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From: scott@applix.UUCP (Scott Evernden)
Newsgroups: comp.graphics
Subject: Re: Amiga World Ray-tracing article...
Message-ID: <444@applix.UUCP>
Date: Mon, 20-Apr-87 11:35:08 EST
Article-I.D.: applix.444
Posted: Mon Apr 20 11:35:08 1987
Date-Received: Tue, 21-Apr-87 01:07:38 EST
References: <239@rocky.STANFORD.EDU> <662@aurora.UUCP>
Reply-To: scott@applix.UUCP (Scott Evernden)
Distribution: comp
Organization: APPLiX Inc., Westboro MA
Lines: 55

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?

The Amiga can display 4096 colors using 6 bit planes in a mode called HAM
(Hold-And-Modify) using the following trick:

Each pixel is represented by a 6 bit value.
If the top 2 bits are 0, then the low-order 4 bits are used as an index
  into a (16 entry x 12 bit wide) color lookup table supplying that pixel's
  color.
If the top 2 bits are NOT 0, then they are 1, 2, or 3 and the low-order
  4 bits are used to modify a primary color component of the preceeding
  (left neighbor) pixel.  (i.e., MODIFY 4 bits of the 12 bit color value
  HELD from the last pixel).

     +-----------------------+
     | 0 | 0 | X | X | X | X |
     +-----------------------+
              \_____________/
                     \__________ 4 bit index into color lookup table;
                                 lookup table has 4 bits/primary (= 12 bits)

     +-----------------------+
     | N | N | X | X | X | X |
     +-----------------------+
      \_____/ \_____________/
         \           \           new 4 bit primary value for (eg., RED)
          \           \_________ the pixel's color is the same as the
           \                     preceeding pixel except the (eg., RED)
            \                    4 bit component is directly given here.
             \_ 01 - modify RED 
                10 - modify GRN
                11 - modify BLU

So, with this scheme, you effectively have 12 bit/pixel control, although
you cannot use any 12 bit value anywhere.  Consequently, HAM-mode Amiga
images can tend to be a bit fuzzy in the vicinity of color edges, but
if the color lookup table is designed correctly, then this problem can be
minimized.

DBW_Render actually computes 12 bit color values for the entire picture,
and then computes a histogram of color usage in order to choose a "best"
set of lookup table values to minimize this "fringing".

 - scott