finding speed of vehicle in video [closed]

edit

Note: This will give you the average speed of the car on that particular path.

May you please assist me with a code snippet. I have been able to get the frame rate total number of frames with the blow code

   void video_details(VideoCapture& cap)
  {

cout << "The video has  " << cap.get(CAP_PROP_FRAME_COUNT) << "  frames" << endl;
cout << "The video is playing at  " << cap.get(CV_CAP_PROP_FPS) << "   frames per second." << endl;
 }

I do not know how to get the remaining variables in order to get the speed. Thank you for the response

Edited the answer.

from a mathematical point of view I understand everything you are saying I should do. I just dnt know how to code it. I got this far by following tutorials and examples. the code I have draws boxes around each car after running background subtraction. as each box is drawn a counter counts the number of boxes drawn and returns the total number of boxes hence the number of cars. am very new to opencv and learning as I go. How would I find the time and the frame at which a car is observed and leaves the scene? I understand what I need to do but have struggling with the code.

The only thing I can do is point you in the right direction here. If you can draw a bounding box around the cars it means at each frame you know the coordinates of such rectangles. If you keep the data that describes each rectangle (x, y, width, height) for each frame and compare them to the detections on the next frame, you can verify area of overlap between bounding boxes. If a detection in one frame has a great overlap with a detection on the next frame it probably means that the object is the same, so you can increment a "life-time" counter for that particular object. If there is no overlap on an object it means that it is new. This requires some skills in general programming, more than in OpenCV.. good luck.

If a car is detected as soon as it enters the frame and stops being detected only when it leaves the frame, you only need to count how much frames it is on the scene to calculate its speed.