Configure libaom AV1 Partition Limits in FFmpeg

This article explains how to configure partition limits in the libaom AV1 encoder using FFmpeg. By adjusting these limits, you can balance encoding speed and compression efficiency, allowing for faster encodes or higher-quality video output depending on your system’s capabilities.

Understanding AV1 Partitions

The AV1 codec compresses video by dividing frames into blocks called partitions. These partitions can range in size from \(128\times128\) pixels down to \(4\times4\) pixels.

• Smaller partitions (\(4\times4\), \(8\times8\)) allow the encoder to capture fine details and complex motion, improving compression efficiency but significantly increasing encoding time.
• Larger partitions (\(64\times64\), \(128\times128\)) speed up the encoding process because the encoder has fewer blocks to analyze, though this may result in a slight loss of detail in complex scenes.

By default, libaom dynamically chooses partition sizes. However, you can manually restrict these limits to optimize for speed or quality.

Configuring Partition Limits in FFmpeg

To set the minimum and maximum partition sizes in FFmpeg, you must pass the min-partition-size and max-partition-size parameters to the libaom-av1 encoder using the -aom-params flag.

The syntax for this flag is:

-aom-params min-partition-size=WxH:max-partition-size=WxH

Valid block sizes include: 4x4, 8x8, 16x16, 32x32, 64x64, and 128x128.

Configuration for Faster Encoding

If you want to speed up your encoding process, you can restrict the encoder from analyzing very small partitions. For example, setting the minimum partition size to 16x16 and the maximum to 64x64 prevents the encoder from spending time on ultra-fine details:

ffmpeg -i input.mp4 -c:v libaom-av1 -crf 30 -cpu-used 4 -aom-params min-partition-size=16x16:max-partition-size=64x64 output.mkv

Configuration for Maximum Quality and Efficiency

If encoding speed is not a concern and you want to achieve the highest possible compression efficiency, you can allow the encoder to use the full range of partition sizes from 4x4 up to 128x128:

ffmpeg -i input.mp4 -c:v libaom-av1 -crf 30 -cpu-used 2 -aom-params min-partition-size=4x4:max-partition-size=128x128 output.mkv

Recommended Best Practices

While manual partition configuration gives you precise control, it is highly recommended to pair these settings with the -cpu-used parameter (typically set between 0 and 8). The -cpu-used setting acts as a general speed/quality preset that automatically scales partition search depth. Manually defining partition limits with -aom-params should be used when you need to enforce strict constraints that the default presets do not satisfy.