Correct Camera Lens Distortion with FFmpeg

Wide-angle lenses and action cameras often produce “barrel distortion,” a visual warping where straight lines curve outward near the edges of the frame. This guide provides a direct, step-by-step tutorial on how to use FFmpeg’s built-in lenscorrection filter to fix this distortion and flatten your video footage.

The lenscorrection Filter Syntax

The lenscorrection video filter in FFmpeg uses a mathematical model to adjust pixels based on their distance from the center of the image. The basic syntax for the filter is:

ffmpeg -i input.mp4 -vf "lenscorrection=cx=0.5:cy=0.5:k1=-0.1:k2=0.05" output.mp4

Understanding the Parameters

To effectively correct your footage, you need to adjust four key parameters:

• cx (Center X): The horizontal center of distortion, expressed as a fraction of the image width (ranging from 0.0 to 1.0). For most cameras, the distortion center is the exact middle of the frame, so this is set to 0.5.
• cy (Center Y): The vertical center of distortion, expressed as a fraction of the image height (ranging from 0.0 to 1.0). This is also typically set to 0.5.
• k1 (Correction coefficient 1): The primary factor for correcting the innermost distortion. This parameter is the most important for fixing barrel distortion.
• k2 (Correction coefficient 2): The secondary factor for correcting distortion near the outer edges of the frame.

How to Find the Right Values

Correcting barrel distortion requires finding the right negative values for k1 and k2.

1. For Barrel Distortion (curving outward): Set k1 to a negative value (e.g., -0.1 to -0.3).
2. For Pincushion Distortion (curving inward): Set k1 to a positive value (e.g., 0.1 to 0.3).
3. Fine-Tuning: Start by setting k2=0 and adjusting k1 until the lines near the center-middle of the screen are straight. Once the center is corrected, adjust k2 (usually a small negative or positive value) to straighten the lines at the extreme outer edges of the frame.

Practical Examples

Standard GoPro/Action Camera Correction: Action cameras often require moderate correction. The following settings are a common starting point for wide-angle 1080p footage:

ffmpeg -i input.mp4 -vf "lenscorrection=cx=0.5:cy=0.5:k1=-0.227:k2=0.022" output.mp4

Heavy Fisheye Correction: For extreme fisheye lenses, you will need a stronger negative k1 value:

ffmpeg -i input.mp4 -vf "lenscorrection=cx=0.5:cy=0.5:k1=-0.350:k2=0.050" output.mp4

Workflow Tip

Because finding the perfect coefficients requires trial and error, test your settings on a short, 5-second clip first to save processing time:

ffmpeg -ss 00:00:10 -i input.mp4 -t 5 -vf "lenscorrection=cx=0.5:cy=0.5:k1=-0.18:k2=0.012" test.mp4

Adjust the k1 and k2 values in the command until straight architectural lines (like walls or doors) in your test clip appear perfectly straight.