Assign an ID (integer) to a object

Hello everyone,

I am doing a doing human speed tracking using image subtraction. There would be multiple objects in the camera view (fixed stationary camera). I would like to ask how do you assign an ID number in the algorithm? As to ensure that I could differentiate the different objects' speed in the camera view. I have read how Motion-Based Multiple object tracking assign integer ID to individual track to identify individual objects. https://www.mathworks.com/help/vision/examples/motion-based-multiple-object-tracking.html .

For an example, There are two objects in the camera view, Object A and Object B. Both object are moving simultaneously but different speed and direction. So how would I label object a and object b to differentiate them?

Also, I would like to ask how to I compute the area when their is two calibration factors as my calibration factor for x axis differ from the y axis.

 L = 21; % // actual length(cm) of one pixel in the x axis 
pixel_x = 2100; % // the number of pixel map to the length(cm) in the x axis 
T = 10; % // 10 cm map pixel_y in the y axis  
pixel_y = 2100; % // the number of pixel map to the actual length in the y axis
distanceInCm_x = distanceInPixels * calibrationFactor_x; % // actual distance in x axis 
distanceInCm_y = distanceInPixels * calibrationFactor_y; % // actual distance in y axis

Thanks in advance for all the response.

Cheers Felicia