Vulkan: Decal 'Normal Fade' feature appears incorrect on scaled meshes

[RandomCatDude]

Godot version

v4.0.dev.20211210.official [21b8199]

System information

Windows 7, Vulkan Clustered, RX 570 (21.5.1)

Issue description

When a mesh is scaled either with its own Scale transform or the Scale of any parent nodes, the Normal Fade feature of Decals applied to it will misbehave.

The Decal's normal fading appears inconsistent and buggy.
When the mesh node's scale is even (all fields have the same value), there is a visible pattern: When the mesh is scaled up, the Decal appears too opaque. And vice versa for meshes that are scaled down, the Decal appears too transparent. Otherwise, there isn't any pattern I can discern.

The Decal's place in the scene tree does not matter, including its own scale.

Three unshaded SphereMeshes, identical aside from their Scale. All of these Decals have their Normal Fade set to 0.5. Their Vertical Fade was also lowered to 0, to not interfere with the test.

I expect that all the Decals appear just like the middle one, fading the exact same amount based on the normal.

Steps to reproduce

1. Create a MeshInstance3D, preferably with a SphereMesh Mesh
2. Create a Decal, give it any Albedo texture. Set the Normal Fade to a value above 0. I also recommend setting both Vertical Fade properties to 0, so that they do not cause confusion.
3. Move the Decal so that it's applied to the mesh. Make it a child of the mesh node so that it's in the same position on the mesh, and covers the same area on it.
4. Scale the mesh node. Observe the decal's transparency changing. Drag the Decal's Normal Fade property around, to clearly see how it's been affected.

Minimal reproduction project

DecalFadeTest.zip

[Calinou]

I can confirm this on commit 8f6cba3.

MRP with some changes: DecalFadeTest_1.zip

I've placed all spheres on the same height, adjusted decals' extents to be very tall (100 units each) and adjusted decal sizes to look visually identical:

If you reduce decals' Y extents to be short (0.5 local units each), they will end at different distances since they're being scaled by their transform:

If you reduce decals' Y extents to be short (0.5 global units each), they will end at the same distance, but you can clearly see Normal Fade acting differently:

[RandomCatDude]

Thanks for improving the minimal reproduction project, wasn't sure mine was very good. That illustrates the problem much better.

[Calinou]

The original issue is still reproducible as of 4.0.rc 493e932. Screenshot taken with decal extents' Y increased to 100 to emphasize the issue:

Updated MRP: DecalFadeTest_2.zip