How to Use the FFmpeg Smartblur Filter

The FFmpeg smartblur filter is a powerful video processing tool used to reduce noise and smooth out details in a video while preserving sharp edges. This guide explains how the smartblur filter works, details its customizable parameters (including radius, strength, and threshold for both luma and chroma channels), and provides practical command-line examples to help you integrate it into your video editing workflow.

Understanding the Smartblur Parameters

The smartblur filter applies a selective blur. It calculates the difference between pixels, and if the difference is below a specified threshold, it applies the blur. If the difference is above the threshold, it assumes the pixel is part of an edge and leaves it sharp.

The filter uses the following syntax: smartblur=luma_radius:luma_strength:luma_threshold:chroma_radius:chroma_strength:chroma_threshold

You can also use the shorthand named parameters: * luma_radius (lr) / chroma_radius (cr): Allowed range is 0.1 to 5.0. This defines the size of the blur variance. A higher radius results in a wider blur area. * luma_strength (ls) / chroma_strength (cs): Allowed range is -1.0 to 1.0. Positive values blur the image, while negative values act as a sharpener. * luma_threshold (lt) / chroma_threshold (ct): Allowed range is -30 to 30. This determines how different a pixel must be from its neighbors to be considered an edge. A higher value means more pixels are blurred; a lower (or negative) value preserves more edges.

If chroma parameters are not specified, they default to the values set for the luma parameters.

Practical Command Examples

Here are three common ways to use the smartblur filter in your terminal.

1. Basic Noise Reduction (Smoothing)

To apply a moderate blur that smooths out skin tones or background noise while keeping the main outlines sharp, use:

ffmpeg -i input.mp4 -vf "smartblur=lr=1.5:ls=0.5:lt=-3" output.mp4

In this command: * lr=1.5 sets a moderate blur radius. * ls=0.5 applies a half-strength blur. * lt=-3 ensures that even subtle edges are protected from being blurred.

2. Advanced Independent Chroma and Luma Control

If you want to blur the color (chroma) channels more aggressively than the brightness (luma) channel to fix color bleeding:

ffmpeg -i input.mp4 -vf "smartblur=lr=1.0:ls=0.3:lt=-2:cr=3.0:cs=0.8:ct=-5" output.mp4

This applies a light, edge-preserving blur to the luma channel, but a much wider and stronger blur to the chroma channel to blend colors smoothly.

3. Image Sharpening

Because the strength parameter accepts negative values, you can use smartblur to sharpen an image without amplifying high-frequency noise:

ffmpeg -i input.mp4 -vf "smartblur=lr=1.0:ls=-0.8:lt=-5" output.mp4

By setting ls=-0.8 (negative strength) and a low threshold of -5, FFmpeg sharpens the existing edges in the video while leaving flat areas untouched, avoiding the introduction of unwanted grain.