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JamesandClaude Opus 4.7
39bc3749b4
Tag-based alpha detection (alpha_mode / ALPHA_MODE / pix_fmt yuva*) is fundamentally brittle. Failure modes seen in the wild: - case-sensitivity across ffmpeg versions (alpha_mode vs ALPHA_MODE) - older muxers that omit the sidecar tag entirely - mp4-as-webm rewraps that drop the tag - ffprobe reporting yuv420p for VP9-with-alpha because the alpha plane lives in a Matroska BlockAdditional sidecar, not the main pix_fmt Each of those silently strips alpha at extraction time. The bug doesn't surface until the rendered output is missing layers — frustrating to debug, silent in stdout. The previous case-insensitive fix patched one of the failure modes; this commit removes the class. The robust alternative is codec-based: any bitstream that CAN carry alpha (VP9, VP8, ProRes 4444) gets the alpha-aware decoder and PNG output by default, regardless of what the tag says. The cost is a small file-size increase on opaque VP9/VP8 sources (cached PNGs vs JPGs); the benefit is no class of silent alpha loss from tag misdetection. - Adds codecMayHaveAlpha() + decoderForCodec() helpers and exports them. - Updates extractVideoFramesRange to force libvpx-vp9 / libvpx for VP9 / VP8 unconditionally (was: only when metadata.hasAlpha). - Updates resolveFrameFormat to default to PNG for any alpha-capable codec (was: only when metadata.hasAlpha). - +4 unit tests covering the codec table. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>