Enchanted Land parallax scroll

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Foxie
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Enchanted Land parallax scroll

Postby Foxie » Fri Jan 12, 2018 5:38 pm

Apologies if this has been brought up already. Couldn't find any hard details with search.

How on earth did they do the parallax scrolling on the intro screen of Enchanted Land? Are they just brute-forcing it with a software blitter?

I can't see how you can do parallax scrolling with sync scroll without resorting to tricks like a single-plane background or palette cycling. Enchanted Land does neither.

Why don't they use the same technique in the game? I can only guess it must be a very dirty trick.

Also it works in all wakestates. Isn't that unusual for 1990? Are they just using a very conservative sync scroll that works every time? I don't think they knew how to detect all four wakestates in 1990?

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Re: Enchanted Land parallax scroll

Postby troed » Fri Jan 12, 2018 5:50 pm

Foxie wrote:Also it works in all wakestates. Isn't that unusual for 1990? Are they just using a very conservative sync scroll that works every time? I don't think they knew how to detect all four wakestates in 1990?


It's _very_ conservative. It needs 20 scanlines, and uses only right overscan, right+left overscan and normal line (i.e, 3 combinations). They left out left overscan completely, likely because they couldn't stabilize the Shifter for the following non-overscan screen.

Enchanted Lands was thus more conservative than SoWatt which used 4 different lines (I don't think I've verified Cuddly, but it likely also uses 4).

A modern sync scroll, which needs to know about wakestates, has 12 possible line lengths and only needs 4 scanlines to reach all scroll positions.

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Re: Enchanted Land parallax scroll

Postby Foxie » Fri Jan 12, 2018 9:23 pm

troed wrote:It's _very_ conservative. It needs 20 scanlines, and uses only right overscan, right+left overscan and normal line (i.e, 3 combinations). They left out left overscan completely, likely because they couldn't stabilize the Shifter for the following non-overscan screen.


Interesting, thanks for the info. I don't think I understand entirely about stabilising the shifter. Of course I've seen stabilisation code in overscans before. I don't really know how that would interact with a non-overscanned line following.


troed wrote:A modern sync scroll, which needs to know about wakestates, has 12 possible line lengths and only needs 4 scanlines to reach all scroll positions.


I saw the slides you made describing that technique. Very impressive. ^.^

Do shifter wakestates come into this at all? I don't know much about wakestates really. From what I understand, shifter wakestate not GLUE-MMU is responsible for the Spectrum 512 dots? Can it ever affect scroll/overscan too?

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Re: Enchanted Land parallax scroll

Postby alien » Sat Jan 13, 2018 4:21 am

An amusing fact for Troed: If I recall correctly, they actually had adaptive code to vary the timing of the the resolution/frequency switches around, just in case there was an Atari out there that had different timings. They read the video counter to see whether the desired line lengths had been obtained. Of course that wouldn't help with the stabilizers. I think at that time, it was still all "magic" because we didn't understand what was happening inside the video chips.
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Re: Enchanted Land parallax scroll

Postby mlynn1974 » Sun Jan 14, 2018 1:55 pm

I never quite understood sync scroller. Paul from DigiTech and Neil from Flair tried to explain it to me, but I didn't quite understand it fully at the time (1991). There is also the fairly well documented sync scroller in the source code for the main menu of the Mindbomb demo. That's good.

I would expect the sync scroller in Enchanted Lands to be conservative and "safe" because it was a commercial product and they could only test it on a limited number of mmu\shifter combinations at the time.

In Enchanted Lands yes sync scrolling is amazing but they must still be blitting a heck of a lot for the overlaid message in the middle, the sprites and some of the layers.

One thing I don't get is that some people say that Grodan and the Kvack Kvack in the So Watt demo also used a sync scroller but when I ripped the graphics from it using Ultimate Ripper the screen line lengths weren't variable and the graphics could be ripped in normal video mode without stabilizers.

Sync scrolling is an amazing technique but even as a programmer I found it convoluted and not well documented.
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Re: Enchanted Land parallax scroll

Postby troed » Sun Jan 14, 2018 3:24 pm

mlynn1974 wrote:I never quite understood sync scroller.


Hopefully (?) my little chapter on the subject in the Breakin' the Borders book helps a few more to understand how it works, even though I was pressed for space :)

Mostly what was important for Enchanted Lands was that the following scanlines were non fullscreen. That means any issue with left over words in the Shifter ("non stabilized") would affect the whole screen. Most sync scrollers are otherwise fullscreens where every line has its own stabilizer.

(I know this was a HUGE pain for me when I did the LoSTE demo, which additionally used a very advanced 1+3 line sync scroller)

alien wrote:An amusing fact for Troed: If I recall correctly, they actually had adaptive code to vary the timing of the the resolution/frequency switches around, just in case there was an Atari out there that had different timings. They read the video counter to see whether the desired line lengths had been obtained.


Thanks, yes, that piece of code has actually been studied in detail since it ... doesn't work :D By mistake when hunting for the left border position they trigger a "full" scanline by skipping hsync and since they obviously realized that wasn't the correct position they then add an empirically tested number of cycles ;)

From my studies of the "tv-snow" screen in SNYD1 I'm guessing Nick had a computer which mostly booted in WS2 (that's why the disting logo is only centered in that wakestate, it's off center in the others). WS2 is one nop off compared to the others for the left border so he probably assumed that position couldn't be relied upon always being the same between computers.

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Re: Enchanted Land parallax scroll

Postby Foxie » Mon Jan 22, 2018 12:08 am

I suppose that nobody knows how the parallax scroll on the title screen was achieved then?

I'd be fascinated to know how they did it, because it would seem to be impossible. Surely they don't have enough cycles to completely re-draw the foreground layer while sync scrolling the background? The foreground sometimes covers the entire screen, so that's a whole lot of copying.

Some games like James Pond II on the Amiga draw their parallax background on only a single plane to make it fast enough. But Enchanted Land doesn't do this - it looks like 16 colours in both foreground and background.


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