libaom AV1 Encoding Guide
libaom-av1 is the AOMedia video encoder for AV1, an open source & royalty-free video codec.
libaom-av1 can save about 30% bitrate compared to VP9 and H.265 / HEVC, and about 50% over H.264, while retaining the same visual quality.
To install FFmpeg with support for
libaom-av1, look at the Compilation Guides and compile FFmpeg with the
libaom offers the following rate-control modes which determine the quality and file size obtained:
- Constant quality
- Constrained quality
- 2-pass average bitrate
- 1-pass average bitrate
For a list of options, run
ffmpeg -h encoder=libaom-av1.
libaom-av1 has a constant quality (CQ) mode (like CRF in x264 and x265) which will ensure that every frame gets the number of bits it deserves to achieve a certain (perceptual) quality level, rather than encoding each frame to meet a bit rate target. This results in better overall quality. If you do not need to achieve a fixed target file size, this should be your method of choice.
To trigger this mode, you must use a combination of -crf and -b:v 0. -b:v MUST be 0.
ffmpeg -i input.mp4 -c:v libaom-av1 -crf 30 -b:v 0 av1_test.mkv
The CRF value can be from 0–63. Lower values mean better quality and greater file size. 0 means lossless.
libaom-av1 also has a constrained quality (CQ) mode that will ensure that a constant (perceptual) quality is reached while keeping the bitrate below a specified upper bound or within a certain bound. This method is useful for bulk encoding videos in a generally consistent fashion.
ffmpeg -i input.mp4 -c:v libaom-av1 -crf 30 -b:v 2000k output.mkv
The quality is determined by the
-crf, and the bitrate limit by the
-b:v where the bitrate MUST be non-zero.
You can also specify a minimum and maximum bitrate instead of a quality target:
ffmpeg -i input.mp4 -c:v libaom-av1 -minrate 500k -b:v 2000k -maxrate 2500k output.mp4
In order to create more efficient encodes when a particular target bitrate should be reached, you should choose two-pass encoding. For two-pass, you need to run
ffmpeg twice, with almost the same settings, except for:
- In pass 1 and 2, use the
-pass 2options, respectively.
- In pass 1, output to a null file descriptor, not an actual file. (This will generate a logfile that ffmpeg needs for the second pass.)
- In pass 1, you can leave audio out by specifying
ffmpeg -i input.mp4 -c:v libaom-av1 -b:v 2M -pass 1 -an -f null /dev/null && \ ffmpeg -i input.mp4 -c:v libaom-av1 -b:v 2M -pass 2 -c:a libopus output.mkv
Average Bitrate (ABR)
libaom-av1 also offers a simple "Average Bitrate" or "Target Bitrate" mode. In this mode, it will simply try to reach the specified bit rate on average, e.g. 2 MBit/s.
ffmpeg -i input.mp4 -c:v libaom-av1 -b:v 2M output.mkv
Use this option only if file size and encoding time are more important factors than quality alone. Otherwise, use one of the other rate control methods described above.
Controlling Speed / Quality
-cpu-used sets how efficient the compression will be. Default is 1. Lower values mean slower encoding with better quality, and vice-versa.
-row-mt 1 enables row-based multi-threading which maximizes CPU usage. To enable fast decoding performance, also add tiles (i.e.
-tiles 4x1 or
-tiles 2x2 for 4 tiles). Enabling
row-mt is only faster when the CPU has more threads than the number of encoded tiles.
When encoding in HDR it's necessary to pass through color information;
-color_primaries. For example, Youtube HDR uses
-colorspace bt2020nc -color_trc smpte2084 -color_primaries bt2020
-crf 0 for lossless encoding. Because of a bug the first frame will not be losslessly preserved (was fixed in 288ca1c66740640d5742f2921efbf502d83f8b6a). You may use
-aom-params lossless=1, which is truly lossless.
The range of options are similar to that of libaom (aomenc). It is supposed to be faster than libaom while having comparable quality.
Rav1e claims to be the fastest software AV1 encoder, but that really depends on the setting.
More info about AV1 can be found here: