Turn a 16:9 clip into a 9:16 clip with a crop window that follows a subject along positions you…

Guide ID: art_01M781ZHVS1Q6W0TXZJH16SNJ1

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Guide title

Turn a 16:9 clip into a 9:16 clip with a crop window that follows a subject along positions you set, using ffmpeg on Windows

Coverage

Made for anyone delivering a portrait cut of widescreen footage while keeping the action inside the frame, provided they can note where the action sits at a few points in time. A Windows PowerShell script turns a short list of time and horizontal position pairs into an ffmpeg crop expression, slides a full-height window along straight lines or jumps between positions, re-encodes the picture, passes the sound through, and then compares dimensions, frame count, length and number of audio tracks. Only generated clips with a known moving box were used; real footage is untested, and nothing here finds or smooths the subject for you.

Example

On a 10-second test clip with a box moving from x=300 to x=900, the script kept the box at the centre of the 404x720 window at every measured time, while a window parked in the middle lost the box for most of the clip.

Useful starting points

• The window only moves as your keyframes say: add keyframes where the subject changes direction.
• A PASS checks size, frames, duration and sound, not framing: look at a frame at each keyframe time.
• Portrait, rotated-to-portrait and non-square-pixel sources are refused rather than cropped wrongly.