When post-processing enabled on camera, Unity ignores custom projection matrix?

I'm using the following code to set oblique near clipping plane on a camera (not the main camera):

camera.ResetProjectionMatrix();
camera.projectionMatrix = camera.CalculateObliqueMatrix(clipPlaneCameraSpace);

This works fine as long as no post processing is applied to the camera. However, when I add a PostProcessLayer to the camera, suddenly objects in front of my oblique near plane are rendered. Using the Frame Debugger, I can see that in this case the projection matrix (well, unityMatrix_VP) that is being sent to the shaders is that is as if I never set my custom projectionMatrix. That is, Unity is calling ResetProjectionMatrix internally. In fact, it seems to be calling it in camera.Render() (my camera is disabled and I render it manually). Known issue? Intentional?

Unity 2018.3.6f1

[edit: it seems to be happening sometime between OnPreCull and OnPreRender]

 

Aha, looks like I answered my own question... I didn't realize the source code for the post processing stuff was available:

Code (CSharp):

1.  
2. void OnPreCull()
3.         {
4.             // Unused in scriptable render pipelines
5.             if (RuntimeUtilities.scriptableRenderPipelineActive)
6.                 return;
7.  
8.             if (m_Camera == null || m_CurrentContext == null)
9.                 InitLegacy();
10.  
11.             // Resets the projection matrix from previous frame in case TAA was enabled.
12.             // We also need to force reset the non-jittered projection matrix here as it's not done
13.             // when ResetProjectionMatrix() is called and will break transparent rendering if TAA
14.             // is switched off and the FOV or any other camera property changes.
15. #if UNITY_2018_2_OR_NEWER
16.             if (!m_Camera.usePhysicalProperties)
17. #endif
18.                 m_Camera.ResetProjectionMatrix();
19.             m_Camera.nonJitteredProjectionMatrix = m_Camera.projectionMatrix;

 

Answering, because I ended up here using Google to solve my problem:
setup an oblique frustum (custom cam matrix) + using Post Process Stack V2

Solution: use the (new) physical camera - "Lens shift" (see Camera inspector) instead of a custom matrix in code..

 

i don't care who my hero was. but from this moment on, you are my hero. mua~

 

so how to fix this issue?
I'm using the following code to mirror screen (main camera):
void OnPreCull()
{
camera.ResetWorldToCameraMatrix();
camera.ResetProjectionMatrix();
Vector3 scale = new Vector3(flipHorizontal ? -1 : 1, 1, 1);
camera.projectionMatrix = camera.projectionMatrix * Matrix4x4.Scale(scale);
}
This works fine as long as no post processing is applied to the camera too.

 

Did you try the advice in the posts above you?

 

This seems to be fixed in the latest 3.2.2 post-processing package.