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Encoding videos for the modern web using ffmpeg

Table of contents

About video codecs and containers

Video files consist of 3 seperate components that define how data is stored and decoded:

  1. Container format (for example mp4 or webm): Stores the video/audio streams, as well as some metadata like subtitles.
  2. Video stream: The data of the video itself (just visual frames, no audio), encoded using a video codec like h264 or vp9.
  3. Audio stream: The audio data encoded using an audio codec like aac or opus.

A single video file will be a specific combination of these 3 components. For web videos, we typically have vp9-encoded video streams with opus-encoded audio streams in a webm container, and h264 video streams with aac audio in mp4 containers as a fallback for older devices.

Encoding a video as WEBM

Our primary video format is VP9 video with Opus audio in a Webm container. It is recommended to encode VP9 using double-pass to enable some quality-enhancements not available in single-pass. For most cases, you will want to encode video using VP9's Constant Quality encoding mode:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 0 -crf 33 -pass 1 -row-mt 1 -an -f null /dev/null
ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 0 -crf 33 -pass 2 -row-mt 1 -threads 16 -speed 2 -tile-columns 2 -tile-rows 2 -frame-parallel 1 -g 240 -auto-alt-ref 1 -lag-in-frames 25 -deadline good -c:a libopus -b:a 64k -f webm output.webm

The options explained (second pass only):

  • -i input.mp4: your input file
  • -c:v libvpx-vp9: encode video as VP9 using libvpx
  • -b:v 0: turn off video bitrate limiting to allow VP9 to vary bitrate in order maintain the taret quality level
  • -crf 33: Constant rate factor (aka "Quality") setting. Valid values are 0-63, sane values are 15-35; 31 is recommended to 1080 video.
  • -row-mt 1 Enable row-based multithreading
  • -threads 16: Use up to 16 threads for encoding (4 tile rows * 4 tile columns)
  • -speed 1: Encoding speed from -16 to 16, higher is faster with less quality. 1 is a good compromise between quality and speed, as output is very close to 0 but much faster. Resolutions of 720p or higher should set this to 2 as the quality gain of 1 is not humanly recognizable for them.
  • -tile-columns 2: Log2 number of VP9 tile columns to use from -1 to 6 (careful with log2 formatted values: 1 means 2 columns, 2 means 4 columns!). Values >0 and threads>1 enable multithreaded encoding. Maximum value for 1920x1080 video files is 2, larger horizontal resolutions allow higher column counts.
  • -tile-rows 2: Log2 number of VP9 tile rows to use from -1 to 2 (careful with log2 formatted values: 1 means 2 rows, 2 means 4 rows!). Maximum number of rows is 4 independent of video size.
  • -frame-parallel 1: Enables parallel frame decoding
  • -g 240: Number of frames allowed between keyframes. Larger values allow for more efficient placement of keyframes resulting in better quality.
  • -auto-alt-ref 1: Enables use of alternative reference frames (double-pass optimization feature)
  • -lag-in-frames 25: Number of frames to look ahead for alternative ref frames (see above)
  • -deadline good: Amount of time to spend encoding, affects quality and encoding time; values are good, best, realtime
  • -c:a libopus: encode audio as opus using libopus
  • -b:a 64k: set audio encoding target bitrate to 64k
  • -f webm: output to webm container format

If you need to stay below a specific bitrate at the expense of quality, you can use the VP9 Constrained Quality mode by setting a specific bitrate target for -b:v. If you are not sure whether you need this mode, then you likely don't.

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -crf 33 -pass 1 -an -f null /dev/null
ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -crf 33 -pass 2 -threads 8 -speed 2 -tile-columns 6 -frame-parallel 1 -auto-alt-ref 1 -lag-in-frames 25 -deadline good -c:a libopus -b:a 64k -f webm output.webm

Encoding a fallback video as MP4

As a fallback video format for older devices we use the widely-supported h264 video codec with AAC audio encoding in an MP4 container. This encoding does not need a double-pass unless you want to use constant-bitrate encoding, which is a bad choice unless you already calculated a specific target bitrate that you need to maintain at the cost of quality. For our encoding, we use Constant Quality Encoding (CRF), as we did for VP9 above:

ffmpeg -i inputfile.mkv -c:v libx264 -vf format=yuv420p -crf 18 -preset veryslow -movflags +faststart -c:a aac outputfile.mp4

The options explained:

  • -i inputfile.mkv: your input file
  • -c:v libx264: use h264 video codec with fast community library
  • -vf format=yuv420p: use yuv 4-2-0 pixel encoding
  • -crf 18: constant rate factor (visual quality setting). Sane values are between 18 and 28 where 18 is visually lossless and 28 may produce minor visual artifacts
  • -preset medium: encoding preset (heavily impacts encoding speed; faster = faster encoding but worse quality). Recommended settings: general optimization: medium / slow; long term storage: veryslow; short-lived content: faster / fast / medium; transcoding: ultrafast / superfast / veryfast / faster / fast / medium
  • -movflags faststart: Moves the MOOV atom to the beginning of the file, eliminating unnecessary seek times
  • -c:a aac - sets audio codec to AAC
  • -f mp4: output to mp4 container format

Additional flags for specific use cases

In addition to the general video processing above, you sometimes need to further edit the video in some cases:

  • -an: Mutes audio (removes audio stream). Reduces filesize, good for videos playing muted in the background or that have no audio.
  • -r 30: Limits max framerate to 30 FPS. Limiting framerate can drastically reduce filesize.
  • -vf "scale=1920:720,setsar=1": Scale video to 1920x720 dimensions. Remember sides need to remain divisible by 2 to use YUV420 pixel format. Set width or height to -2 to scale only one side while maintaining the original video's aspect ratio

If you have already processed a video and just want to use one of those additional options on the video, you don't need to re-encode it entirely. You can just copy the stream you are not changing (using -vcodec copy for video or -acodec copy for audio)

For example. to mute an already processed mp4 video without touching video/container data:

ffmpeg -i input.mp4 -vcodec copy -an output.mp4

This will remove the audio stream while only copying the video stream (thus not doing the expensive video encoding again)

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