apr-87 08:07:14 EST
References: <239@rocky.STANFORD.EDU> <662@aurora.UUCP>
Reply-To: upl@puff.WISC.EDU (Future Unix Gurus)
Distribution: comp
Organization: U of Wisconsin CS Dept
Lines: 40

In article <662@aurora.UUCP> jbm@aurora.UUCP (Jeffrey Mulligan) writes:
>in article <239@rocky.STANFORD.EDU>, ali@rocky.STANFORD.EDU (Ali Ozer) says:
>+ 
>+ In article <647@puff.WISC.EDU> beilke@puff.WISC.EDU (Matthew Beilke) writes:
>+>Make that 320X400 6-bit H.A.M.  The Amiga doesn't have the capability to
>+>display > 6 bit planes.
>+ 
>+ Despite that (just to set the record straight for comp.graphics 
>+ readers not familiar with the Amiga), the Amiga can display 4096 
>+ colors at once, using 6 bit planes.
>
>Could you elaborate a bit on how they achieve this remarkable feat?
>(No sarcasm intended.)
>
>With the systems I have used (a set not including the Amiga):
>
># colors that can be displayed AT ONCE = 2 ** ( # of bit planes )
>
># of "distinct" colors = 2 ** ( r_DAC_bits + g_DAC_bits + b_DAC_bits )

I can answer this one. The Amiga uses a cleaver dodge called Hold And
Modify (or HAM) mode. What it does is to take the 6 bits it can have
for a given pixel and devided into two fields like so:
	   xx xxxx
	   /    \
    Control    Data

The top 2 bits define how the bottom 4 bits are interpreted.
Your four choices are: New Red Value, New Blue Value, New Green Value,
or an offset in a 16 entry color table. If you use any of the first three
choices, it takes the value of the previous pixel and substitutes in the
new red, green, or blue value. Obviously, if these were your only choices,
it would take up to 3 pixels to change from one color to another (depending
on whether they share any common red, green or blue values). In order to
smooth this out, you can allocate a pallete of 16 absolute colors, accessed
through the foourth mode.. In any case, the pixel color generated is a 12 bit
value (4096 combinations). Clever people, those Amiga developers......

Jeff Kesselman
upl@puff.cs.wisc.edu