[zfleeman]

Closes #140.

ffmpeg writes an MP4's index (the moov box) after the video data, because it doesn't know where each frame lands until the encode finishes. A player streaming the file from a link (Discord, a browser) has to fetch the end of the file before it can start. -movflags +faststart makes ffmpeg move the index to the front when it finishes, so playback can start from the first few kilobytes.

Change

TwoPass._generate_params() adds "movflags": "+faststart" to the second pass for every MP4 codec. It uses the same "vp9" in codec rule as _output_filename(), so the two can't disagree about which files are .webm. The first pass writes to a null output and doesn't get the flag. +faststart has been in ffmpeg since 2013, so there's no version check.

The flag shows up in the printed and --dry-run commands on its own.

Checked

A real 5-second x264 encode of a phone clip lists its top-level boxes as ftyp, moov, free, mdat (before: ftyp, free, mdat, moov).

Tests

test_twopass.py: for every codec in CODEC_ENCODERS, the last pass has -movflags +faststart unless the codec is vp9, and no earlier pass has -movflags.