High Definition Render Pipeline (Custom) - Single Pass Stereo Mod

USE AT YOUR OWN RISK

This is a hack for instanced single pass stereo with custom views in HDRP for supporting stereoscopic displays and video projections that need user dependent perspective rendering.

In broad strokes, the approach is to add a custom XRPass to XRLayout during HDRenderPipeline.Render() and use OnEndCameraRendering to blit to a SBS texture. "Normally" a XRDisplaySubsystem from a XR Provider plugin is supposed to provide the XRPass.

The officially recommended solution is to write and compile your own OpenXR provider (NOT a XR Provider plugin as has been previosly stated). Yes, writing a plugin ... just to do the equivalent of the (now obsolete) camera.SetStereoViewMatrices() and camera.SetStereoProjectionMatrices() methods.

Based on Unity 6000.1.0f1 and HDRP 17.1.0 April 22, 2025. BEWARE that Unity is not necessarily updating their sematic versioning or change log with changes, for example 6000.1.7f1 also comes with HDRP 17.1.0, but the date is June 03 2025 and it DOES contain changes.

Known issues

• When building, UI targeting additional displays will be visually flipped, while the UI input coordinates will not. My hunch is that this has to do with the fact that we don't have a XRDisplaySubsystem defined.

Instructions

• 1. Modify Scriptable Render Pipeline Core.
  • Install High Definition Render Pipeline using the package manager. This will automatically install Scriptable Render Pipeline Core.
    • Close the Unity project and move com.unity.render-pipelines.core from /Library/PackageCache to /Packages (removing the @hashcode). Then open the project again and make these edits to the package:
      • /Runtime/XR/XRView.css
        • make the struct and the constructor public.
      • /Runtime/XR/XRPass.css
        • Make all fields of XRPassCreateInfo public.
        • Since we don't have a XRDisplaySubsystem:
          • Make the isHDRDisplayOutputActive property always return false. Setting it to true will give missing color space conversion errors in build.
          • Make the hdrDisplayOutputColorGamut property always return ColorGamut.sRGB.
          • Make the hdrDisplayOutputInformation always return new HDROutputUtils.HDRDisplayInformation( -1, 1000, 0, 160f ).
        • Make the AssignView() method public.
        • Make the AddView() method public.
      • /Runtime/XR/XRLayout.css
        • make the AddPass mathod public.
• 2. Install OffAxisCamera
  • Get it on (Asset Store). Alternatively moddify this hack and use your own view calculations.
• 3. Install MockHMD
  • Install XR Plugin Management using the package manager or in the Project Settings->XR window. Don't enable "Initialize XR on Startup".
  • Install Mock HMD Loader. If this is not present in your project, stereo instancing will be stripped from all shaders and the STEREO_INSTANCING_ON keyword will be undefined.
• 4. Add custom HDRP
  • Close the Unity project and drop this repo at some path. Then edit your package manifest.json file to point to that path. For example: "com.unity.render-pipelines.high-definition": "file:../../PackageRepos/com.unity.render-pipelines.high-definition.custom-single-pass-stereo",
  • Cross your fingers and re-open your project.
• 5. Setup your scene
  • Add a OffAxisCamera component to your main camera and give it a reference to a "Window Transform".
  • Add StereoHackEnabler component to an object in the scene.
  • Assign a RenderTexture asset to StereoHackEnabler with the same format as HDRP, so either R16G16B16A16 or B10G11R11_UFloatPack32 format.
  • Render the the texture somehow. For example by assigning it to a RawImage in a Canvas.

What was changed in HDRP?

• Modified HDRenderPipeline.cs and HDRenderPipeline.RenderGraph.cs. Changes are marked with "// CEC EDIT".
• Added reference to OffAxisCamera in the Unity.RenderPipelines.HighDefinition.Runtime.asmdef.
• Added some files in Runtime/StereoHack/.
• Added this readme.

Notes

Here is the Unity Discussions thread that lead to the soltuion.