How to Encode Specific Frames Using Libaom?

Encoding a specific subset of frames from an input video file using libaom (the reference encoder for the AV1 video format) allows you to test encoding settings, generate short previews, or split processing workloads without rendering the entire file. This article covers how to define exact frame ranges, control keyframe placement, and utilize wrapper tools like FFmpeg to manage your libaom encoding boundaries efficiently.

Frame Control with FFmpeg and Libaom

Because libaom is typically used as an underlying library via FFmpeg rather than as a standalone CLI for daily workflows, the most precise way to target specific frames is by leveraging FFmpeg’s input and output seek parameters.

1. Selecting a Frame Range by Count

To encode a specific number of frames starting from the very beginning of a file, use the -frames:v (or -vframes) flag.

ffmpeg -i input.mp4 -c:v libaom-av1 -frames:v 150 output.mkv

This command forces the libaom encoder to stop immediately after processing the first 150 frames.

2. Seeking to a Specific Starting Point

If your target subset resides in the middle of a video, combine seeking flags (-ss and -to, or -t) to isolate the exact segment.

Flag	Purpose	Placement
`-ss`	Seeks to the start timestamp or frame.	Before `-i` (fast/input seek) or after `-i` (accurate/output seek).
`-t`	Specifies the duration of the clip to encode.	After the input file.
`-to`	Specifies the stop timestamp of the clip.	After the input file.

For maximum precision when dealing with specific frames, you can pass frame expressions directly into the evaluation filter before handing the frames off to the libaom encoder:

ffmpeg -i input.mp4 -vf "select='between(n,300,450)'" -vsync vfr -c:v libaom-av1 output.mkv

between(n,300,450): Tells FFmpeg to only pass video frames 300 through 450 to the libaom encoder.
-vsync vfr: Ensures variable frame rate compliance so dropped frames do not leave blank gaps.

Internal Libaom Parameters for Chunked Encoding

When encoding a subset of frames that will eventually be merged back into a larger video, maintaining reference frame continuity is critical. Libaom features internal parameters to handle how boundaries are treated.

Keyframe Controls (`-g` and `strict_gop`)

When encoding a subset, ensure that the first frame of your subset is forced as an Intra-frame (I-frame) so it can decode independently.

-g (GOP Size): Sets the maximum distance between keyframes.
-aom-params strict_gop=1: Enforces strict Group of Pictures (GOP) boundaries, preventing libaom from looking outside your specified frame subset for temporal references.

ffmpeg -ss 00:02:00 -i input.mp4 -c:v libaom-av1 -frames:v 300 -aom-params strict_gop=1 output.mkv

By constraining the input via FFmpeg’s frame-selection filters and enforcing strict GOP structures within libaom, you can successfully isolate and encode any precise subset of frames without quality degradation at the boundary points.