Copper-generate WB Backdrop

[commodorejohn]

I've wondered how many colors the Copper can change, how quickly - be kind of fun to make a low-res true-color bitmap for a backdrop

[commodorejohn]

I hope this not off topic.
How fast can the colors be changed in the 32 color registers?

I looked back over some of the documentation just now - if I'm not mistaken, Copper MOVEs take 560ns (four cycles) to execute. Running with my true-color idea, that would make a bitmap of 80x(screen height) theoretically possible, though I don't know how stable it'd be in practice.

[commodorejohn]

There were a few programs for displaying copper  coloured pics on the workbench on AmiNet years ago (can't remember their  names though) but they were very tricky to edit/create pics with and  the results were pretty poor, think thats why they didn't catch on...

Well, the resolution would be pretty crap, but on the other hand, you'd have 12-bit true color. Pictures shouldn't be that hard to generate, so I'd blame it more on poor tools than any actual complexity of the task (really, all you'd be doing is a straight chain of MOVEs with WAITs at the end of each line.) Probably work better with visually simple, color-heavy images - the Windows XP "Bliss" wallpaper, for example, would look pretty good.

I was wondering if one could use a single bit  plane for higher resolution and switch out the single color register  with a new color. I am not for sure how it would be timed or even be  fast enough.

That was basically what I was thinking of doing, but it's fairly slow - you could only achieve 80x(height), and even that might not be reliable.

You're pretty much right, as far as I remember. The  caveat is that anything else that can steal DMA cycles will cause the  copper to miss one or more colour changes, and the blitter and CPU will  pretty much stall on any accesses to chip RAM while you're doing this.  In other words, any disk, audio, display or sprite DMA can or will  interfere.

Bummer. Probably not very stable, then - 40-wide might be a better shot, but then you're getting into ridiculously low resolution...

[commodorejohn]

You could also use DynamicHires, changing palettes per scanline, still gives very nice results, at a decent resolution. Several art programs will output this mode (HAMlab, ADpro and, I think IFFPro on the PC).

You can indeed - I used to use a similar approach for boot screens on my Apple IIgs. I just thought it would be an amusing novelty, is all.

Actually, the IIgs approach might be better overall - it used custom palettes shared over multiple similar lines, selected at conversion time. On the IIgs this was due to hardware limitations, but on the Amiga it'd help to cut down on the Copper bandwidth (which I gather from one of the threads on the subject is a limiting factor, at least on machines with no fast RAM.)