Video Cropper
Crop video framing to remove black bars or change aspect ratio directly in your browser. Adjust crop boundaries locally with no uploads and zero data limits.
MP4, WebM, MOV, MKV, AVI, FLV, 3GP · up to 1 GB
Tips
- The video engine downloads about 30 MB once on your initial visit and runs offline afterward.
- Keep crop width and height set to even pixel values to prevent H.264 encoder errors.
- Use the automatic detection option to identify and remove hardcoded letterbox borders.
How to use it
- 1 Open or drag your video file into the designated processing area.
- 2 Adjust the crop rectangle over the preview or use automatic border detection.
- 3 Verify that the output width and height are even numbers.
- 4 Process the video and download the cropped file to your device.
What you should know
Video footage often contains unnecessary background space, letterboxing from mismatched display standards, or awkward framing when converting horizontal widescreen recordings for vertical mobile feeds. Standard video streams rely on chroma subsampling schemes like YUV 4:2:0, where colour information is stored across 2x2 pixel blocks. Cropping allows you to isolate the primary subject or eliminate hardcoded black bars without stretching pixel aspect ratios. Because re-encoding modifies spatial pixel grids, selecting the proper target frame size ensures sharp playback across modern operating systems and web browsers while cutting down overall file size by dropping unused frame areas.
This tool processes your video locally using a client-side WebAssembly engine, so raw footage never leaves your computer. You can define exact rectangular dimensions or detect outer borders to eliminate letterboxes. Always maintain even width and height values, as odd pixel counts trigger encoder failures in standard H.264 codecs. Cropping requires re-encoding the visual track, which can slightly alter bitrate and discard non-standard metadata tags or subtitle tracks, though primary audio streams are preserved. Because processing runs in a single browser thread, high-resolution and long videos will take multiple times their playback duration to render.
Frequently asked questions
Why does cropping require re-encoding the video?
Video codecs store frames using spatial inter-frame compression and shared macroblocks. Changing the visible frame boundaries requires decoding the video stream, slicing off pixels, and recompressing the remaining frame data into a new container.
Why must crop dimensions be even numbers?
Most video encoders use YUV 4:2:0 chroma subsampling, which groups colour channels into two-by-two pixel blocks. Setting an odd width or height splits these chroma blocks and causes the video encoder to fail.
Why is exporting high-resolution video slow in the browser?
The WebAssembly video engine operates in a single thread inside your browser tab without direct hardware graphics acceleration. Processing times depend on video resolution and length, often taking several times the video's actual duration.
Guides
Articles and format references related to this tool.
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