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it's quite a puzzle here, but what happens:

• you draw something into your frame
• then you assign it to last_frame
• in the next round, you absdiff last_frame(with the drawing in it) to frame, so this already has your previous drawing in it !

remedy: you need to copy to last_frame before you draw into it, also, it needs a "deep copy", not just an assignment:

 binary = cv2.absdiff(last_frame, frame)
 binary = cv2.cvtColor(binary, cv2.COLOR_BGR2GRAY) 
 ret, binary = cv2.threshold(binary,50,255,cv2.THRESH_BINARY) 

 last_frame = frame.copy()

 # now draw into your frame, as you like !

it's quite a puzzle here, but what happens:

• you draw something into your frame
• then you assign it to last_framelast_frame (unfortunately, with the drawing in it)
• in the next round, you absdiff last_frame(with the drawing in it) last_frame to frame, so this already has your previous drawing in it !

remedy: you need to copy to last_frame before you draw into it, also, it needs a "deep copy", not just an assignment:

 binary = cv2.absdiff(last_frame, frame)
 binary = cv2.cvtColor(binary, cv2.COLOR_BGR2GRAY) 
 ret, binary = cv2.threshold(binary,50,255,cv2.THRESH_BINARY) 

 last_frame = frame.copy()

 # now draw into your frame, as you like !