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 --enable-libaom option.

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 or check FFmpeg's online documentation.

Note: Users of libaom older than version 2.0.0 will need to add -strict experimental (or the alias -strict -2).

Constant Quality

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.

Constrained Quality

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

Note: When muxing into MP4, you may want to add -movflags +faststart to the output parameters if the intended use for the resulting file is streaming.

Two-Pass

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 1 and -pass 2 options, 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 -an.

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

Note: Windows users should use NUL instead of /dev/null and ^ instead of \.

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.

Keyframe placement

By default, libaom's maximum keyframe interval is 9999 frames. This can lead to slow seeking, especially with content that has few or infrequent scene changes.

The -g option can be used to set the maximum keyframe interval. Anything up to 10 seconds is considered reasonable for most content, so for 30 frames per second content one would use -g 300, for 60 fps content -g 600, etc.

To set a fixed keyframe interval, set both -g and -keyint_min to the same value. Note that currently -keyint_min is ignored unless it's the same as -g, so the minimum keyframe interval can't be set on its own.

For intra-only output, use -g 0.

HDR

When encoding in HDR it's necessary to pass through color information; -colorspace, -color_trc and -color_primaries. For example, Youtube HDR uses

-colorspace bt2020nc -color_trc smpte2084 -color_primaries bt2020

Lossless encoding

Use -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.

SVT-AV1

libsvtav1 is the Intel x86-64 codec for AV1. Compile with --enable-libsvtav1. See ​ffmpeg doc and ​upstream doc.

The range of options are similar to that of libaom (aomenc). It is supposed to be faster than libaom while having comparable quality.

rav1e

librav1e is the Xiph encoder for AV1. Compile with --enable-librav1e. See ​ffmpeg doc and ​upstream CLI options.

Rav1e claims to be the fastest software AV1 encoder, but that really depends on the setting.

More Info

More info about AV1 can be found here:

• ​Nwgat's AOMedia's AV1 Codec Overview