# Remove Black Bars from a Video Auto-detect and crop letterbox or pillarbox black bars from a video. Two-pass ffmpeg cropdetect runs in your browser — nothing is uploaded. ## Run it - **CLI:** `gizza tool video-autocrop-bars 'url=https://example.com/input' 'threshold=24' 'round=2'` - **Web:** https://gizza.ai/tools/video-autocrop-bars/ - **Agents:** machine-readable descriptor (parameters JSON Schema) at https://gizza.ai/tools/video-autocrop-bars/tool.json ## Inputs - `file` — Video _(file; accept: video/*)_ - `threshold` — Black threshold (0–255) _(field)_ - `round` — Snap crop to multiple of _(field)_ ## Output - Cropped video (video) ## Query parameters Open the tool pre-filled and auto-run via URL: - `threshold` — Black threshold (0–255) - `round` — Snap crop to multiple of - `url` — fetch the input file from a public URL (CORS-permitting) Example: `https://gizza.ai/tools/video-autocrop-bars/?url=https://example.com/input&threshold=24&round=2` --- ## About this tool Black bars appear when a video's aspect ratio doesn't match the frame it was packaged in: **letterboxing** (bars above and below, e.g. a widescreen film inside a 4:3 frame) and **pillarboxing** (bars left and right, e.g. a phone clip inside a 16:9 frame). They waste pixels and bitrate, look bad on re-uploads, and defeat auto-cropping thumbnails. This tool removes them automatically, in two passes — the same technique video engineers use with ffmpeg by hand: 1. **Detect** — ffmpeg's `cropdetect` filter scans **every frame** of the clip and keeps the union of all picture content it sees. Because the box only ever grows, a fade-from-black opening or a dark scene can't trick it into cropping real content. 2. **Crop** — the detected rectangle is applied with `crop=W:H:X:Y` and the video is re-encoded with H.264 at CRF 18 (visually lossless tier). Audio is stream-copied untouched when the container allows it. Everything runs in your browser via WebAssembly ffmpeg — the file is never uploaded. ### Worked example A **320×240** clip that's really a **320×180** widescreen picture with 30-pixel black bars top and bottom (classic 16:9-inside-4:3 letterbox): - the detect pass settles on `crop=320:180:0:30` — 320×180 of real picture, starting 30 px down; - the crop pass re-encodes just that rectangle; - the result is a **320×180** video with the bars gone. If the clip had no bars at all, the tool says so instead of silently re-encoding it. Leave the defaults (**threshold 24**, **snap 2**) for normal black bars. If your source was heavily compressed and the "black" is really dark grey, raise the threshold to around **48**. Pick **snap 16** if a downstream encoder or player prefers macroblock-aligned dimensions (the crop may keep up to 15 extra bar pixels per axis). ## Limits and edge cases - Input is capped at **25 MB** in chat/CLI; in the browser the practical limit is your device's memory. - The crop rectangle is **one box for the whole clip** — content that changes aspect ratio mid-video (e.g. a trailer switching between scope and flat) gets the union box, keeping everything visible. - **Threshold 0** is the strictest setting: typical "black" bars in limited-range video sit at luma 16, so threshold 0 usually reports *no bars detected* — that's the tool refusing to guess, not a bug. - **Threshold 255** makes every pixel count as black; the tool reports that the whole frame reads as black and asks you to lower the threshold. - Cropping requires re-encoding (an ffmpeg stream copy cannot crop), so the video track is always re-encoded; CRF 18 keeps that visually transparent. ## FAQ
How does it know where the bars are? It runs ffmpeg's `cropdetect` filter over the whole clip. Each frame, the filter measures the smallest rectangle that contains everything brighter than the black threshold; with the running-maximum mode used here, those rectangles are merged into a union box across all frames. The final box is the picture; everything outside it is bars. The crop values are read from the detection log, then applied in a second pass.
Why did it say "no black bars detected"? The detected picture box covered the entire frame. Either your video genuinely has no bars, or the bars aren't dark enough to count as black at the current threshold — heavily compressed sources often have noisy, dark-grey bars. Raise the **black threshold** (try 48, then 64) and run again. The tool prefers telling you over re-encoding a video it wouldn't change.
Does it lower my video quality? The crop itself is pixel-exact, but the video track must be re-encoded (ffmpeg filters can't run in stream-copy mode). It re-encodes with H.264 at **CRF 18**, which is generally considered visually lossless. Audio is copied through untouched when the input container is MP4, MOV, M4V or MKV; other containers (e.g. WebM) are rewritten to MP4 with AAC audio.
Can it remove white or colored borders too? No — detection is luma-based black detection (that's what `cropdetect` measures), which covers the letterbox/pillarbox bars that video players and encoders add. For colored borders you know the size of, use a manual crop tool and give it the exact rectangle.
What do the two settings actually do? **Black threshold (0–255)** is how dark a pixel must be to count as bar: 24 (the ffmpeg default) matches true black bars; higher values catch grey-ish bars but risk eating into dark scenes. **Snap crop to multiple of** rounds the cropped width/height down to a multiple of 2, 4, 8 or 16: 2 removes the most bar while keeping dimensions H.264 legal, 16 yields the most encoder-friendly dimensions at the cost of possibly keeping a sliver of bar.
## Related tools - [Pad a Video to an Aspect Ratio](https://gizza.ai/tools/video-aspect-pad/): Letterbox or pillarbox a video to 9:16, 1:1, 16:9 or any preset ratio with colored bars or a blurred background — free, in your browser, nothing is uploaded. - [Crop a Video](https://gizza.ai/tools/video-crop/): Crop a video to any rectangle in your browser — set width, height, and an optional x/y offset (centered by default). Re-encodes locally with ffmpeg, free. - [Convert MKV to MP4](https://gizza.ai/tools/mkv-to-mp4/): Free MKV to MP4 converter that runs in your browser. Lossless stream-copy remux for H.264/HEVC clips, or re-encode VP9/AV1. Nothing is uploaded. - [Convert MOV to MP4](https://gizza.ai/tools/mov-to-mp4/): Free MOV to MP4 converter that runs in your browser. Lossless container remux (stream-copy) for H.264/HEVC clips, or re-encode ProRes. Nothing is uploaded. - [Convert MP4 to MKV](https://gizza.ai/tools/mp4-to-mkv/): Free MP4 to MKV converter in your browser. Lossless stream-copy remux — keeps video, audio and subtitle tracks, nothing re-encoded or uploaded.