Re: Apple IIgs video modes?

[hochstrasse@urz.unibas.ch]

In article <carl.203.0@busop.cit.wayne.edu>, carl@busop.cit.wayne.edu (carl@busop.cit.wayne.edu) writes:
> 
> I am new to Apple IIgs computers and I was wondering if someone could list 
> its video mode specs, like resolution and color.  From what I remember I 
> heard it could do a maximum of 256 colors on screen, but only 16 different 
> ones per scanline out of a palette of 4,096.  But then I've seen these 
> pictures that claim to be 3,200 colors.  So what's the scope?
> 
> Thanks
> 
> -
> Carl Mueller
> Wayne State University
> email: carl@busop.cit.wayne.edu

The facts in your post are all correct but I'll try to put it systematically:

RESOLUTION (GS specific)
320x200, every pixel 4 bits deep, so 16 principally 16 colors possible
640x200, every pixel 2 bits deep, so 4 colors here in principal

But you aren't restricted to the same choice of colors all over the entire
screen. Instead, the GS uses CLUTs (color look-up tables) called palettes, 
to determine the hue to display.
Standard display modes defined by Apple can use the following number of
palettes (one scan line can only have one palette at a time):

- 320x200 ('320 mode') 16 palettes per screen so 16*16=256 different
  hues per screen
- 640x200 ('640 mode') 16 palettes also. But since each pixel only
  uses 2 bits, you have space for more 'positions' inside the palette:
  4 'mini-palettes'. The first stands for the 1st, 5th, 9th pixel column,
  the second for the 2nd, 6th, 10th pixel column and so on. By using
  dithering (two adjacent colors blend into a new color) and clever
  minipalettes you can get 16 different colors per palette. Since you
  have 16 palettes as in 320 mode, you can also get 256 different hues here.

(Unfortunately many paint programs only support a screen to have one
palette, so the the entire screen is restricted to 16 different hues.)

Now we come to the number of colors we can choose from: The GS hardware
supports 16 different levels of each basic video color red, green and
blue (RGB). Makes 16*16*16=4096 different hues.

So much for the standards: 256 colors out of 4096 per screen.

The trick for the 3200-color screen modes is, that the program displaying
those pictures changes palettes on the fly, i.e. while the electron beam
moves from one scan line to the next. Since you have 200 scan lines with
16 hues each you get 16x200=3200 which stands for very realistic pictures
if the palettes are chosen carefully. The disadvantage is, that a un-
accelerated GSes are kept busy almost only to switch palettes.

Huh, that was a long one. I hope it was clear enough...

-- Urs

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