[rescue] Weird Silicon Graphics o2 graphics issues

Curtis H. Wilbar Jr. rescue at hawkmountain.net
Thu Oct 2 15:30:06 CDT 2003

On Thu, 2003-10-02 at 16:11, Nathan Raymond wrote:
> On Thu, 2 Oct 2003, Andrew Weiss wrote:

You forgot the Atari 400/800/1200XL/600XL/800XL/etc ...

The 8 bit atari had a display list.  In memory there was a entry for
each scan line.  One of the things that could be done is to define
an interupt on any given scan line (or all of them).  The work you
could do was limited by the time it took for the beam to move back
to the left side of the screen, but you could alter your color
palette for each scan line.

This is not as advanced as the Amiga 500s HAM (which I had not known
about as I never had the money to get into Amigas back then).... but
in some ways was more flexible (as you didn't need to worry about
chagning the R, G, or B from the pixel to your left).

If anyone here ever has seen the demo atari wrote which is an Atari
Fuji logo and the word ATARI under it in beautiful cycling colors
(I think they appeared moving up the screen... or they had the
illusion of doing so) on a black backround...  this was done with
display list interrupts cycling the color palettes on each scan line
which gave the atari logo it's moving banded color rainbow effect.

I'm sure many games made use of display list interrupts.

The whole video architecture of the Atari 8 bit amazes me still to
this day....

-- Curt

> For instance the Amiga 500:
> http://www.obsoletecomputermuseum.org/amiga500/
> "The other special mode was called Hold And Modify, or HAM for short. HAM
> mode let you have all 4096 colors on the screen at once. It performed this
> trick by creating a 16 color palette. A pixel could be one of those 16
> colors, or it could copy two color values from the pixel to its left and
> define the third color. For example, a pixel could copy the red and green
> values from its neighbor to the left, and define its own blue value. This
> let you have all 4096 colors, but limited which colors could be next to
> each other. Plus, changing one pixel could potentially change the color of
> every pixel to the right of it. Despite these limitations, HAM mode screen
> could be gorgeous, for beyond what either IBM or Mac machines could do.
> HAM mode was used mainly for still screens, but a few games actually
> utilized it."
> And the Apple IIgs:
> http://www.old-computers.com/museum/computer.asp?c=71
> "320x200 with 3200 colors: in this mode, the CPU is used to swap palettes
> into and out of video memory such that a separate 16 color palette can be
> used on each of the 200 scan lines for 3200 possible colors. This mode is
> often used for viewing graphics... 640x200 with 16 dithered colors: in
> this mode, the pixels in the graphic screen are grouped into even and odd
> columns. The even columns can have a palette of 4 pure colors out of a of
> 4096 possible. The odd columns can have a second palette of 4 pure colors.
> The GS dithers the adjacent colors for 4x4=16 dithered colors. This mode
> is widely used in productivity programs and also in Apple's Finder for the
> GS... Combinations and variations: the Apple IIGS has scan line
> interrupts. Part of the screen can be in 640x200 mode and part of it can
> be in 20x200 resolution. Such split modes are sometimes used in paint
> programs, where the menu bar is in 640x200 while the graphic is in
> 320x200."
> --
> Nathan Raymond
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