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

First of all, Lucas-Kanade is NOT a sparse optical flow technique. The reason so many believe it is, is due to a wide spread misunderstanding. The misconception became an accepted truth since the very first implementation of Lucas-Kanade in OpenCV was labelled as SPARSE, and still is to this day. The arguments to why Lucas-Kanade should be called sparse, apply to any dense flow algorithm. If you insist that Lucas-Kanade is sparse, then all flow algorithms are sparse and there is no point in distinguising them.

Sparse flow is the same as point tracking, dense flow consists of vectors over the video, indicating estimates of motion of fixed positions.

You can read more about all of this in this tutorial that I wrote, where I also show how Lucas-Kanade is just as dense as any other algoritm out there (although not as accurate).