From 78e84a9ca8de3ba490f70a065fa8e1781c7847fb Mon Sep 17 00:00:00 2001 From: Vance Ingalls Date: Wed, 9 Sep 2026 00:00:24 -0700 Subject: [PATCH] docs(render): explain alpha-aware WebM inspection --- docs/guides/rendering.mdx | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/docs/guides/rendering.mdx b/docs/guides/rendering.mdx index 4c23fc9a45..8a96bdf74a 100644 --- a/docs/guides/rendering.mdx +++ b/docs/guides/rendering.mdx @@ -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