dust effect displays green triangular shapes issue

[joelolopez]

im playing this Brave story new traveler game using vulkan backend when i notice that the dust effects displays wrong images (green triangular shapes)

OS android 7.0 , Octa-core, 2360 MHz, ARM Cortex-A53, 64-bit, 16 nm , Mali-T830 MP2 , 4 GB RAM

[Leopard20]

Did you try changing the options? Like disabling the stencil test?

[joelolopez]

yup i tested it and still displaying the same image

[unknownbrackets]

If I remember correctly, this is rendered using flat shading. It might be impacted in some way by render-to-CLUT, though.

Stencil test won't help. Does this happen with both OpenGL and Vulkan?

If it happens with OpenGL, did it work in v1.5.4? Is it broken in the latest git build?

-[Unknown]

[joelolopez]

this only happen with vulkan, im using v. 1.5.4 from the ppsspp. Org

[unknownbrackets]

The problem here is that flat shading on Vulkan picks a different vertex for the triangle strips:

GLES: i + 2 (default GL_LAST_VERTEX_CONVENTION)
Everyone else: i

Brave Story uses the wrong colors if you follow the wrong convention. So my guess (without testing) is that the PSP used the OpenGL default convention.

In theory, we could pass the offset vertex into the vertex shader and pull the correct color, I guess. Not sure what the most efficient solution is here.

-[Unknown]

[hrydgard]

Oh, that makes sense. Ouch.

Well, depending on the primitive type there can be somewhat easier solutions, like for example if it's TRIANGLE_LIST then we can "just" copy the color from the third to the first vertex in each triangle... Unfortunately that does mean reading back from decoded vertices, which can be expensive on some platforms as we write verts directly to uncached VRAM (remember that slowness bug..). Or we need to make the vertex decoder a lot smarter. Or simply always software-transform FLAT triangles, then we can easily poke around in the vertex data to fix it up and will handle all the types easily, at the cost of losing hardware transform for these triangles. This will also let us simulate FLAT on platforms that don't support it...

[unknownbrackets]

In this specific case it's a triangle strip. Really, the first two colors are garbage, it's correct after that.

Always software transforming flat really doesn't sound too terrible to me. They're pretty infrequent, although I could imagine finding some crazy game that uses flat for complicated many-vertex shape rendering or something...

-[Unknown]

[hrydgard]

Yeah, I think I like that idea best too. Bit of a minor perf regression risk as you say, but probably not common.