Video Prep: crop, find cuts, cover a logo, trim and mute in your browser

Reference-video and recast models copy everything in the clip you give them except the people: black bars, a corner logo, burnt-in captions and the original soundtrack all end up in every video made from it. Cropping the bars and the watermark, keeping a single shot of 5–15 seconds and dropping the audio gives the model a clean template. The crop is a straight rectangle – it keeps the full scene inside it rather than cutting out a person – so the look of the original stays intact.

The clip is cut to your start and end points, cropped to the rectangle you set, scaled down only if it is taller than 1080p, and re-encoded as H.264 MP4 at 30 fps with the “faststart” flag, so it plays and uploads anywhere. With “Remove sound” on the audio track is dropped; off, it is kept as AAC. Dimensions are rounded to even numbers, which most video models require.

Reference-video models want a single shot of at most 15 seconds and a clip that starts and ends on a cut, so the motion is complete. “Find cuts” runs a scene-change scan on a small copy of the clip (a 40-second 1080p clip takes about half a minute on a phone) and lists the cut times. Tap a cut to start there, then “15 s to the last cut” to end on the last cut inside the limit – or tap another cut with “end” selected. The 4:3 centre crop keeps both people and the middle of a wide 16:9 performance clip; 9:16 is for vertical clips that are already framed for phones.

Everything runs in your browser with ffmpeg.wasm, so long or very large clips take longer – a 15-second 1080p clip exports in roughly a minute on a recent phone. Keep source clips under about 60 seconds and 100 MB. If the browser cannot open the file, convert it to MP4 first; HEVC/H.265 videos from some phones do not play in every browser. Black-bar detection samples three frames and looks for near-black rows and columns; if the bars are dark grey or have a logo in them, set the crop by hand.