FFmpeg Smartblur Edge-Preserving Smoothing Guide

The smartblur filter in FFmpeg is an edge-preserving smoothing filter designed to reduce visual noise, compress flat textures, or soften skin without degrading the sharp lines and boundaries in a video. This guide explains how the filter functions, details each of its parameters, and provides practical command-line examples for video processing.

Understanding the Parameters

The smartblur filter works by calculating local variance: regions with low contrast are blurred, while areas with high contrast are recognized as edges and retained. The filter accepts six primary parameters:

  • luma_radius (lr): Controls the size of the blur footprint on luminance. Allowed values range from 0.1 to 5.0. A larger radius smooths wider areas. (Default: 1.0)
  • luma_strength (ls): Determines the intensity of the blur. Allowed values range from -1.0 to 1.0. Positive values blur flat regions, while negative values invert the effect to produce edge sharpening. (Default: 1.0)
  • luma_threshold (lt): The edge detection sensitivity threshold. Allowed values range from -30 to 30. Pixels with differences smaller than this threshold are smoothed, while differences exceeding it are treated as edges and left untouched. (Default: 0)
  • chroma_radius (cr): Sets the blur radius for the color channels (0.1 to 5.0).
  • chroma_strength (cs): Sets the blur strength for the color channels (-1.0 to 1.0).
  • chroma_threshold (ct): Sets the edge threshold for color channels (-30 to 30).

If chroma parameters are omitted, they automatically inherit the values defined for luma.

Basic Syntax

To use smartblur, pass it through FFmpeg's video filter flag (-vf):

ffmpeg -i input.mp4 -vf "smartblur=lr=1.5:ls=0.8:lt=-5" -c:a copy output.mp4

Practical Use Cases

1. Subtle Denoising and Skin Softening

For light noise removal or skin smoothing, use a moderate radius, a positive strength, and a negative threshold. A negative threshold prevents the filter from smoothing over subtle gradient shifts that define facial features or texture boundaries.

ffmpeg -i input.mp4 -vf "smartblur=lr=2.0:ls=0.6:lt=-8" -c:v libx264 -crf 18 -c:a copy output.mp4

2. Stylized Painterly Effect

Increasing both the radius and the threshold causes the filter to treat broader tonal variations as flat areas, producing a flat, painterly aesthetic while keeping distinct outlines intact.

ffmpeg -i input.mp4 -vf "smartblur=lr=5.0:ls=1.0:lt=15" -c:v libx264 -crf 18 -c:a copy output.mp4

3. Edge-Preserving Sharpening

By setting luma_strength to a negative value, the filter sharpens details that cross the defined threshold instead of smoothing them.

ffmpeg -i input.mp4 -vf "smartblur=lr=1.5:ls=-0.7:lt=-10" -c:v libx264 -crf 18 -c:a copy output.mp4

Best Practices for Tuning

  • Adjust lt first: Keep lr around 1.5 and ls at 1.0, then adjust lt between -15 and 10 until only the intended flat regions blur.
  • Preview short clips: Use -ss and -t (e.g., -ss 00:00:10 -t 5) to render short test segments to fine-tune the parameters before processing long files.