Rendering glitch in Lego Harry Potter Y1-4

[hrydgard]

Reported by perhaps on Discord.

Game: Lego Harry Potter Y1-4

Device: Galaxy A21, Vulkan

Tested on 1.14.4 and latest build.

Nearby save:

ULUS10500DATA00.zip

Occurs in the Chamber of Secrets (2nd door), quest Collecting Ingredients

[unknownbrackets]

A GE frame dump would be more helpful. Might be some kind of Z fighting...

-[Unknown]

[mayhaps3500]

First time creating a dump. Let me know if I've done something wrong.

ULUS10500_0003.ppdmp.zip

[unknownbrackets]

Even in a PSP playback of that framedump, something funny is happening:

The scene initially is drawn like this, which looks nice and all (2382/4468):

But it then darkens everything one by one (4425/4468) and that's when we've gotten the artifact:

The initial darkening of that specific area occurs at 3414/4468. This actually looks more or less right to me:

But then it touches the same area again at 3437/4468 and 3438/4468, which makes it nearly black and gives it that weird pattern. To confirm, skipping those two draws makes it look as one would expect:

The first darkening is a simple triangle strip, made up of two triangles:	X	Y	Z
290.119141	19.013916	2996.390625
279.134521	61.373779	2979.410156
243.680420	24.455322	3011.021484

X	Y	Z
243.680420	24.455322	3011.021484
279.134521	61.373779	2979.410156
231.450928	65.311646	2994.353516

The second two draws are one triangle each with:	X	Y	Z
231.450928	65.311646	2994.353516
290.119141	19.013916	2996.390625
279.134521	61.373779	2979.410156

X	Y	Z
243.680420	24.455322	3011.021484
290.119141	19.013916	2996.390625
231.450928	65.311646	2994.353516

While these are technically the same points, the triangle is drawn in a different order (first is TR BR TL, TL BR BL, second is BL TR BR, TL TR BL.) This is done as well with a >= depth test, and I think the depth ends up being slightly different because of the order (actually doesn't matter because the texture would make it ugly anyway just on == depth.) Anyway, the texture is different and I think this was intended to be the other side. Both are the same clockwise order, so there's no face culling.

And just to confirm, I tried quickly stripping these two triangles out and running on a PSP, which does look good:

It might be there's some CPU emulation bug causing the game to do these extra triangles, but to me this looks like a game bug.

-[Unknown]

[hrydgard]

Interesting. If it's indeed a game bug like tha, but the Z fighting glitches just look a bit different than on the real PSP, yeah, there's probably not much worth doing here...

[hrydgard]

Somewhat oddly, neither pixel nor vertex depth rounding does anything to improve the look of it on PowerVR, and it does look weirdly worse there. Still, I don't see that we have any good options here, and given that it also does appear to be a game bug, I'm going to close this.