標準のバイナリでは、webp は有効になっていません。 Mac で ffmpeg ビルドするならば、以下のように、homebrew でできます。 brew install ffmpeg --with-fdk-aac --with-ffplay --with-freetype \ --with-libass --with-libquvi --with-libvorbis --with-libvpx --with-opus \ --with-x265 --with-webp $ brew info ffmpeg ffmpeg: stable 3.1.2 (bottled), HEAD Play, record, convert, and stream audio and video https://ffmpeg.org/ /usr/local/Cellar/ffmpeg/3.1.1 (236
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