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2018-09-22 00:30

Compyx

Registered: Jan 2005
Posts: 631

Unexpected sprite behaviour

While trying to hack together a linecruncher I ran into a bit of an odd situation: sprites appear where they shouldn't.

My code currently doesn't actually line crunch, since I haven't got the timing correct yet, I implemented a stable raster and basically copied the line crunch code from an old demo part I once wrote.

The weird thing is that although I only poke #$32 at the sprite Y-positions, I get those sprites at raster position 0 as well. Y-stretched since I my line crunch routine uses a $d017 sideborder stretcher to allow for something a little more interesting than just a blank space.

I figured this was a bug in VICE, but it also happens on the real deal, and even x64 emulates this correctly. So there must be some logic to this.

Anyway, here's a link to the code: https://www.dropbox.com/s/ftu9jty6r32xj9a/linecrunch-fuckup.tar..
Just run linecrunch.prg or run make if you have 64tass installed.

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2018-09-22 23:03

Golara
Account closed

Registered: Jan 2018
Posts: 212

I made the same thread few months ago. Funny that people are still getting cought by this. And yes, they will wrap around from old to new frame if you stretch it. I tried sprite crunching and that usually makes the sprites very tall. All I had to do was to crunch it every 4-5 files to make it full screen. If your sprite data is filled with the same byte of graphics ($80,$80,$80 * 21) it doesn't even matter where you crunch it and it's perfect for straight lines (pseudo vertical rasterbars)

2018-09-24 22:05

TheRyk

Registered: Mar 2009
Posts: 2076

There's tons of old demos with open top/bottom border and bottom sprite stuff happening unwanted in top border, too. Back in the CRT days we didn't notice/see.

When I first opened top/bottom I faced the same but simply solved the problem by setting new Y-pos just after bottom border action but before start of new frame. However, it took some time till I discovered full/debugging borders as you can see here hehehe Neotunes 2 Light (standalone) bottom stuff not happening in top but vice versa by switching Y-pos too early (apart from not really so stable rasters in side borders as I thought)

Since I've got a good real CRT (30 kg Sony Trinitron which can do full border in "overscan" mode and even NTSC), things are easier

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