-Closure- tricks

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big alec
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-Closure- tricks

Postby big alec » Sat Mar 26, 2016 3:53 am

Ok, I meant to ask this several weeks ago, after finally seeing a video cap of SYNC's brilliant -Closure- demo, and I'm seriously surprised that nobody has asked this before, since I don't think it is obvious by any means:

How the hex do they do a fullscreen x-dister on a regular ST? It should be impossible, so it's sensible to suspect a trick. One idea that came to my mind was that there is no actual bitmap distortion going on, but instead the effect is based on color palette modifications using 12 cycle move instructions. One clue to that is that is the observation that all the disting fullscreen graphics are zoomed, i.e. really big, with about 41 "pixels" per scan line.

But then the question arises: How can the timing of the color changes be delayed with 1 pixel precision (at least that's what it looks like from inspecting a frame from the youtube capture)? Is there some new Shifter/GLUE trick that can distorts video timing in a way that the location of a palette switch is shifted? Is it related to Alien/ST-Cnx' 4-bit hardscroll trick?

Either way, my hat is off to you, Troed! Congratulations from an amazed old school scener. Achievement unlocked! :)

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npomarede
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Re: -Closure- tricks

Postby npomarede » Sat Mar 26, 2016 1:27 pm

Hi
as you suspect, the 1 pixel precision is obtained by having some kind of bitmap mask. So, you can get 4 pixels precision with the usual NOP as in most color plasma, but then you add an additional layer of bitmap, so instead of having only color 0 only as the background, you have a background made with stripes of different colors.
The tricky part is then to change each color 0-15 at the correct position to get the amazing visual result.

For a slightly smaller demo using a similar effect, you can have a look at the dragonnels demo menu (but this was also used in several demos too, in one of the No Cooper plasma for example). Closure certainly gives the best result using this technique :)



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troed
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Re: -Closure- tricks

Postby troed » Sat Mar 26, 2016 5:16 pm

(Emulator writers should be disqualified from spilling the beans ;))

It should be impossible, so it's sensible to suspect a trick


Thanks Big Alec! It was our goal to leave people confused at STNICCC, the old school way :D Unfortunately whether people were confounded or not largely depended on how recent their own involvement in the scene was :) I remember giving Omega and Electra a preview in our room and that only resulted in a long discussion having to juggle memories on how much four bpl bitmap data you really could movem in a VBL.

As to your 12 cycle hypothesis - you're correct in that 41 (or maybe 42, depends on wakestate) virtual pixels are visible per line. So they cannot be 12 pixels wide. Also, in the fullscreen XY dist screen there would be no time to do a per-column lookup for the Y-dist.

Closure does however use the very latest sync-trick knowledge to be able to fit all the code needed into the fullscreens. Even with trickery :P

/Troed


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