使用ffmpeg / avconv将视频转换为WebM

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.

你必须指定-minrate，-maxrate以及比特率-b:v，以使编码器使用CBR。它们都必须具有相同的值-否则，它将选择不同的目标比特率，并进行VBR编码，但质量较差。

声音的

VP9编码当前选择的音频编解码器是Opus。FFmpeg将默认选择必要的编码器及其选项。如果要显式设置-c:a libopus，也可以这样做。有关更多选项，请参考libopus文档。