Path: utzoo!utgpu!watserv1!watmath!att!mcdchg!mcdphx!hrc!viking!campbellb
From: campbellb@viking.UUCP (Brian Campbell)
Newsgroups: comp.sys.apple
Subject: Response to Amiga HAM question
Keywords: Amiga, HAM, screen graphics
Message-ID: <48e6d609.f759@viking.UUCP>
Date: 27 Feb 90 22:32:30 GMT
Organization: gte
Lines: 29


Because of some interest on the apple news group,
what follows is a technical description of the Amiga HAM mode.
This info is based on discussions from my "Amigoid" friend, but I can not guarentee
it accurateness.

In the HAM mode the screen is represented by a six bit plane, i.e. six bits per pixel.
Two of these bits is used to represent one of four control values, and the other four
bits is used to represent a color value, whose meaning depends upon the control value.
One of the control values means that the color value represents one of 16 colors
from the color pallette (like on the IIGS).  The other three control values represent
Red, Green, and Blue modifications, and the pallette is ignored (at least directly).
That is, one of the color (R, G, or B) components is defined directly by the color
value, and the other two color components are borrowed from the previous pixel.
For example, with a red modification control value, the pixel assumes blue and green
values of the previous pixel, and assumes a red value defined in the pixel's four bit
color value.

What this means (if this explanation is correct) is that the Amiga can NOT arbitrarily
depict 4K colors anywhere on the screen, as implied by others comparing the IIGS and
the Amiga.  It can display any one of 4K colors WITH restrictions.  This mode is good
when adjacent pixels have similar color values.  But what if you want adjacent pixels
to differ in more than one color component.  You can't do it, unless one of the colors
is represented in the color pallette.  To change from one arbitrary color to another
arbitrary color, you need, in general, three pixels to do it.  The colors of the two
intervening transition pixels will not necessary blend in well with the first and
fourth pixels, particulary if the first and fourth pixels represent contrasting colors.
This may account for the "rainbow effect" reported by others on the net.