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scifista42

Sprite clipping near slopes?

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This screenshot was taken in Recurring Nightmare - Demo Beta in Zandronum, but the same issue appears in ZDoom too. Notice how the Chainsaw clips wrongly:



I have edited the map (MAP31) to get rid of the slope (simply removing the "Plane - Align" linedef action), and now it clips correctly, both in ZDoom and Zandronum.



How exactly does stuff like this happen? Shouldn't the renderer be proof of such basic clipping errors?

For that matter, how does ZDoom's slope rendering algorithm work?

Doom engine's rendering algorithms are well explained on the doom wiki, but there's no info about slope rendering on zdoom wiki, and just a very brief info on the doom wiki.

I understand that it's probably more than elaborate, but could somebody knowledgeable be able to explain the principle, similarly like Doom engine's rendering process is explained here? (or anyhow else)

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Spite clipping in the Doom engine is a mess. Slopes only make it worse.
I have seen far, far worse glitches with slope-less maps. This one's relatively harmless by comparison.

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I trust you. I agree that this glitch is comparatively not so terrible. I'm still curious about the algorithm.

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Funny thing is GZDoom seems to have an opposite effect. It clips it so "well" that part of it's disappeared into the wall next to it. But then, that's probably to be expected when you have it in a small corner that it logically shouldn't be able to fit in. XD


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OpenGL doesn't concern me, it's an advanced non-Doom-related tech that always clips everything 'technically properly', even if it means that sprites will be sunken into floors, walls and ceilings. Software rendering, with its hacky quirks in favor of fast performance, is what I'm enthusiastic about.

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Not knowing how ZDoom's slopes work off hand, I think I can make somewhat of an educated guess as to what's going on. First let's look at the area that is getting clipped:

Sprites are drawn after geometry, so they rely on drawn segs to clip the sprites. It looks like here that they are doing just that, as expected. However, the front seg (the one facing relative the camera) is getting missed. So it seems that something in the slope behaviour can cause a seg to be missed/skipped. Don't really know why at this stage.
Maybe Randi could provide a clearer answer if someone presented this to him.

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Uhhhh.... TBQH I can't see what's wrong with either of the two first screenshots.

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