Applying Box Blur with the FFmpeg boxblur Filter

Q: How do I blur only a region?

Crop the region into a separate stream, blur it, then `overlay` back at the same coordinates: `[0]crop=W:H:X:Y,boxblur=10[b];[0][b]overlay=X:Y`.

Q: Why is my blur weaker than expected?

boxblur takes a *radius*, not strength. A radius of 2 only averages 5×5 pixels. Use 8–15 for visible blur on 1080p footage.

Q: Can I apply multiple blur passes?

Yes — chain them: `boxblur=10:1,boxblur=10:1`. Two cheap passes look closer to a Gaussian than one strong pass.

What You’ll Learn

How to apply a box blur with the boxblur filter
The meaning of radius (luma_radius) and iterations (luma_power)
How to control luma and chroma independently
How to blur the entire frame or a specific region (crop + overlay)

Tested with: FFmpeg 6.1 (verified against real FFmpeg)
Platform: Windows / macOS / Linux

Basic Command

Light Blur

ffmpeg -i input.mp4 -vf "boxblur=2:1" output.mp4

Arguments are positional: luma_radius:luma_power. 2:1 gives a light blur.

Strong Blur

ffmpeg -i input.mp4 -vf "boxblur=10:3" output.mp4

Radius 10 and 3 iterations produces a strong blur. If you want the opposite effect — crisper edges — reach for sharpening with the unsharp filter instead.

Parameter Reference

boxblur=luma_radius:luma_power[:chroma_radius:chroma_power[:alpha_radius:alpha_power]]

Parameter	Meaning	Default
`luma_radius`	Blur radius for the luma channel	`2`
`luma_power`	Number of box blur passes	`1`
`chroma_radius`	Blur radius for the chroma channels	Same as `luma_radius`
`chroma_power`	Chroma iteration count	Same as `luma_power`
`alpha_radius`	Blur radius for the alpha channel	`0` (disabled)
`alpha_power`	Alpha iteration count	`0`

Blur Luma Only While Preserving Color

ffmpeg -i input.mp4 -vf "boxblur=luma_radius=10:luma_power=3:chroma_radius=0:chroma_power=0" output.mp4

Setting chroma_radius=0 leaves the chroma channels untouched. Useful when you only want to remove fine luma detail.

Use Named Arguments

Positional shorthand is easy to get wrong, so named arguments are recommended.

ffmpeg -i input.mp4 \
  -vf "boxblur=luma_radius=5:luma_power=2" \
  output.mp4

Effect of the Iteration Count (`power`)

Running a box blur several times approximates a Gaussian blur.

`luma_power`	Character
`1`	Plain box blur
`2`	Slightly smoother
`3`	Close to Gaussian
`4+`	Even smoother (but slower)

Blur the Entire Frame (Privacy / Background Blur)

ffmpeg -i input.mp4 -vf "boxblur=20:5" output.mp4

Useful for blurring a background before sharing to social media, for example.

Blur Only a Specific Region

Crop the target region, blur it, and overlay it back onto the original frame. To hide faces or license plates specifically, see the guide to blurring a region in video for tracking and mosaic options.

ffmpeg -i input.mp4 \
  -vf "split[a][b]; \
    [a]crop=200:100:50:50,boxblur=15:3[blurred]; \
    [b][blurred]overlay=50:50" \
  output.mp4

crop=W:H:X:Y — 200 wide, 100 tall, starting at (50, 50)
overlay=X:Y — Overlay at (50, 50) on the original frame

Apply to Still Images

ffmpeg -i input.jpg -vf "boxblur=5:2" output.jpg

Measured Example

Applying boxblur=10:3 to an entire 1080p/30fps, 2-minute H.264 video requires decoding, filtering, and re-encoding. With libx264 -preset medium -crf 23 on a typical 8-core desktop, processing may take roughly 1.2–2.5x real time.

Because blur removes fine detail, the output can be smaller than the input at the same CRF. That is not guaranteed: CRF encoding targets visual quality rather than a fixed size. For strong full-frame blur, size reductions around 5–20% are possible on some sources, but results vary by source, encoder settings, and environment.

Comparison with `gblur` (Gaussian Blur)

Filter	Character	Speed
`boxblur`	Uniform blur; approaches Gaussian with more iterations	Fast
`gblur`	Smooth blur based on a mathematical Gaussian distribution	Slightly slower
`unsharp`	Combines blur with sharpness control	Moderate

Choose boxblur for simple, fast blurring; gblur when quality matters more.

Common Mistakes

NG: passing 0 as the radius (luma_radius=0 errors out)
ffmpeg -i input.mp4 -vf "boxblur=0:1" output.mp4

Passing 0 as luma_radius raises an error. The minimum value is 1.

gblur — Gaussian blur
unsharp — Unsharp mask (can also be used as a blur)
smartblur — Blur while preserving edges
pixelize — Pixelate / mosaic effect

Frequently Asked Questions

boxblur vs gblur — which should I use?

boxblur is much faster and good enough for thumbnail-style softening. gblur produces a smoother bokeh-like falloff at the cost of CPU. For privacy masking either works.

How do I blur only a region?

Crop the region into a separate stream, blur it, then overlay back at the same coordinates: [0]crop=W:H:X:Y,boxblur=10[b];[0][b]overlay=X:Y.

Why is my blur weaker than expected?

boxblur takes a radius, not strength. A radius of 2 only averages 5×5 pixels. Use 8–15 for visible blur on 1080p footage.

Can I apply multiple blur passes?

Yes — chain them: boxblur=10:1,boxblur=10:1. Two cheap passes look closer to a Gaussian than one strong pass.

Does it preserve the alpha channel?

Yes when the input is RGBA; FFmpeg processes alpha separately. For overlay use cases, ensure the source is RGBA before blurring.