joska wrote:I had a look at the intro now, and from what I can see only about 1/3 of the screen is parallax scolling. And the background looks like 2 bitplanes only. I believe they're syncscrolling the screen and redrawing the background. I'm not very familiar with syncscrolling - can you sync-scroll only parts of the screen?
I don't think it's possible to scroll only part of the screen, but I'm no expert on sync scrolling. AFAIK there's no way to reload the video address counter mid-frame, which is a shame because that precludes lots of effects. It would be wonderful if someone figured out a way of doing that. I think that latch is wired directly to the vsync signal, so you'd need some way of generating a false vsync mid-frame. I wonder if the 68k's reset instruction could be abused to force an MMU/glue/shifter reset mid-screen?
I've no idea how full-screen distorters work if they can't change the scrolling on each scanline. I've heard they use lots of different screens but I'm struggling to understand how that helps.
I suppose the Enchanted Land background could be two planes. The foreground looks like more though - all four? That means they can't "own" the two background planes like James Pond II does with its background plane. So they must be cleverly masking it when redrawing to avoid overwriting the foreground. I'm amazed that's even possible at 50fps!
Also there's some per-line scrolling on the floor. It might be possible to scroll 0, 4, 8, or 12 pixels on each line by switching to medium res? I'm not entirely sure. The "big" scrolling AFAIK can only be done at the top of the screen since it takes many scanlines to advance the video address counter to the desired position. I think Troed's method uses only four scanlines, but that's still four scanlines too many to do mid-screen (unless you cleverly arranged it with a break in the screen I suppose).
Come to think of it, perhaps you could put a sync-scroll mid-screen, and then cover up the four lines of artifacts by just blitting those lines.
The two weirdest things about Enchanted Land are that the parallax scroll doesn't happen in the game itself, and they didn't go fullscreen. Actually I can forgive them for not going full screen horizontally because of the need to write a blitter routine that's synced. But they could have easily gone vertical which would have looked nice.