FFmpeg Cookbook
JA

Black Mask, Mosaic & Blur Video

Black out, pixelate or blur a fixed video region. Set its coordinates, dimensions, time interval and whether to remove audio.

Your file is not uploaded
Drop a video, or tap to select
MP4 / MOV / WebM
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Preview (drag to move; corner handles to resize)
Fixed region, first frame. Review movement throughout the output.
The rectangle is fixed throughout the clip (no face tracking). For moving subjects, run the tool multiple times on different time ranges.

Blur and pixelation do not guarantee unreadability. Use an opaque mask for sensitive text and review the entire clip. Audio needs a separate check.

Processing… 0%

Equivalent FFmpeg command

CLI command for your current settings. Updates live as you change controls.

ffmpeg -y -i input.mp4 -filter_complex "[0:V:0]scale=1920:1080:flags=bilinear,setsar=1,pad=ceil(iw/2)*2:ceil(ih/2)*2,format=yuv420p,drawbox=x=480:y=270:w=640:h=360:color=black:t=fill[v]" -map "[v]" -map "0:a?" -c:a aac -b:a 128k -map_metadata -1 -map_metadata:s -1 -map_chapters -1 -c:v libx264 -preset fast -crf 23 -pix_fmt yuv420p -movflags +faststart output.mp4

Checks on a synthetic fixture

On 2026-09-10, native FFmpeg 8.1 processed a 3-second 320x180 test pattern with two tone tracks, subtitles, a chapter and synthetic tags. This is not browser verification.

Synthetic source
Synthetic source
Black mask
Black mask
Pixelated region
Pixelated region

These checks do not certify real faces, documents, speech, phone recordings or audio codec transition boundaries. The pixelated frame is a visual example, not proof of unreadability.

Conditions, results, commands and SHA-256 · Pre-sharing checks

Popular uses
Hide faces or platesProtect screen-share detailsBlur selected areas
Next tools
Crop Video Tool Video Compressor Video Watermark Tool

How to use

  1. Check the first frameSelect a video and its first frame appears. If the preview cannot be shown, processing stays off, because the region cannot be checked.
  2. Cover the subject with a marginAdjust the rectangle. Choose black masking for names or addresses and check audio separately.
  3. Watch the whole outputCheck movement, reflections, other scenes, burned-in captions and conversations before sharing.

Recommended settings

Email, address or notification in a recording
Black mask with a margin, then full playback review
Light blurring may preserve letter shapes and lengths.
Style a background
Blur or pixelation
Useful visual effects, but not guarantees that personal information is unreadable.
A face moves across the screen
Cover its whole path or use a tracking-capable editor
This tool does not track subjects. A mask on the first frame may miss them later.

When it goes wrong

The subject becomes visible later

Cause: The rectangle is fixed and does not follow faces or objects.

Fix: Cover the entire path or use an editor with tracking.

No preview appears

Cause: An unsupported browser codec, damaged media or a preview timeout may be responsible.

Fix: Convert to a supported MP4 and select it again. Processing without a visible region is disabled.

Text remains recognizable after blurring

Cause: The region and effect may retain information.

Fix: Switch to black masking, include a margin and review the entire output.

Questions

Does strong pixelation guarantee privacy?

No. Text, faces, context and audio can still support identification. Prefer black masking for confidential text and consider whether sharing is appropriate at all.

Can I mask several places at once?

There is one rectangle per run. You can process the output again, but each pass re-encodes the video. An editor with multiple masks is better for complex work.

Does it track faces automatically?

No. The preview is the first frame and processing uses fixed coordinates. Watch the whole output to check that the subject never leaves the mask.

Are audio and subtitles retained?

Audio remains unless you select removal. Separate subtitles, data, attached pictures and chapters are excluded. Burned-in captions remain outside the masked area.

What are the output format and quality?

The video is re-encoded as H.264 (CRF 23) MP4, so it is not identical to the original. The size matches how the browser displays it, and an odd edge is padded by one pixel. HDR and other 10-bit video comes out as 8-bit.

More detail (settings, limits, FFmpeg commands)

What each setting means

Rectangle position and size
A fixed position chosen on the first frame. Processing expands outward to even coordinates instead of shrinking the selection.
Effect and strength
Black masking fills the region with opaque black. Blur and pixelation strength (Low, Medium, High) only changes the look; it does not guarantee the content becomes unreadable.
Time interval
Unchecked applies throughout. Otherwise enter 0-7200 seconds with end after start. Frames outside the interval remain unmasked.
Remove all audio
Checked excludes every audio track. Otherwise all audio is retained and re-encoded to AAC.

Browsers and limits

  • Maximum 500 MB; the browser must be able to decode the first frame.
  • One fixed rectangle; no automatic tracking or simultaneous multiple masks.
  • Output is re-encoded. HDR and other 10-bit video becomes 8-bit.
  • The black-mask measurements cover a region of a synthetic video; they do not prove that real people or documents become unidentifiable.

The same job with the FFmpeg command line

CLI example for a fixed black mask
ffmpeg -i input.mp4 -vf "drawbox=x=36:y=18:w=84:h=48:color=black:t=fill" -map 0:V:0 -map 0:a? -c:v libx264 -crf 23 -preset fast -pix_fmt yuv420p -c:a aac -b:a 128k -map_metadata -1 -map_metadata:s -1 -map_chapters -1 -movflags +faststart redacted.mp4
These coordinates target a 320x180 synthetic fixture. Adjust them for your own video.

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