Configure FFmpeg Lagfun Filter Decay Factors

This article explains how to configure the decay factor for the luma and chroma channels using the FFmpeg lagfun video filter. You will learn the specific parameters used to target individual video planes and see practical command examples to achieve custom temporal fading and motion trail effects.

The lagfun filter is a temporal effect in FFmpeg that creates a trail or “ghosting” effect by slowly fading out pixels from previous frames. The speed of this fade is controlled by the decay factor, where a value of 0.0 means instant decay (no effect) and 1.0 means no decay (the pixel remains indefinitely).

To configure the decay factors for the luma and chroma channels individually, you must target the specific plane parameters of the filter:

Basic Syntax

The basic syntax for specifying these channels in your filtergraph is:

lagfun=decay1=value:decay2=value:decay3=value

Practical Examples

1. High Luma Lag with Fast Chroma Decay

If you want a strong, bright motion trail but want to prevent color bleeding or rainbow-like smearing, you can set a high decay for the luma channel and a low decay for the chroma channels:

ffmpeg -i input.mp4 -vf "lagfun=decay1=0.95:decay2=0.5:decay3=0.5" output.mp4

In this command: * decay1=0.95 keeps the brightness/detail details on screen for a long time. * decay2=0.5 and decay3=0.5 force the color information to fade quickly, keeping the trails mostly monochromatic or naturally colored.

2. High Chroma Lag with Fast Luma Decay

To create an artistic, psychedelic color-bleeding effect where colors smear across the screen but the sharp details of the video update normally, reverse the settings:

ffmpeg -i input.mp4 -vf "lagfun=decay1=0.2:decay2=0.98:decay3=0.98" output.mp4

In this command: * decay1=0.2 ensures the black-and-white structural details of the video do not lag. * decay2=0.98 and decay3=0.98 create extreme, long-lasting trails for the color channels.