Advantages and disadvantages of each algorithm:

- (1) great for tracking particles. simulates "freezing by heating". information about density can be extracted from the force flow of the particles. tracking movement of objects can tell us they go around dense areas (don't know if its a wall or a group of people though).
- (2) can tell us that a change in velocity occurred (possibly signaling that a dense object has formed.  Also, if the velocity never changes, it is possible there is a wall there).
- (3) measures density based on the positions of the particles. using the information provided by (1) and (2), one can possibly estimate the density very well.

In summary:
- (1) tells you that you are moving around something dense.
- (2) can possibly tell you if the object they're moving around is a stationary object, or dense group of people.
- (3) can possibly measure the density by using (1) and (2).
