What is the difference between sparse and dense optical flow?

Question

Lots of resources say that there are two types optical flow algorithms. And Lucas-Kanade is a sparse technique, but I can\'t find the meanings of sparse and dense? Can some one

Answer 1

Sparse optical flow gives you the velocity vectors for some interesting (corner) points, these points are extracted beforehand using algorithms like Shi-Tomashi, Harris etc. The extracted points are passed into your [optical flow function] along with the present image and next image. Any good optical flow function should check the optical flow in the forward direction using the above corner points and also back track to cross check if it is following the same points.

On the other hand, dense optical flow can referred from here: http://www.cs.toronto.edu/~fleet/courses/cifarSchool09/flowChapter05.pdf