Alpha Processing for Post Processing
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Summary
In Unity 6 Preview, URP now has a setting for Alpha Processing (URP Asset > Post-processing > Alpha Processing). With this enabled, URP will process alpha when rendering the post processing stack as opposed to discarding it (replaced alpha values with 1 before).
The render target requires a format with the alpha channel. The camera color buffer format must be RGBA8 for SDR (HDR off) or RGBA16F for HDR (64-bits). You can configure the format using the settings in URP Asset > Quality.
Example use cases for this feature
- Rendering the game UI (such as HUD) with post-processing configuration different from the in-game scenes.
- Character customization screen, where the background interface and the 3D character are rendered with different post-processing effects and blended using the alpha channel
- Depth of Field in Layered Environments: Ideal for side-scroller games with multiple layers, allowing for advanced visual effects that selectively blur and focus elements across layers without impacting each other.
- Post Processing on transparent billboard animations
- XR applications that make use of post processing and need to support video pass-through (Mixed Reality)
- Rendering scenes that make use of post processing to image files with a transparent background
Notes on Camera Stacking in URP
Camera stacking in URP involves multiple cameras rendering on the same frame, where the base camera writes to a target and overlay cameras add geometry on top. The stacking allows partial isolation between cameras / layers, so it might be used as layer system for some special cases if you are using the cameras and effects in the right order, but it is not a true layering system. Post processing volumes applied to any of the overlaid cameras also gets applied to every camera underneath. Camera stacking can also have a significant performance impact (e.g. culling overhead) and should be used with care, especially on resource-constraint hardware like mobile devices.
Please test the feature with your desired use case in mind and give us feedback on how we could further improve it. Especially the combinations with camera stacking can be challenging to test fully in our test suite. We appreciate your findings.