It is documented: seethis. Note that you have to look at the documentation forvision.videoFileReader in the Computer Vision System toolbox, and not the documentation forVideoReader.
Note also that there isn't a "step instruction" in MATLAB.step here is a method of thevision.videoFileReader class.
So far, Mathworks has avoided using step() as the name of any toolbox routine. However, it is used as the name of amethod in a number of different location, so you have to look at the object type of what you are calling step() on to determine what effect it has.
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