Boxblur is the fastest blur in libavfilter and is the right tool when you need to anonymize a face, hide a license plate, or soften a background. It is also the building block of many other effects — a soft vignette, a focus-pull, or a face-blur before a privacy review.

The command

ffmpeg -i in.mp4 -vf "boxblur=10:1" -c:v libx264 -crf 20 out.mp4

What it does

The two numbers are luma radius and luma power. A radius of 10 means each output pixel is the average of a 10-pixel neighborhood around it; the power of 1 means the average is taken once. Increase the radius for a stronger blur; increase the power (2 or 3) for a more uniform result that takes longer to compute. Add a second pair `boxblur=luma_r:luma_p:chroma_r:chroma_p` to also blur the color channels.

Requirements

Any video input. The filter is in libavfilter core. CPU-only, no hardware acceleration. For privacy-grade face blurring, increase the radius to 20 or higher so the region is unrecognizable.

Things to avoid

  • Using a radius below 5 for privacy. Small blurs leave enough structure to recover the original image, especially on a high-resolution source.
  • Blurring the entire frame. The `boxblur` filter has no region selector; if you need to blur only part of the frame, use the `crop` and `overlay` filters around it.
  • Combining boxblur with stream copy. The blur is a video filter, so the encoder must run. Use `-c:v libx264` (or your chosen encoder) explicitly.
  • Confusing boxblur with gblur. gblur is a Gaussian blur with a different falloff curve, which may be what you want for an organic look.