[HDRP][Path Tracing] Added alpha channel to path traced results

[eturquin]

Addresses https://fogbugz.unity3d.com/f/cases/1304187/

The alpha channel of path-traced images was always set to 1.
Now, if there is no sky, it will use the alpha value of the clear color on camera ray misses (set to 0 by default), to match rasterization behaviour.

This change has been tested with and without presence of sky, and with a clear color with alpha going from 0 to 1:

• with a sky: output alpha is 1
• without a sky: output alpha is equal to alpha of the clear color
• if a ray hit a geometry, alpha is 1 (no matter if the material has refraction or not)

Note that for now, volumetrics are ignored and won't affect the alpha.

[pmavridis]

Perhaps it is worth backporting this to 10.x? The PR seems simple-enough/back-portable.

[sebastienlagarde]

Agree for backporting to 10.x.
The PR miss a changelog
and we should update the doc about alpha to say that it also work with path tracer

https://docs.unity3d.com/Packages/com.unity.render-pipelines.high-definition@10.2/manual/Alpha-Output.html

[eturquin]

The PR miss a changelog
and we should update the doc about alpha to say that it also work with path tracer

Ok, adding both.

[remi-chapelain]

Tested with a camera outputting the path traced result to a render texture and then using the texture in a transparent and/or a opaque alpha clipped material. ✔️
Tested that it only worked with Sky setup to None ✔️
Don't know if "worth doc mentioning" but worth noting a few things even though they're expected :

• HDRP Color buffer needs to be setup with an alpha channel (R16B16G16A16) for this PR to have any effect
• Doesn't work with Unity Recorder (Recorder does not care about alpha)
• Doesn't work with CaptureScreenshot (Game view discards alpha)

Thanks to @pmavridis who helped me test this.

606e1293d2254aaf49f5c66979c4f30e.mp4