How to Use the FFmpeg gblur Filter

This guide provides a straightforward explanation of how to use the gblur (Gaussian blur) filter in FFmpeg to apply a smooth blur effect to your videos. You will learn the basic syntax of the filter, understand its key parameters like sigma and steps, and view practical command-line examples for achieving different levels of blur intensity.

Understanding the gblur Filter

The gblur filter applies a Gaussian blur to an input video. It is highly effective for softening images, obscuring sensitive details, or creating artistic background effects. The filter is controlled primarily by three parameters:

• sigma (or s): Determines the horizontal blur strength (standard deviation of the Gaussian formula). A higher value results in a stronger blur. The default value is 0.
• steps: Defines the number of steps used to approximate the Gaussian blur. More steps result in a smoother, higher-quality blur but require more processing power. The default is 1.
• sigmaV: Determines the vertical blur strength. If this is not specified, it defaults to the same value as sigma.

Basic Syntax

To apply a basic Gaussian blur to a video, use the -vf (video filter) flag followed by gblur and your desired parameters.

ffmpeg -i input.mp4 -vf "gblur=sigma=10" output.mp4

In this command, sigma=10 applies a moderate, uniform blur across both the horizontal and vertical axes of the video.

Practical Examples

1. Applying a Light Blur

For a subtle softening effect, keep the sigma value low.

ffmpeg -i input.mp4 -vf "gblur=sigma=3:steps=1" output.mp4

2. Applying a Heavy Blur

For a strong blur where details are completely obscured, increase the sigma value.

ffmpeg -i input.mp4 -vf "gblur=sigma=30:steps=3" output.mp4

Note: Increasing the steps to 3 ensures the heavy blur remains smooth without looking pixelated, though rendering will take longer.

3. Asymmetric Blur (Horizontal Only)

If you want to blur the video horizontally while keeping the vertical axis sharp, set sigma for horizontal strength and sigmaV to 0.

ffmpeg -i input.mp4 -vf "gblur=sigma=20:sigmaV=0" output.mp4

4. Blurring a Specific Portion of the Video (Timeline Editing)

You can use FFmpeg’s timeline editing capability to apply the blur filter only during a specific timeframe of the video using the enable option.

ffmpeg -i input.mp4 -vf "gblur=sigma=15:enable='between(t,5,10)'" output.mp4

This command applies a blur with a sigma of 15 only between the 5-second and 10-second marks of the video.