Contact Shadow Flickering

[Nolram12345]

Hello!
We've noticed during that development that there is a frequent issue with contact shadow flickering on sloped surfaces.

Here's the issue visualised in a test scene in the GLTF viewer:
https://github.com/google/filament/assets/37701550/ad080d02-4ffa-44e2-8caa-d4811a64aff8

1. Is this a known issue?
2. Is there a workaround?

To me personally this seems like something that could be addressed with a slope bias for contact shadows.

[Nolram12345]

Update: Further experimentation also reveals that the issue gets significantly worse with higher sample counts for the contact shadows.

gltf_viewer_D9NNnXsTM8.mp4

[Nolram12345]

Here's a GLB and the settings used to reproduce the issue.
contact_shadow_issue.zip

Visual output:
https://github.com/google/filament/assets/37701550/1e126af7-fab1-44ed-abdf-54c93debb1fe

[romainguy]

Thanks! I don't get any flickering (macOS, OpenGL, Vulkan, and Metal) :/

[Nolram12345]

Then it may be platform related. Might need someone else on Windows & Linux to try it out and attempt to reproduce.
Might be a precision issue, so I do think some kind of slope bias/normal bias for contact shadows could help address that too - if that actually is the issue (but I do think it is seeing as it mainly happens at shallow shadow angles on sloped surfaces).

[suzp1984]

which version are you works on?
The maximum allowed shdow map size is 1024 at current master branch.

filament/libs/viewer/src/ViewerGui.cpp

Lines 908 to 910 in 2763931

	ImGui::Checkbox("Enable shadows", &light.enableShadows);
	int mapSize = light.shadowOptions.mapSize;
	ImGui::SliderInt("Shadow map size", &mapSize, 32, 1024);

Here is my Imgui config:

I check the git log the 4096 Shadow map size config is long time ago, at least one years ago.

[Nolram12345]

I'm using a customised version of the viewer to allow me to tweak some settings that aren't exposed by default such as bias.
However, in regards to resolution, you can just enter a custom number into the shadow map size field, even on the current official version.

I've not changed any rendering code, just the imgui layer to allow me to tweak more settings to see what Filament is capable of and how it performs.

Besides that, this is using the current master.

[Nolram12345]

I might escalate this to an issue, as this issue is quite persistent and common when using contact shadows, making them hard to use in production.

[Nolram12345]

I discovered that this only happens when TAA is on.

[Nolram12345]

Reproduced under Linux & OpenGL using latest release (1.51.3).
Could this perhaps be some driver inconsistency? I'm on NVIDIA drivers 552.12, the latest as of time of writing.
CC @romainguy

msrdc_YX7Pr9669Y.mp4