FFmpeg framerate Filter: Convert FPS with Blending

The FFmpeg framerate filter allows you to alter a video’s frame rate by generating intermediate frames through temporal blending rather than simply duplicating or dropping frames. Unlike basic frame rate changers, it features built-in scene change detection that prevents cross-fade artifacts across shot transitions. This guide explains how the framerate filter works, details its primary parameters, and provides practical FFmpeg commands for achieving smooth frame rate conversion.

How the framerate Filter Works

When converting video from one frame rate to another (for example, from 24 fps to 60 fps), standard filters like -r or fps repeat or drop entire frames, which can introduce stutter or judder.

The framerate filter solves this by:

  1. Linear Blending: Creating intermediate frames by blending adjacent frames according to configurable interpolation windows.
  2. Scene Detection: Monitoring pixel variation between consecutive frames. If a transition exceeds a specified threshold, the filter marks it as a scene change and disables blending for that transition, preventing "ghosting" between unrelated scenes.

Key Filter Parameters

The framerate filter accepts several parameters configured in a key=value format:

  • fps: The target output frame rate. Can be specified as an integer (e.g., 60), float (e.g., 29.97), or rational expression (e.g., 30000/1001). Default is 50.
  • scene: The scene change detection threshold as a percentage (from 0.0 to 100.0). A frame difference higher than this value is treated as a cut. Default is 8.2. Lower values make the filter more sensitive to scene changes.
  • interp_start: The point within the transition window (from 0 to 255) where blending begins. Default is 15.
  • interp_end: The point within the transition window (from 0 to 255) where blending ends. Default is 240.
  • flags: Processing flags. You can set flags=scenecd to use scene change detection (enabled by default).

Basic Syntax

To perform a basic conversion, pass the filter into the -vf (video filter) argument:

ffmpeg -i input.mp4 -vf "framerate=fps=60" -c:a copy output.mp4

This command takes input.mp4, uses the default blending and scene change settings, and outputs a 60 fps video to output.mp4 while copying the audio stream without re-encoding.

Adjusting Scene Change Sensitivity

If you notice ghosting across cuts, decrease the scene threshold to make the detection more aggressive. If rapid motion inside the same scene is mistakenly triggering scene cuts (preventing smooth blending), increase the value.

ffmpeg -i input.mp4 -vf "framerate=fps=60:scene=6.0" -c:v libx264 -crf 18 -c:a copy output.mp4

In this command:

  • fps=60: Outputs at 60 frames per second.
  • scene=6.0: Decreases the detection threshold from the default 8.2 to 6.0, ensuring sharper cuts are respected.
  • -c:v libx264 -crf 18: Encodes the output with high visual quality using H.264.

Fine-Tuning Blending Behavior

The interp_start and interp_end parameters dictate the weight of the blend. By default, values below 15 show only the previous frame, and values above 240 show only the next frame, avoiding excessive blur when motion is minimal.

To create wider blending transitions across intermediate frames:

ffmpeg -i input.mp4 -vf "framerate=fps=59.94:interp_start=0:interp_end=255:scene=8.0" -c:v libx264 -preset slow -crf 20 -c:a copy output.mp4
  • interp_start=0 and interp_end=255 enforce full-range interpolation, ensuring every newly generated frame is a composite blend of its surrounding frames.