How to Use Contrast Adaptive Sharpening in FFmpeg

This article provides a straightforward guide on how to use the Contrast Adaptive Sharpening (cas) filter in FFmpeg. You will learn the basic syntax of the filter, understand its primary parameters, and see practical command-line examples to help you sharpen your videos while preserving natural details and minimizing visual artifacts.

Understanding the CAS Filter

The cas filter in FFmpeg is based on AMD’s FidelityFX Contrast Adaptive Sharpening technology. Unlike traditional sharpening filters that can introduce ugly halos or amplify compression noise, CAS adapts to the local contrast of the image. It applies stronger sharpening to softer areas and less sharpening to already detailed edges, resulting in a cleaner, more natural-looking enhancement.

Basic Syntax

To use the CAS filter in FFmpeg, you apply the video filter flag (-vf or -filter:v) followed by cas.

The basic syntax is:

ffmpeg -i input.mp4 -vf "cas=sharpness" output.mp4

Parameters

• sharpness: This parameter controls the strength of the sharpening effect.
  • Value Range: 0.0 to 1.0.
  • Default: 0.0 (which provides no sharpening).
  • A value of 1.0 applies the maximum level of sharpening. For most videos, a value between 0.3 and 0.7 yields the best balance between clarity and noise control.

Practical Examples

1. Applying Moderate Sharpening

To apply a moderate, clean sharpen to your video, use a sharpness value of 0.5:

ffmpeg -i input.mp4 -vf "cas=0.5" -c:v libx264 -crf 18 -c:a copy output.mp4

2. Combining Scaling and Sharpening

Contrast Adaptive Sharpening is highly effective when applied after upscaling a video. In this example, the video is upscaled to 1080p and then sharpened with CAS to restore lost edge clarity:

ffmpeg -i input.mp4 -vf "scale=1920:1080,cas=0.6" -c:v libx264 -crf 18 output.mp4

3. Maximum Sharpening for Soft Footage

If your source footage is very soft or slightly out of focus, you can push the filter to its maximum limit:

ffmpeg -i input.mp4 -vf "cas=1.0" -c:v libx265 -crf 20 output.mp4

Using these commands, you can easily integrate Contrast Adaptive Sharpening into your FFmpeg transcoding workflows to improve the visual quality of your video library.