# 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.