41 lines
3.5 KiB
Markdown
41 lines
3.5 KiB
Markdown
# Alpha Output
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To maximize performance and minimize bandwidth usage, HDRP by default renders image frames in the **R11G11B10** format. However, this format does not include an alpha channel, which might be required for applications that want to composite HDRP's output over other images.
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To configure HDRP to output an alpha channel, open your [HDRP Asset](HDRP-Asset.md) (menu: **Edit > Project Settings > Graphics > Scriptable Render Pipeline Asset**)), go to the **Rendering** section, and set the **Color Buffer Format** to **R16G16B16A16**. However, note that enabling this option incurs a performance overhead. In HDRP, opaque materials always output 1 in the alpha channel, unless you enable [Alpha Clipping](Alpha-Clipping.md). If you want to export the alpha of an opaque material, one solution is to enable **Alpha Clipping** and set the Threshold to 0.
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Furthermore, when post-processing is enabled, the *Buffer Format* for post-processing operations should also be set to *R16G16B16A16* in order to apply post-processing operation in the alpha channel. This can be selected from the post-processing section of the HDRP asset. If the post-processing format is set to **R11G11B10**, then HDRP will output a copy of the alpha channel without any post-processing on it.
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The following table summarizes the behavior of HDRP regarding the alpha channel of the output frames.
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Rendering Buffer Format | Post-processing Buffer Format | Alpha Output
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**R11G11B10** | **R11G11B10** | No alpha output
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**R16G16B16A16** | **R11G11B10** | Alpha channel without post-processing (AlphaCopy)
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**R16G16B16A16** | **R16G16B16A16** | Alpha channel with post-processing
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Note that alpha output is also supported in [Path Tracing](Ray-Tracing-Path-Tracing.md).
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## DoF and Alpha Output
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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 is not processed, compositing will result in a sharp cut-off of an object that should appear blurred. This is better is illustrated in the images below:
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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).
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An out-of-focus sphere composited over a solid blue background using *AlphaCopy*. In this case, DoF is NOT 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).
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## Temporal Anti-Aliasing and Alpha Output
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When Temporal Anti-Aliasing (TAA) is enabled it is highly recommended to enable post-processing for the alpha channel (*R16G16B16A16* format for both rendering and post-processing). If the alpha channel is not post-processed, then the alpha mask will be jittered, as shown in the images below:
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A sphere rendered with TAA using *AlphaCopy*, composited over a solid blue background using the alpha channel. The alpha channel is not temporally stabilized by TAA, resulting in jittering on the final image.
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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.
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