Copper color-changing restrictions?

[Photon]

Well, it's not a trick really. Just two different copper lists swapped at 50 Hz. You should be able to see the 'flicker' on an LCD screen, I guess the trick part is that it works on CRTs and gives a shade 'between' RGB value 5,10,11 and 5,10,12, for example. It's the same thing Atari ST demos used to get more than 512 colors, but much smoother and less noticable since Amiga had smaller steps between shades.

It doesn't have to be copper shades (gradients), I use the same 50Hz color-changing in the XMas 2007 demo to get 27971 possible colors, but in that demo the shading is calculated rather than hard-coded.

Allegedly, a coder from The Gang did it first, in part 3 of Gigademo 3, in this demo it's about picture palettes(?) but it's hard to tell if it's done right. I only say that cos it looks a bit like a HAM pic in interlace, and the colors are a bit washed out to tell for sure. (But the RGB lines below it uses the method ofc)

Then again, Andrew Braybrook and probably demo coders before him (?), did it on C64 (f.ex. the Alleykat game text screens), so again, it's only a way of making something look smoother it could just as well be done with a stippled bitplane to 'mix close hues'.

If the 27971 color effect starts at the top of the screen, you can set interlace to improve the effect further, on scandoublers/flickerfixers/capable LCD TVs/WinUAE you will then get what looks like a smooth gradient at twice the vertical resolution.

[Photon]

I also think some groups (Empire? Digitech?) used 'horizontal shaking' to get 4px resolution and twice the number of shades in some scroll early on. Ie. the copper code that changes scrollfont color register every 8px gets its start wait position modified by 4px every other frame.

[Rebel-CD32]

pmc, if you download that program CopColEd you can use it to test the effects of a flickering Copper gradient in real time. Although it does look interlaced (even with an Indivision flicker fixing everything else) the colour gradients become smoother. Lionheart and Agony use the flickery Copper gradient to give the illusion of more colours in the sky.

CopColEd saves out Assembly code so if you make your gradient/picture in it, you could export it and play around with it further.

[pmc]

@ Photon - I'd never noticed any flicker watching your Blitter Sweet demo so it never occurred to me that you might be switching copper lists. I suppose selection of the colours you flick between can either soften or pronounce the flicker so choosing the right colours gives the best result. Interesting stuff - nice one.

@ Rebel-CD32 - Cheers for the pointer to that utility, appreciated.

I think I'll avoid using it to save out code from it but that's just me - coding something to do that with a couple of copper lists shouldn't be too hard and I'd rather play with this stuff myself - it's more fun that way.

[Photon]

Actually, an editor to help coders make more palatable gradients isn't a bad thing I've sometimes used DPaint to make a gradient and just save out the palette to get the RGB values.