Spec allows resolveTarget to be 3d but doesn't allow selecting a depth slice

[kainino0x]

In #4254 we allowed view to be 3d and added depthSlice to select which slice of view to render to.

However the change to allow 3d views in the "renderable texture views" algorithm also applies to resolveTarget. So the spec now says resolveTarget can be 3d, but there is no way to select a depth slice of it.

3d textures cannot be multisampled, but there is one scenario in which this matters: view is 2d multisampled, and resolveTarget is 3d. I do not know if this is actually possible, but the spec seems to allow it.

There are several options here depending on what the backends can support:

• Disallow resolveTarget from being 3d
• Similarly, require view and resolveTarget to have the same dimensionality, then this scenario doesn't work
  • (but it could be an issue later if 3d textures become multisample-able)
• Add a resolveTargetDepthSlice

cc @haoxli

[magcius]

I imagine resolving directly into a 3D texture is something that's relatively untested on most devices, especially on mobile devices and tilers where I would expect resolveTo matters the most. Making the user do their own copy (resolve 2DMS -> 2D, and then use copyTextureToTexture to copy 2D -> 3D slice) seems fine.

[Kangz]

GPU Web WG 2024-05-08 Pacific-time

• KN: (summary)
• HL: Not sure if any motivation for a 2d multisampled render target with a 3d resolve target.
• …
• KG: Jasper and others’ instinct is that this may not work well. There are most likely devices we want to target that would have bugs here. Would want to be cautious about adding this feature, test carefully. So disallow for now.
• (No objections)
• Consensus: Disallow 3d textures in resolveTarget.
• HL: Once we allow 2d array textures in view, want to allow it in resolveTarget?
• KG: I expect that 2d arrays would work fine here, because that’s what VR would do normally. It’s just that 3d is unlikely to work well enough on already-deployed devices.
• KN: Require resolveTarget to be 2d for now (as the spec bugfix) and then allow 2d-array when allowed as part of Allow 2d-array texture view as renderable texture view #4607?
• KG: As long as end state allows 2d-array in both places.
• MM: Who will do the analysis to find out if 3d works?
  • KN: Honestly don’t think it’s important enough anyone needs to investigate it yet.
  • KG: “Investigations welcome” though.