ffmpeg / avconvを使用してビデオをWebMに変換する

Note: This information is based on the FFmpeg Wiki on VP9 encoding. Please refer to that article for more information – it will be continuously maintained and extended. For the best results you should use the a recent version of ffmpeg by downloading it from their website (a static build will suffice; it contains the libvps-vp9 encoder).

Variable Bit Rate

VBR encoding gives you the optimal overall quality, since the encoder can freely choose how many bits to assign to a frame. Choose this mode unless you are preparing videos for constant-bitrate streaming.

Option 1: Constant quality encoding

Typically, if you do not want to target a specific file size, you should let the bitrate vary freely, as that will lead to the highest quality. You can do this by setting the bitrate to 0 and the constant rate factor (CRF) to the target quality level:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 30 -b:v 0 output.webm

CRF value can be from 0–63. Lower values mean better quality. Recommended values range from 15–35, with 31 being recommended for 1080p HD video. Google has a guide with more info on that.

Option 2: Two-pass encoding with a target bitrate

If you want your file to have a specific target bitrate or file size, you need to specify the rate and use two-pass encoding (which will ensure an optimal quality distribution). Here we're choosing 5 MBit/s, which should be enough for 1080p content.

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 5M -pass 1 -f webm /dev/null && \
ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 5M -pass 2 output.webm

Windows users need to use NUL instead of /dev/null, and a ^ instead of \.

Constant Bit Rate

First of all, libvpx offers constant bitrate and variable bitrate encoding modes. Constant bitrate should be avoided whenever possible (unless you target a specific file size or streaming scenario), since the average quality per file size will be worse. Still, you could try setting a constant bitrate if nothing else works for you, e.g. with 1 MBit/s:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -minrate 5M -maxrate 5M -b:v 5M output.webm

Look at the output and increase or decrease the bit rate to your liking (or file size constraints). For example, you can use 500K or 5M, et cetera.

エンコーダがCBRを使用するには-minrate、-maxrateとビットレートを指定する必要があります-b:v。それらはすべて同じ値である必要があります。そうでない場合は、代わりに別のターゲットビットレートを選択してVBRエンコーディングを実行しますが、品質は低くなります。

オーディオ

VP9エンコーディングに現在選択されているオーディオコーデックはOpusです。FFmpegはデフォルトで必要なエンコーダーとそのオプションを選択します。明示的に設定したい場合は-c:a libopus、それも可能です。その他のオプションについては、libopusドキュメントを参照してください。