Configure HEVC AMF Rate Control and Quality in FFmpeg

This article provides a practical guide on how to configure the rate control and quality settings for the AMD Advanced Media Framework (AMF) HEVC encoder (hevc_amf) in FFmpeg. You will learn about the available rate control modes, quality presets, and specific command-line arguments required to optimize your hardware-accelerated video encodes for streaming, archiving, or recording.

Rate Control Modes in hevc_amf

Rate control determines how the encoder allocates bitrate across the video frames. The hevc_amf encoder supports several rate control modes via the -rc parameter.

1. Constant QP (CQP)

Constant QP mode encodes every frame using a fixed quantization parameter. This mode prioritizes image quality over file size and is ideal for local recording and archiving. Lower QP values result in higher quality and larger file sizes.

• Command Example:

  ffmpeg -i input.mp4 -c:v hevc_amf -rc cqp -qp_i 22 -qp_p 22 output.mp4

• Key Parameters:

  • -qp_i: Sets the QP for I-frames (keyframes).
  • -qp_p: Sets the QP for P-frames.

2. Constant Bitrate (CBR)

CBR maintains a strict, consistent bitrate throughout the video. This is best suited for live streaming environments where bandwidth is limited and predictable data flow is required.

• Command Example:

  ffmpeg -i input.mp4 -c:v hevc_amf -rc cbr -b:v 5M output.mp4

• Key Parameters:

  • -b:v: Sets the target video bitrate (e.g., 5M for 5 Mbps).

3. Variable Bitrate (VBR Peak)

VBR adjusts the bitrate depending on the complexity of the scene, while ensuring the bitrate does not exceed a specified peak limit. This is highly efficient for general video playback and local storage.

• Command Example:

  ffmpeg -i input.mp4 -c:v hevc_amf -rc vbr_peak -b:v 5M -maxrate 8M output.mp4

• Key Parameters:

  • -b:v: Target average bitrate.
  • -maxrate: The maximum peak bitrate allowed for complex scenes.

Quality Presets and Optimization

To balance encoding speed and output quality, hevc_amf offers a direct quality preset option alongside other tuning parameters.

The -quality Parameter

You can set the overall encoding quality preference using the -quality option. This adjusts internal encoder settings to favor either execution speed or compression efficiency.

• speed: Prioritizes fast encoding and low latency; ideal for live streaming or older hardware.
• balanced: The default setting, providing a compromise between speed and compression quality.
• quality: Prioritizes visual fidelity and compression efficiency; ideal for archiving.

ffmpeg -i input.mp4 -c:v hevc_amf -quality quality output.mp4

Additional Quality Adjustments

For further optimization, you can combine rate control and quality presets with the following parameters:

• GOP Size (-g): Sets the keyframe interval. For streaming, a GOP size equal to double the framerate (e.g., -g 120 for a 60 fps video) is recommended.
• Profile (-profile:v): Sets the HEVC profile. Use main for standard 8-bit video, or main10 for 10-bit HDR content (if supported by your AMD GPU).

Comprehensive High-Quality Example

This command uses VBR with the highest quality preset, a defined keyframe interval, and a 10-bit color profile:

ffmpeg -i input.mp4 -c:v hevc_amf -rc vbr_peak -b:v 6M -maxrate 10M -quality quality -profile:v main10 -g 120 output.mp4