Cross correlation will let you pick similar obejcts out between two images.

You should learn about fourier transforms to understand some of the details.

Books

Jaehne Digital image processing 1991 Springer

Pratt Digital image processing 1991 Wiley

Gonzalez, Woods Digital image processing 1992 Adison Wesley

and Lim

To experiment you could start with subrtaction, use an offset to stop numbers going negative.

One way of looking at motion is that you are looking at tited objects in the 3D space of images. (stationary objects form pillars in this space, the same in every plane)

Good motion detection is critical to Mpeg4 encoding, so it is a very current field.

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