While that can be useful for streaming, it reduces quality significantly as frames with a lot of motion will not have the bandwidth needed to look good.Īnother part of the reason you’re losing quality is that webm (VP9) prefers to encode in two passes for best results. If you do not specify a video bitrate, it defaults to something low which gets you Constrained Quality mode, where each frame has a maximum bitrate. It is important to set the video bitrate to zero. For VP9, the CRF can range from 0 (best quality) to 63 (smallest file size).

Setting the video bitrate to zero while also specifying the CRF (Constant Rate Factor) enables Constant Quality mode which targets a certain perceptual quality level.

We discard the video using /dev/null because the first pass's only purpose is to generate a log file of statistics. If you're using Microsoft Windows™ instead of UNIX, change /dev/null to NUL. Use two-pass Constant Quality mode: ffmpeg -i input.mp4 -b:v 0 -crf 30 -pass 1 -an -f webm -y /dev/nullįfmpeg -i input.mp4 -b:v 0 -crf 30 -pass 2 output.webm

Convert mp4 to webm ffmpeg