How to find the distance travel by particle using optical flow function ?

[@mi]

Hello All,

I am working on a project where I have to calculate the velocity of the moving particle and then have to move the sample stage with same velocity in a opposite direction of particle motion.

What I am doing?

I wrote programme in LabVIEW

whats it does?

1)Open camera and load the configuration

2)Take first image, lets assume it a previous image 

3)Take second image and assume it as a current image

4)Do image processing

5)Use both current and precious image and apply to the optical flow function to find the distance travel by             particle.

what is the problem?

1) Optical flow VI (LKP), calculate the particle distance only from the defined pixel location in feature in tag at optical flow VI , see image below., but I my case particle can be anywhere and I have to find the distance travel by the particle to find the velocity, so do any one know how to find the distance travel by particle from the random locations.

2) can any one explain me in which unit distance array return the travel distance of the particle?

3) What each parameter does in options at optical flow VI? such as window size, iteration and level?

I have attached VI for more information.

Please let me if you know the answers of any above asked questions.