A frame is not a moment.

A rolling-shutter sensor does not expose a picture, it sweeps one. The rows are read one after another from the top, so the bottom of the frame is a later instant than the top. The gap between them is the readout time — and it is not the shutter speed, which decides how long each row is open, and not the frame rate, which decides how often the whole thing repeats. It is a third clock, it appears on no slate, and every artefact below is it multiplied by something.

The engine is free and needs no account. Which means an ordinary 130 degrees-a-second whip on an ordinary 16 ms sensor leans every vertical in the frame by 5.07 degrees, and you can know that before the day rather than in the grade. One multiplication does the work — displacement = image velocity × readout — and it is a lean when the motion is sideways and a stretch when it is vertical, which is why those two are one fault and not two. The same clock explains the banding: the readout decides how many bands a flickering light puts across the frame and the shutter decides how deep they are, independently. Run it backwards and a frame you already have measures the sensor. Every lane ships a worked example that costs nothing. Writing the report needs credits.