[atari7800] Re: New Kid on the Block...

Palette handling in 160B (160x4) mode is a little tricky. In this mode
the palletes are used in two groups, 0-3 and 4-7 selected by the high
bit of the palette field in the display list. The four color bits in
each pixel are used to select one of the colors in those palette groups.
For example:

Display List Palette = 0
Pixel bits = 1
Palette used: P0C1

Display List Palette = 0
Pixel bits = 4
Palette used: P1C1

Display List Palette = 4
Pixel bits = 1
Palette used: P4C1

If the pixel data is 0, 4, 8, or 12 then the pixel is transparent.

Dan

> -----Original Message-----
> From: atari7800-bounce@xxxxxxxxxxxxx 
> [mailto:atari7800-bounce@xxxxxxxxxxxxx] On Behalf Of Manuel Rotschkar
> Sent: Saturday, October 30, 2004 6:40 AM
> To: atari7800@xxxxxxxxxxxxx
> Subject: [atari7800] New Kid on the Block...
> 
> 
> Hi there!
> 
> Ah... the magical attraction of contests...
> 
> Ok, I started reading some of Dan Boris docs and such and am 
> still wondering about 7800 things but my first task should be 
> something
> simple: A title screen
> 
> If I simply wanted to display a 160*200 graphic with 16 
> colors to the 7800, it seems like "160B" was the best choice.
> 
> As I understand it, a title screen would basically be one 
> single giant sprite. No, wait. An object can be 32 "Byte 
> Wide", so that is 64 pixels in "160B", right? So I'd have 2 
> 64 and 1 32 pixel wide sprites.
> 
> I understand Dans docs thus far that I can have 4 color bits 
> here per pixel in "160B", but nowwhere it says which bit 
> combinations target which of the 24 color registers. I can 
> set one of 8 palettes in the display list, but that doesn't 
> seem to make too much sense to me, I guess it gets somehow 
> overridden by the 4 color bits?
> 
> Is there sort of like a standart GFX format available for 
> full screen 7800 images? Any tool that'll produce the 
> required data? Any code available that displays a picture in 
> this standard format? Any examples?
> 
> Greetings,
>       Manuel
>

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