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Convert straight alpha to premultiplied alpha with the FFmpeg premultiply filter
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What this guide covers
For editors and compositors with a straight-alpha foreground and a next step that expects premultiplied alpha. They end up with a documented way to premultiply from a separate matte or the clip's own alpha, and a way to check it. Based on the FFmpeg manual; no command was run.
More about this guide
Guide ID: art_01M6W0XM26HG2G2ZT9KJAHVS75
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Guide title
Convert straight alpha to premultiplied alpha with the FFmpeg premultiply filter
Coverage
For editors and compositors with a straight-alpha foreground and a next step that expects premultiplied alpha. They end up with a documented way to premultiply from a separate matte or the clip's own alpha, and a way to check it. Based on the FFmpeg manual; no command was run.
Example
Hypothetical and not run: compare composites of a soft-edged keyed foreground, one straight and one premultiplied, each declared correctly, plus a deliberately mismatched case.
Useful starting points
- Do not premultiply a clip that is already premultiplied; applying it twice darkens the soft edges.
- Inspect the saved file with ffprobe: a pixel format without an alpha component means transparency was lost.
- Edges over black are not a reliable test; compare composites over a contrasting background on the same frame.
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