Occlusion Culling on OpenXR is unstable (meshes flickering) in Godot4.4-stable

[jamie-pate]

Tested versions

• Reproducible in Godot v4.4-stable, Godot v4.4.1-rc1

  • Meta Quest 3
  • X11+SteamVR/OpenXR Vulkan (mobile and forward+)
    • OpenXR: Running on OpenXR runtime: SteamVR/OpenXR 2.9.6
    • OpenXR: XrGraphicsRequirementsVulkan2KHR:
      • minApiVersionSupported: 1.0.0
      • maxApiVersionSupported: 1.2.0

• Not Reproducible in Godot v4.3-stable

System information

Godot v4.4.stable - Ubuntu 22.04.5 LTS 22.04 on X11 - X11 display driver, Multi-window, 1 monitor - Vulkan (Mobile) - dedicated NVIDIA GeForce RTX 3050 Laptop GPU - 12th Gen Intel(R) Core(TM) i7-12700H (14 threads)

Issue description

bf99a2c79bb434c96e9287fe94412687.mp4

When Occlusion Culling is enabled in Godot 4.4-stable some meshes are mysteriously culled. Only Reproducible when using OpenXR and root.use_xr == true

Steps to reproduce

• Extract minimal reproduction in OpenXR
• install Godot Openxr vendors addon in 4_4/addons and 4_3/addons
• Run 4_4/project.godot or 4_3/project.godot

Minimal reproduction project (MRP)

xr-ocull-broken.zip

[jamie-pate]

Also noticed that occlusion culling only runs on one of the two eyes when using stereoscopic rendering.. probably fine?

bisect:
v4.4.dev3.official [f4af820] GOOD
v4.4.dev4.official [36e6207] GOOD
suspects:

• Fix Occlusion Culling Buffer occasionally getting corrupted (GH-98758).
  0f0e240 BAD
  0f0e240^ GOOD

v4.4.dev5.official [9e60984] BAD
v4.4.dev7.official [46c8f8c] BAD

[Rudolph-B]

I'm struggling to set up the project (first time using VR). Is it required to have hardware for the stereoscopic view? I'd appreciate any help.

Additionally does disabling jitter (Rendering->Occlusion Culling->Jitter Projection) have any affect?

[dsnopek]

Hm. I wonder if this relates to how the projection from OpenXR can have an "asymmetric fov"? Basically, the center of the projection may not be the center of the viewport.

The comment in Projection::get_viewport_half_extents() seems to indicate that this could be a problem:

Vector2 Projection::get_viewport_half_extents() const {
	// NOTE: This assumes a symmetrical frustum, i.e. that :
	// - the matrix is a projection across z-axis (i.e. is invertible and columns[0][1], [0][3], [1][0] and [1][3] == 0)
	// - the projection plane is rectangular (i.e. columns[0][2] and [1][2] == 0 if columns[2][3] != 0)
	// - there is no offset / skew (i.e. columns[2][0] == columns[2][1] == 0)
	real_t w = -get_z_near() * columns[2][3] + columns[3][3];
	return Vector2(w / columns[0][0], w / columns[1][1]);
}

[jamie-pate]

Hm. I wonder if this relates to how the projection from OpenXR can have an "asymmetric fov"? Basically, the center of the projection may not be the center of the viewport.

Pretty sure this is it, but I don't understand the projection matrix for OpenXR well enough yet to figure out how to fix the current implementation

oh, the code is using the camera's transform and guessing the coordinates of the near plane rectangle based on that..

does the perspective transform add some translation for OpenXR for left vs right eye on top?

Normal

CAM_PROJ [X: (0.733064, 0.0, 0.0, 0.0), Y: (0.0, 1.303225, 0.0, 0.0), Z: (0.0, 0.0, -1.000025, -1.0), W: (0.0, 0.0, -0.100001, 0.0)]
cam center tfm (-5.982937, 2.307015, 0.085401)
top corner world (-6.051144, 2.345381, 0.035401)
right corner world (-5.91473, 2.268649, 0.035401)

projection.get_endpoints(p_cam_transform, endpoints);
proj: ep 4 (top corner world): (-6.051144, 2.345381, 0.035401)
proj: ep 7 (right corner world): (-5.91473, 2.268649, 0.035401)
proj: tcw - ep4 (0.0, 0.0, 0.0)
proj: rcw - ep7 (0.0, 0.0, 0.0)

OpenXR

CAM_PROJ [X: (0.726552, 0.0, 0.0, 0.0), Y: (0.0, 0.835475, 0.0, 0.0), Z: (0.000007, -0.193193, -1.000012, -1.0), W: (0.0, 0.0, -0.046693, 0.0)]
cam center tfm (-5.982937, 2.307015, 0.085401)
top corner world (-6.015212, 2.335723, 0.063205)
right corner world (-5.950646, 2.280228, 0.060944)

projection.get_endpoints(p_cam_transform, endpoints);
proj: ep 4 (top corner world): (-6.003316, 2.337547, 0.106775)
proj: ep 7 (right corner world): (-5.939691, 2.282415, 0.105469)
proj: tcw - ep4 (-0.000015, 0.00533, 0.000275)
proj: rcw - ep7 (-0.000015, 0.00533, 0.000275)

Normal

OpenXR

[jamie-pate]

I'm struggling to set up the project (first time using VR). Is it required to have hardware for the stereoscopic view? I'd appreciate any help.

This issue only happens with OpenXR (e.g. it does not occur with the NativeMobileXR interface which is also stereoscopic and requires no setup)

See below on ideas to simulate hardware

Additionally does disabling jitter (Rendering->Occlusion Culling->Jitter Projection) have any affect?

Disabling Jitter doesn't change anything

For OpenXR you need to at least trick your system into thinking it has VR hardware :D

The Meta XR Simulator might be the best way?

On linux I have had success using ALVR and PhoneVR to create a fake HMD by connecting my android phone to my pc over WIFI or usb tethering

There is also with SteamVR's null_driver but I haven't been able to get that working