How Does HandBrake Use Libaom for AV1?

This article explores how the open-source video transcoder HandBrake integrates the libaom reference library to enable AV1 video encoding. We will look at the history of this integration, how HandBrake utilizes the library’s CPU-based encoding algorithms, and the performance trade-offs users face when selecting libaom over alternative hardware-accelerated encoders.

The Integration of AV1 and Libaom in HandBrake

HandBrake officially introduced AV1 video encoding support with the release of HandBrake 1.6.0. To achieve this, the developers integrated libaom, the official reference software encoder maintained by the Alliance for Open Media (AOMedia). As an open-source, cross-platform transcoder, HandBrake relies on these external, well-maintained libraries to handle the complex mathematics behind modern video compression standards.

When a user selects the AV1 (libaom) encoder from HandBrake’s video settings dropdown, HandBrake acts as a graphical user interface (GUI) and pipeline manager. It decodes the source video file, applies any requested filters (such as deinteracing or color adjustments), and then passes the raw video frames directly to the libaom library. The library processes these frames according to the user’s configuration and returns the compressed AV1 video stream, which HandBrake then muxes into a container like MP4, MKV, or WebM.

How Libaom Operates Within the Transcoding Pipeline

Unlike hardware-accelerated encoders that offload work to dedicated microchips on a graphics card, libaom is a traditional software encoder. It relies entirely on the computer’s central processing unit (CPU).

Within HandBrake, libaom utilizes advanced compression techniques to drastically reduce file sizes while maintaining high visual fidelity. Key mechanisms include:

• Block-Based Prediction: Analyzing spatial redundancies within a single frame and temporal redundancies across sequential frames to store only the changes between images.
• Variable Block Sizes: Dynamically partitioning frames into coding units ranging from 4x4 up to 128x128 pixels, allowing complex scenes to use fine details and simple scenes (like a flat sky) to save data.
• Tiles and Threading: HandBrake configures libaom to split video frames into independent “tiles,” allowing the encoder to distribute the heavy computational workload across multiple CPU cores simultaneously.

Performance and CPU Utilization

Because libaom was built as a reference encoder, its primary goal was strict adherence to the AV1 specification and maximum compression efficiency, rather than raw speed. Consequently, running libaom in HandBrake is notoriously CPU-intensive.

To manage this, HandBrake exposes libaom’s speed presets (typically ranging from 0 to 13). Lower numbers prioritize maximum compression efficiency at the cost of incredibly slow encoding times, while higher numbers speed up the process by disabling some of the most exhaustive search algorithms.

Libaom vs. Alternative AV1 Encoders in HandBrake

While libaom was the pioneering software encoder in HandBrake, the software has since introduced alternative methods for generating AV1 video. Understanding where libaom fits requires a quick comparison:

• SVT-AV1: Developed by Intel and Netflix, SVT-AV1 is another software encoder available in HandBrake. It is highly optimized for modern multi-core CPUs and generally executes transcodes significantly faster than libaom while maintaining comparable quality.
• Hardware Encoders (Intel QuickSync, NVIDIA NVENC, AMD VCE): HandBrake supports dedicated hardware AV1 encoding on compatible modern GPUs. These encoders process video almost instantaneously and use very little CPU, but they lack the granular efficiency and deep optimization tweaks offered by libaom.

Ultimately, libaom remains a critical component of HandBrake’s toolkit, serving as the gold standard for pure, high-efficiency AV1 archiving when encoding speed is not a primary concern.