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Merge pull request #3806 from heygen-com/docs/prinfra-345-webm-alpha-inspection

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docs(render): explain alpha-aware WebM inspection
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Vance Ingalls
2026-09-09 00:47:05 -07:00
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parent 6af3df6f1f 78e84a9ca8
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@@ -81,6 +81,19 @@ MP4 is the normal opaque delivery format. After rendering transparency, inspect
the file over a contrasting background rather than trusting a player that
always shows black behind alpha.
For VP9 WebM, `ffprobe` reporting `pix_fmt=yuv420p` does not by itself mean
transparency was lost. FFmpeg's default VP9 decoder can also produce an opaque
image from a file that contains alpha. To inspect a frame, explicitly select
the alpha-aware decoder **before** the input:
```bash
ffmpeg -c:v libvpx-vp9 -i overlay.webm -frames:v 1 -pix_fmt rgba overlay-frame.png
```
View the PNG over a checkerboard or contrasting background. If it is still
opaque, compare it with the composition's PNG-sequence output to check whether
transparency was already missing before video encoding.
## Input video codecs
Studio, preview, `check`, and published projects normally create cached browser