Crop and speed

What it does

A crop shows part of the source instead of all of it — the centre half, one side, a square out of a wide clip.

A speed plays the picture faster or slower. The line narrated over it is unchanged, and so is how long the scene runs.

The standard behind it

• A crop is stored as fractions of the source, never pixels. The library holds phone video, screen captures and stills at every size, and a pixel rectangle that frames a face in a 4K clip is somewhere in the corner of a 720p one. Fractions survive the source being replaced.
• A rectangle that runs off the edge of the frame is refused rather than clamped. ffmpeg would clamp it — quietly producing a different rectangle from the one you drew, with nothing reporting a problem.
• The crop sides are forced to even numbers. Odd dimensions fail the encode outright, after the seek and the decode have already been paid for.
• Speed changes how much of the source a scene consumes, not how long the scene runs. At double speed a four-second line eats eight seconds of tape; if there is less than that, the last frame holds rather than the scene ending early.
• Speed applies to the picture alone. The line is narrated over the shot, so the source’s own sound is already replaced before any of this — there is nothing to stretch.