New to Image processing, Want to find distance between 2 objects.

[Nachiket]

Dear friends,
I am new to Image processing and ImageJ.

I want to find out distance between 2 spots. And Distance of that spot from edge of rectangle.
(In Java Program,  which keeps monitoring webcam and taking images (interval 5 sec))

Please guide me through process and how (and which feature of ImageJ) to use ImageJ.

As per my understanding,
First findout all the spots.
Second findout distance of each spot to other spot.
Third find out Rectangle (under which spots are located)
Forth find out distance.

Thank you
Regards,