docs/video-export-notes.md

Video Export Notes

Transparent VP8 WebM with ffmpeg.wasm

When transparent VP8 WebM is encoded from a raw RGBA stream, crf can appear to do nothing even though the argument is being passed correctly.

Why this happens:

• The app feeds ffmpeg a rawvideo input stream, not a PNG frame sequence.
• FFmpeg's rawvideo demuxer declares a fixed input bitrate based on width, height, pixel format, and framerate.
• For RGBA video this reported input bitrate is enormous, often hundreds of Mbps.
• If libvpx then sees a much smaller explicit -b:v target, its rate control can collapse into an effectively strict bitrate clamp.
• In that state, output size tracks duration and -b:v much more than crf, so different CRF values can produce nearly identical files.

Practical consequence:

• crf is not reliably meaningful for transparent VP8 WebM when the input is rawvideo and -b:v is set too low.

Why PNG-sequence examples behave differently:

• With an image-sequence input such as frame_%03d.png, the demuxer does not report the same huge fixed source bitrate.
• That means libvpx is less likely to abandon CRF-driven rate control.

Project takeaway:

• For the app's transparent VP8 WebM mode, avoid treating a low -b:v value as harmless.
• If CRF tuning seems ignored, the first thing to check is whether the explicit bitrate target is forcing libvpx into a near-CBR path.