Shadow acne on DirectionalLight3D when shadow blur set to 0

[SoloRobo]

Tested versions

• Reproducable in v4.4.beta4.official [93d2706] and v4.4.rc1.official [8ed125b]

System information

Godot v4.4.rc1 - Fedora Linux 41 (Workstation Edition) on Wayland - Wayland display driver, Single-window, 1 monitor - Vulkan (Forward+) - integrated AMD Radeon 780M (RADV GFX1103_R1) - AMD Ryzen 7 7840U w/ Radeon 780M Graphics (16 threads)

Issue description

On a DirectionalLight3D setting shadow blur to 0 causes visible dithering

Steps to reproduce

Create a simple scene as attached below.

Camera3d
CSGBox
DirectionalLight3D

Enable shadows on DirectionalLight3D, reduce shadow blur to 0, rotate CSGBox until dithering can be seen on Camera preview.

Minimal reproduction project (MRP)

mrp-44.zip

[AThousandShips]

Can confirm, also occurs on 4.3.stable

[Calinou]

This is not dithering but shadow acne due to shadow bias being too low. Shadow bias is automatically adjusted according to shadow blur to give a good result out of the box, but we should clamp the lower bound for automatic adjustment to something like 0.05. SpotLight3D has the same issue, but not OmniLight3D.

Setting shadow blur to 0.01 mostly avoids the issue, and setting it to 0.05 avoids it completely.