How to Use FFmpeg smartblur Filter with Custom Thresholds

This guide explains how to use the smartblur filter in FFmpeg to apply a smooth blur to your videos while preserving sharp edges. You will learn the exact syntax, how to adjust the custom threshold parameters for both luma and chroma channels, and see practical command-line examples to achieve the perfect balance between noise reduction and detail preservation.

Understanding the smartblur Syntax

The smartblur filter works by blurring areas of the video that have low contrast (flat areas) while leaving high-contrast areas (edges) untouched. The basic syntax for the filter is:

smartblur=luma_radius:luma_strength:luma_threshold[:chroma_radius:chroma_strength:chroma_threshold]

You can also use the shorthand parameter names:

• lr / cr (Luma/Chroma Radius): Controls the size of the blur. Allowed range is 0.1 to 5.0.
• ls / cs (Luma/Chroma Strength): Controls the intensity of the blur. Allowed range is -1.0 to 1.0. Positive values blur the video, while negative values sharpen it.
• lt / ct (Luma/Chroma Threshold): The crucial setting that defines what is considered an edge. Allowed range is -30 to 30.

A threshold value of 0 filters everything. A positive threshold filters only flat areas (preserving edges), while a negative threshold filters only edges (preserving flat areas).

Practical FFmpeg Command Examples

1. Basic Luma Blur with Custom Edge Preservation

To apply a moderate blur to the brightness (luma) channel while keeping the edges of objects sharp, use a positive threshold. Run the following command:

ffmpeg -i input.mp4 -vf "smartblur=luma_radius=3.0:luma_strength=1.0:luma_threshold=5" output.mp4

In this command: * luma_radius=3.0 sets a medium-sized blur radius. * luma_strength=1.0 applies the maximum blur effect within that radius. * luma_threshold=5 ensures that any pixel difference greater than 5 (an edge) is not blurred, keeping details sharp.

2. Advanced Blur (Custom Luma and Chroma Settings)

If you want to blur both the brightness details and the color (chroma) channels independently, you can define all six parameters. This is highly useful for smoothing out color blotches or digital noise:

ffmpeg -i input.mp4 -vf "smartblur=lr=2.5:ls=0.8:lt=4:cr=1.5:cs=0.5:ct=2" output.mp4

In this command: * lr=2.5:ls=0.8:lt=4 applies a strong, edge-preserving blur to the luma channel. * cr=1.5:cs=0.5:ct=2 applies a milder, more sensitive blur to the color channels to blend noisy color patches without bleeding over prominent edges.