Atari 8-Bit Graphics Capabilities

[emkay]

On 8/30/2021 at 9:34 PM, PixelCrunch said:

Sorry for the massive Necro, but I’m in need of another answer…

How many video layers does the Atari 8-Bit Series have? How many layers of graphics can there be on the GTIA chip? (Example: the NES has just one background layer.).

In theory the Atari has 2 Layers. There is one layer for high details, and there is one layer for low details.

If you see any of the demonstrations, you always see graphics that use low details on the high detail layer and high details on the low details layer. Resulting in colorless graphics with small moving objects. 

 

If you want to have colorful games with huge moving objects, you'd have to use the high details layer for high details, and the low details layer for low details. 

Sounds simple, huh?

Some time ago , I did some demonstration image showing some possibilities. 

 

The resolution of 390(195)x240 is not fake and would be displayed on a modern TV 

[Synthpopalooza]

On 6/2/2022 at 9:58 PM, Mark loves Stella said:

You might find this interesting. There are multi color hi res modes. But, they may not work the way you are requesting. 

http://atarionline.pl/forum/comments.php?DiscussionID=1099&page=1

Interlace Character Editor - my design.  But it does full frame flicker, so not the best, unless you got one of those modern LCD TV's that does frame blending.

[oky2000]

It's interesting looking at the options of home computer for the early eighties in the UK (Atari 800/800XL, ZX Spectrum, Commodore 64 and Amstrad CPC, Acorn BBC etc). 

 

The only two that really share anything is the Spectrum and C64, which both have separate color memory not just screen memory (1kb Screen RAM and 1kb Color RAM is the most simple mode the C64 offers and this does allow 16 colours per scanline with zero CPU time taken but with restrictions on where they are and how colours can be changed). A good example of what is only possible with Color RAM type system in hi-res bitmap (320x200) 8kb screen is Law of the West but in 2kb mode the only difference is the background colour of each character is fixed and only the foreground can be set for each 8x8 character block without using rasters. 

 

The alternative is the enormous 16kb true bitmap screen of the 8bit Amstrad CPC machines, you can draw any combinations of 16 colours anywhere on a 160x200 screen but just like the custom chip devoid ST the 4mhz Z80 alone has to manipulate the screen 50fps (which is impossible). However in 320x200 medium resolution of the same Amstrad this 16kb screen becomes useless managing 4 colours only....which makes the budget Spectrum/Timex computers more useful at that sort of resolution which can always display a combination of 2 colours per 8x8 block in it's one and only 6.25kb 32 column screen mode.

 

 

 

[+Stephen]

On 7/20/2019 at 8:04 AM, R0ger said:

The palette resulting from PAL mixing itself is close to existing Atari colors. Color $20 and $0f will give something rather close to 2F.

But with flickering, different palette is created. I wouldn't say larger, but different. First the hues. Atari hues lie on a circle in color space. I'm picking 4 colors on the circle. Not even equally spaced. It's more like equilateral triangle for RGB, and yellow stuck between R and G. Flickering then creates hues between 2 base colors. So these new hues are somewhat at different angles, and they all have slightly worse saturation.

 

At the moment I use these colors for hues: 70,B0,E0,20 .. basically RGB are picked to be as strong and as single-component as possible. And then yellow is added. If you imagine the color space, the base colors create polygon. The flickering and dithering then can create virtual colors inside that polygon. So if some picture would be heavily skewed toward some color, the base colors could be picked differently. Though I like the system RGB + one extra color to extend the polygon toward area of interest.

 

Brightness levels are simpler. I use these 6: 0,4,8,10,12,14 .. as you can see, there is more detail in the bright values, as both flickering and dithering are more visible in bright areas. Flickering gives me extra levels very similar to original 2,6,9,11,13.

 

This also brings problem. When I'm dithering the images on PC, I'm using this 11x7=77 color palette, which I then recode into hue/bright values, and into 2 fields for flickering modes. But I have to compute the palette first. I have to simulate the way base colors combine into the resulting colors. I have very simple formula for that, which only picks other Atari palette color, but most of the time I use different method. I create XEX with all colors possible, I make screenshot in Altirra, with PAL artifacts and frame blending, and then I use this screenshot as reference.

 

This is old reference image from 11x7F mode:

 

 

Today I don't mix hues only in neighbor pairs only, I do all combinations. So instead of adding R-Y,Y-G,G-B,B-R I also add R-G and B-Y. So I actually have 10 hues, and the hues can't be ordered nicely like this, so today I use different reference pattern. The old one also had to be processed to remove the scanlines, the new one can be used straight from Altirra.

 

 

There is another level of color interpolation with dithering. And that's complex topic by itself. Rasta can't do it that well, at least it can't adapt dithering to current palette. It has to do dithering first, and then adapt palette to it. Also my approach has basically every color (minus character limits) available everywhere, which helps.

 

Recently re-visited this thread, did the convertor for these images ever get released?

[R0ger]

On 8/1/2022 at 6:06 PM, Stephen said:

Recently re-visited this thread, did the convertor for these images ever get released?

Nope. Problem is I keep fiddling with it But expressing the interest will certainly push it higher on my priorities list.

[Wrathchild]

On 6/1/2022 at 6:00 AM, Cecito said:

There will be no routine that simulates a 40x24 text mode with 8 or 16 colors per character to be used from programs written in basic like commodore or spectrum.

You could opt for it being a VBXE targeted program though? 

Edited August 3, 2022 by Wrathchild

[_The Doctor__]

On 8/3/2022 at 4:35 AM, R0ger said:

Nope. Problem is I keep fiddling with it But expressing the interest will certainly push it higher on my priorities list.

Well in that case one can only make sure to express their interest, with so many pokers in the fire, I hope to help inspire you to complete them and clear the plate so to speak!