FFmpeg FPS Filter: Custom Frame Rate Drop and Dup

This article provides a direct guide on using FFmpeg’s fps video filter to alter a video’s frame rate while controlling exactly how frames are duplicated or dropped. You will learn the syntax of the fps filter, how its rounding mathematical rules determine frame selection, and how to implement these settings in your command-line workflows.

To change a video’s frame rate, FFmpeg must decide which original frames to keep, drop, or duplicate to fit the new timeline. While the basic -r option can change frame rates, the fps video filter (-vf "fps=...") offers precise control over this process by exposing parameters for rounding and timestamp behavior.

Basic Syntax of the fps Filter

The most basic usage of the filter defines only the target frame rate:

ffmpeg -i input.mp4 -vf "fps=fps=30" output.mp4

In this command, FFmpeg converts the input video to 30 frames per second. By default, it uses “near” rounding, which maps input frames to the closest target timestamps.

Customizing Drop and Duplicate Rules with round

To customize how FFmpeg drops or duplicates frames, you must use the round option. The round parameter tells FFmpeg how to handle timestamps that fall between target frame intervals.

The syntax is:

-vf "fps=fps=target_fps:round=rounding_method"

There are five rounding methods available:

• near (Default): Rounds to the nearest target timestamp. This creates the smoothest visual transition by choosing the closest available frame.
• zero: Rounds towards zero.
• inf: Rounds away from zero.
• down: Rounds down towards negative infinity. This is useful if you want to bias the filter toward dropping frames rather than duplicating them.
• up: Rounds up towards positive infinity. This biases the filter toward duplicating frames earlier in the timeline.

Example: Forcing Frame Dropping (Rounding Down)

If you are downsampling a video (e.g., 60 fps to 24 fps) and want to strictly drop frames mathematically rather than letting FFmpeg interpolate to the nearest neighbor, use down:

ffmpeg -i input.mp4 -vf "fps=fps=24:round=down" output.mp4

Example: Biasing Towards Duplication (Rounding Up)

If you are upsampling (e.g., 25 fps to 30 fps) and want to bias the duplicate frames upward to shift the timeline slightly forward, use up:

ffmpeg -i input.mp4 -vf "fps=fps=30:round=up" output.mp4

Managing the Start and End of the Video

To further customize frame rate conversion, you can control how FFmpeg handles the very beginning and the very end of the stream.

Setting the Start Time

By default, the fps filter assumes the first frame starts at timestamp 0. If your input video has a presentation timestamp (PTS) offset, this can cause unexpected frame drops or duplicates at the start. You can manually define the starting timestamp using the start_time option:

ffmpeg -i input.mp4 -vf "fps=fps=30:start_time=0" output.mp4

Defining End of File (EOF) Behavior

You can control how the filter behaves when it reaches the end of the video using the eof_action parameter:

• round (Default): Applies the same rounding rule to the final remaining frame.
• pass: Passes the exact source end timestamp through, which can sometimes result in a variable frame rate for the very last frame to prevent unnecessary duplication.

ffmpeg -i input.mp4 -vf "fps=fps=30:round=near:eof_action=pass" output.mp4