Alpha Output
To maximize performance and minimize bandwidth usage, HDRP by default renders image frames in the R11G11B10 format. However, this format doesn't include an alpha channel, which might be required for applications that want to composite HDRP's output over other images.
To configure HDRP to output an alpha channel:
- Open your HDRP Asset in the Inspector window.
- Go to Rendering > Color Buffer Format.
- Select R16G16B16A16.
Note: Enabling this option incurs a performance overhead.
In HDRP, opaque materials always output 1 in the alpha channel, unless you enable Alpha Clipping. If you want to export the alpha of an opaque material, one solution is to enable Alpha Clipping and set the Threshold to 0.
Furthermore, when you enable post-processing, also set the Buffer Format for post-processing operations to R16G16B16A16 to apply post-processing operation in the alpha channel. You can select this from the Post-Processing section of the HDRP asset. If you set the post-processing format to R11G11B10, HDRP outputs a copy of the alpha channel without any post-processing on it.
The following table summarizes the behavior of HDRP regarding the alpha channel of the output frames.
| Rendering Buffer Format | Post-processing Buffer Format | Alpha Output |
|---|---|---|
| R11G11B10 | R11G11B10 | No alpha output |
| R16G16B16A16 | R11G11B10 | Alpha channel without post-processing (AlphaCopy) |
| R16G16B16A16 | R16G16B16A16 | Alpha channel with post-processing |
Note: Alpha output is also supported in Path Tracing.
DoF and Alpha Output
Another case which might require post-processing of the alpha channel is for scenes that use Depth Of Field. In this case, if the alpha isn't processed, compositing results in a sharp cut-off of an object that appears blurred. This is better illustrated in the images below:
An out-of-focus sphere composited over a solid blue background using a R16G16B16A16 buffer format for both rendering and post-processing. In this case, DoF is applied in the alpha channel, resulting in a proper composition (the output alpha used in the composition is shown in the image inset).
An out-of-focus sphere composited over a solid blue background using AlphaCopy. In this case, DoF isn't applied in the alpha channel, resulting in a sharp outline around the composited sphere (the output alpha used in the composition is shown in the image inset).
Temporal Anti-Aliasing and Alpha Output
When you enable Temporal Anti-Aliasing (TAA), it's highly recommended to enable post-processing for the alpha channel (R16G16B16A16 format for both rendering and post-processing). If the alpha channel isn't post-processed, then the alpha mask is jittered, as shown in the images below:
A sphere rendered with TAA using AlphaCopy, composited over a solid blue background using the alpha channel. The alpha channel isn't temporally stabilized by TAA, resulting in jittering on the final image.
A sphere rendered with TAA (R16G16B16A16 for both rendering and post-processing), composited over a solid blue background using the alpha channel. TAA is also applied in the alpha channel, resulting in a stable composition.