Rolling — the series
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Every row is a different camera
A shutter that reads its rows one after another images each of them from wherever the camera was at that instant, so a frame is a stack of projections indexed by height — a handscroll with the roll running down the picture. Its rays miss their own best centre by the spread of the eye's track, at a ratio of 0.988, and a global shutter's meet to 2 × 10⁻¹⁶ m.
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A turning frame can be straightened; a travelling one cannot
Read a frame row by row while the camera turns at a radian a second and every point is 21 px from where a global shutter would put it, at every depth alike. Turn each row's rays back and every point returns to six trillionths of a pixel, with no depth known. Travel at 3 m/s instead, and the best correction that needs no depth is exact at one distance and 21 px wrong at 2 m.
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A frame's shear knows travel only over depth
Read a frame row by row while the camera turns and travels, and every vertical post leans — the near ones more. The lean is the turn plus the travel over the post's depth, and that sum is all the frame holds: twice the travel past posts twice as far draws the same frame to eighteen decimal places. Two posts cannot separate turn from travel. A facade can, because a turn leans the edges of the frame more than its middle, but the two signals are 99.8 per cent alike, and reading them apart takes a pixel on every row.