Thanks for replying, @ptrblck
Currently all images are together in test, train and val folders. If amount not divisible by 3, dropping the last frame is no problem . The frames from a single video need not be contiguous. It’s fine as long as they are from the same video.
The video folders (I can change my dataset to that format) can be shuffled freely. There’s no order among the videos, and the order of the frames in a single video can also be ignored.