Force Strict Keyframe Interval in libvpx-vp9

This article explains how to configure the libvpx-vp9 encoder in FFmpeg to enforce a strict maximum keyframe interval (GOP size). You will learn the specific command-line parameters required to disable scene-cut detection and lock keyframes to a precise, consistent interval, which is essential for adaptive bitrate streaming protocols like DASH and HLS.

To force a strict keyframe interval when encoding VP9 video with libvpx-vp9, you must define the maximum and minimum keyframe intervals and disable the encoder’s automatic scene-cut detection. Without disabling scene detection, the encoder will insert extra keyframes at scene changes, breaking the strict interval alignment required for multi-bitrate streaming segmenters.

The Core FFmpeg Parameters

You can enforce a strict keyframe interval using the following three parameters in your FFmpeg command:

• -g <value>: Sets the Group of Pictures (GOP) size, which is the maximum interval between keyframes (frames).
• -keyint_min <value>: Sets the minimum interval between keyframes. To force a strict interval, set this to the same value as -g.
• -sc_threshold 0: Disables scene-cut detection. Setting this to 0 prevents FFmpeg and the libvpx-vp9 library from inserting keyframes at arbitrary scene changes.

Step-by-Step Implementation

To calculate the correct -g and -keyint_min values, multiply your video’s frame rate (fps) by your desired keyframe interval in seconds.

For example, if your video is 30 fps and you want a strict keyframe interval of 2 seconds, your target frame interval is 60 frames (\(30 \times 2 = 60\)).

Example Command

Run the following command to encode a video with a strict 2-second keyframe interval at 30 fps:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -g 60 -keyint_min 60 -sc_threshold 0 -b:v 2M output.webm

Alternative Method: Time-Based Keyframe Forcing

If your source video has a variable frame rate (VFR), frame-based intervals might not result in perfectly timed segments. In this case, you can force keyframes at exact time intervals using the -force_key_frames expression:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -force_key_frames "expr:gte(t,n_forced*2)" -sc_threshold 0 -b:v 2M output.webm

In this command, "expr:gte(t,n_forced*2)" forces a keyframe precisely every 2 seconds, regardless of the frame rate, while -sc_threshold 0 ensures no additional keyframes are generated by scene changes.