The algorithm is the second step of the four passway. An algorithm will execute some kind of function on a sequence of images and end up with a black and white image; just like the image below.

The white pixels on the image will indicate the object of interest. For example, when looking for motion, the pixels that have been changed; it depends on what the algorithm is looking for. The black pixels are background pixels.

Black and white image

The only convention that an algorithm should meet is that it should return a black and white image to the next step; the expositor. And of course that it requires a sequence of images as a parameter. Besides those conventions, an algorithm can do everything it wants to do. So you could create your own cat detection algorithm, marker detection, etc.

Differential Images

This is a trivial motion detection algorithm. Three images are compared, and the pixels that have been changed are marked as white pixels, pixels that are black are background images.