Vulkan Mobile: Decal Rendering Issues, Doubling Up?

[SeanH-CB]

Godot version

4.1.1.stable

System information

Windows 11 - Vulkan (Mobile) - NVidia GTX 1070 537.13

Issue description

When placing more decals equal to or around the max of 8 decals for a surface, issues begin to occur.
Setting a decal to be slightly transparent, depending on the viewing angle decals are rendered twice which means they are no longer blended with the surface correctly.

Also when moving the camera around, sometimes a decal will also move around along the surface.
This issues occurs on both StandardMaterial3D & ShaderMaterial.

Screenshot:

Decal.Issue.Example.Compressed.mp4

Steps to reproduce

• Make a New 3D Project
• Set rendering Mode to Mobile
• Add a MeshInstance3D
• Place around 8 decals on the surface
• Move the camera around until you see issues described/shown in video

Minimal reproduction project

Mobile Decal Issues.zip

[Calinou]

cc @BastiaanOlij

#80911 may have surfaced a bug that previously wasn't noticeable.

Place around 8 decals on the surface

Does this occur with only 7 decals? Vulkan Mobile is limited to 8 decals per surface, but there may be an issue preventing 8 decals on the same surface from rendering correctly.

[jcarlosrc]

I have noticed that decals move with the camera without achieving the 8 amount limit.

[HatAndTie]

It also seems to happen when the decal isn't in the cull-mask of the camera, while the mesh it is projected on is.

[clayjohn]

I'm looking into this now. The bug is more pervasive than I expected. It impacts lights, decals, and reflection probes and its an issue in both the Mobile and GL Compatibility renderers.

The root of the issue is that we pass IDs for all lights/decals that affect a mesh to the mesh before drawing. But we only update data for lights/decals that will be visible.

[coobenguy]

Still having this issue in the latest 4.2 beta

[Calinou]

@coobenguy Please open a new issue with a minimal reproduction project attached.