FFmpeg Lagfun Filter: Apply Temporal Lag Effects

This guide explains how to use the lagfun video filter in FFmpeg to create a temporal lag effect, often recognized as ghosting, motion trails, or video echoing. You will learn the filter's syntax, its core parameters, and practical command examples to control how fast or slow motion trails decay in your video projects.

What is the Lagfun Filter?

The lagfun filter computes a temporal decaying sum across video frames. Instead of moving objects disappearing instantly from their previous positions, each pixel value slowly fades over time. This generates an artistic visual trail behind moving elements, making it ideal for simulating long exposure, light painting, or psychedelic motion blur.

Syntax and Parameters

The basic syntax for using the filter is:

-vf "lagfun=decay=<value>:planes=<value>"

The filter accepts two primary parameters:

  • decay: Sets the decay factor for the temporal lag. The value must be a floating-point number between 0.0 and 1.0. The default value is 0.95. A higher number keeps trailing pixels visible for longer, while a lower number makes trails fade quickly.
  • planes: Specifies which color planes to filter. By default, it applies to all planes (15 or all channels). You can limit it to luminance or specific chroma channels if you want stylized color-lag effects.

Basic Command Example

To apply a standard temporal lag effect to a video, run the following command:

ffmpeg -i input.mp4 -vf "lagfun=decay=0.95" -c:a copy output.mp4

In this command, -vf "lagfun=decay=0.95" applies the default decay factor, and -c:a copy preserves the original audio stream without re-encoding.

Adjusting Trail Length

You can customize the intensity and length of the trail by modifying the decay value:

  • Subtle Motion Blur / Short Trails: A lower decay value creates a subtle smearing effect suitable for fast action.

    ffmpeg -i input.mp4 -vf "lagfun=decay=0.7" -c:a copy output_subtle.mp4

  • Extreme Ghosting / Long Light Trails: A value close to 1.0 makes the trails linger for several seconds, which works best on high-contrast videos with light elements against dark backgrounds.

    ffmpeg -i input.mp4 -vf "lagfun=decay=0.99" -c:a copy output_extreme.mp4

Isolating Color Planes

You can apply the lag effect exclusively to specific color components. For example, to apply the effect only to the luminance channel (plane 1 in YUV) while leaving the color unaffected:

ffmpeg -i input.mp4 -vf "lagfun=decay=0.9:planes=1" -c:a copy output_luma.mp4

Alternatively, targeting only the chroma channels (planes 2 and 4, or planes=6) will cause color trails to smear while maintaining sharp edge details in the luminance channel.