How to Animate Vignette Size Over Time in FFmpeg

This article demonstrates how to animate the vignette effect in FFmpeg by using mathematical expressions within the vignette filter. You will learn how to leverage built-in variables like time (t) and frame number (n) to create dynamic visual effects, such as pulsing vignettes or gradual fades, directly from the command line.

The FFmpeg vignette filter uses the angle parameter to control the size and reach of the dark vignette border. By default, this parameter is a static value (usually PI/5), but you can pass an equation wrapped in quotes to make it change dynamically as the video plays.

The Math behind the Animation

To animate the filter, you must write an expression using FFmpeg’s internal evaluation variables: * t: The current timestamp of the frame in seconds. * n: The sequential frame number (starting at 0). * w & h: The width and height of the input video. * PI: The mathematical constant Pi (~3.14159).

In the vignette filter, a larger angle value increases the size of the dark vignette area (bringing the darkness closer to the center), while a smaller value pushes the darkness to the outer edges.

Practical Examples

1. The Pulsing “Breathing” Vignette

To make the vignette smoothly expand and contract over time, you can use the sine wave function (sin). The following command oscillates the vignette angle around a base value of PI/5:

ffmpeg -i input.mp4 -vf "vignette='angle=PI/5+sin(t)*0.15'" -c:a copy output.mp4

• sin(t) fluctuates between -1 and 1 over time.
• Multiplying by 0.15 scales the intensity of the pulse so the effect remains subtle.

2. Gradual Fade-In Vignette

If you want the vignette to start completely invisible and slowly darken the edges of the video over a set number of seconds, you can use a linear multiplier paired with the min function to cap the maximum darkness:

ffmpeg -i input.mp4 -vf "vignette='angle=min(PI/3, t*0.1)'" -c:a copy output.mp4

• t*0.1 increases the angle by 0.1 radians every second.
• min(PI/3, ...) ensures that once the vignette reaches PI/3 (a moderately strong vignette), it stops growing and remains constant.

3. Frame-Based Fast Flicker

For a more chaotic or high-energy look (such as a horror film or old projector effect), you can base the math on the frame count (n) instead of time. This creates rapid transitions on every frame:

ffmpeg -i input.mp4 -vf "vignette='angle=PI/5+random(1)*0.05'" -c:a copy output.mp4

By substituting these expressions into your FFmpeg commands, you can customize the timing, frequency, and intensity of your vignette animations without needing to use external video editing software.