Vulkan: BaseMaterial3D Proximity Fade exhibits a visible line regardless of distance if opaque surfaces are in front

[Calinou]

Godot version

4.0.alpha7

System information

Fedora 34, GeForce GTX 1080 (NVIDIA 510.60.02)

Issue description

BaseMaterial3D Proximity Fade exhibits a visible line regardless of distance if opaque surfaces are in front.

This looks similar to #54978, but happens even if neither VoxelGI nor SDFGI are enabled.

Proximity Fade disabled

Proximity Fade enabled

This also occurs without any lights or environment (although the difference is more subtle). You can notice it by Ctrl + Tabbing between those images opened in new tabs:

Proximity Fade disabled

Proximity Fade enabled

In comparison, there is no such visible line in 3.5.beta4's GLES3 renderer:

Steps to reproduce

• Add a small mesh without any material override.
• Add a large mesh with a material that has Proximity Fade enabled (any distance will do).
• Place the large mesh behind the small mesh (relative to the camera).

Minimal reproduction project

• master: test_proximity_fade_line_master.zip
• 3.x: test_proximity_fade_line_3.x.zip

[somnathsarkar]

I took a look at the project in a frame debugger, and it seems like the issue lies in the computation of screen_uv.

Godot 3.x:
vec2 screen_uv = gl_FragCoord.xy * screen_pixel_size;

Godot 4.0:
vec2 screen_uv = (gl_FragCoord.xy * scene_data.screen_pixel_size) + (scene_data.screen_pixel_size * 0.5);

I don't believe we need to manually correct for the center of the pixel since gl_FragCoord returns the center by default anyway: https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/gl_FragCoord.xhtml

Using an incorrect value of screen_uv causes the wrong texel of the depth buffer to be sampled, which is most noticeable at the boundary of another object. When I change this computation of screen_uv back to the Godot 3.x version, the black border disappears:

This also appears to fix the related issue: #54978

[Calinou]

@somnathsarkar Nice catch 🙂

I wonder the proposed fix also resolves #56792.

[somnathsarkar]

The blur in #56792 is eliminated after applying the fix

Old screen_uv:

New screen_uv:

[Calinou]

cc @clayjohn