What causes this discontinuity? A bug in bicubicresize?

Thanks for the observation, spent quite a bit of time on it. So when specific conditions are met, resize can fallback to a single Crop: and Now the question is that this is good or not. Seems that it was a planned behaviour.

It should be fine to fall back to a crop, because an integral (sub)pixel shift would mean all coefficients will be zero, except for one of them which will be 1, making it the same as a crop. But the other frames shouldn't be this blurry. I assume spline16resize similarly reduces to a crop when possible, but it doesn't exhibit this problem.

For 32bit float format I can see the same difference by looking at the Histogram so it's not a rounding issue. The exact peaks of histogram are smeared a bit for non-integer shifts giving the image a blurry look.

Yes, disabled the crop, at least it became consistently blurry. I don't think that this edge case speed gain is worth vs. consistent behaviour.

Probably the least disruptive solution, if not my preferred one :)