vehicle counting using virtual line detection; how to get the pixel changing status of the virtual line

I don't want to use optical flow. I want to use a virtual rectangle to simplify the problem. Once the vehicle get through the rectangle, the intensity of the rectangle changes greatly. Detect the changing number of peaks of the intensity's changing then we have the number of cars.

I may do it in the follow steps:

1) read the video and convert it in gray scale, then convert it into binary scale.

2) detect the intensity of virtual rectangle and count the peaks of the intensity's changing.

however, I do not know how to do the second step in detail. Can anyone help me out? Thank you