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From: farren@hoptoad.UUCP
Newsgroups: comp.graphics
Subject: Re: Amiga World Ray-tracing article...
Message-ID: <2009@hoptoad.uucp>
Date: Sat, 18-Apr-87 18:10:24 EST
Article-I.D.: hoptoad.2009
Posted: Sat Apr 18 18:10:24 1987
Date-Received: Sun, 19-Apr-87 08:29:22 EST
References: <239@rocky.STANFORD.EDU> <662@aurora.UUCP>
Reply-To: farren@hoptoad.UUCP (Mike Farren)
Distribution: comp
Organization: Nebula Consultants in San Francisco
Lines: 38

In article <662@aurora.UUCP> jbm@aurora.UUCP writes:
>in article <239@rocky.STANFORD.EDU>, ali@rocky.STANFORD.EDU (Ali Ozer) says:
>
>+ 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.)
>

There are sixteen "base" colors in a color table.  The 6 bit planes
are divided into two functional groups: the low-order 4 bits are used
for color selection, and the high-order 2 bits are used for function
selection.  If the high order bits are both zero, then that pixel will
be displayed in the color selected from the color table by the
low-order bits (1 of 16, of course).  If the high-order bits are NOT
both zero, then the color of the pixel is selected by taking the color
of the previously displayed pixel (usually the one directly before
this pixel) and substituting the low-order 4 bits for either the R, G,
or B value previously used.  The color which is changed is determined
by which of the high-order bits is set.  The mode is called Hold and
Modify (HAM), because the preceding pixel's color is held and then
modified to produce the current pixel's color.

The net result of this is to allow display of all 4096 possible colors
on one screen, but the screen's real color resolution will depend on
the exact arrangement of the colors.  To go from full black to full
white, for example, will require intermediate steps of, say, red and
purple before the full white appears.  Unless carefully managed, this
can produce significant "fringing" effects.  These can be minimized by
careful selection of the sixteen base colors.

-- 
----------------
                 "... if the church put in half the time on covetousness
Mike Farren      that it does on lust, this would be a better world ..."
hoptoad!farren       Garrison Keillor, "Lake Wobegon Days"