ord（'q'）和0xFF的用法

Question

I am not able to understand the following code snippet-

if cv2.waitKey(0) & 0xFF == ord('q'):
break

in this code-

    1 import numpy as np
    2 import cv2
    3 
    4 cap = cv2.VideoCapture(0)
    5 
    6 while(True):
    7     # Capture frame-by-frame
    8     ret, frame = cap.read()
    9 
   10     # Our operations on the frame come here
   11     gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
   12 
   13     # Display the resulting frame
   14     cv2.imshow('frame',gray)
   15     if cv2.waitKey(1) & 0xFF == ord('q'):
   16         break
   17 
   18 # When everything done, release the capture
   19 cap.release()
   20 cv2.destroyAllWindows()

What does ord('q') and 0xFF mean? How is it being used here?

Answer 1

• ord('q') returns the Unicode code point of q
• cv2.waitkey(1) returns a 32-bit integer corresponding to the pressed key
• & 0xFF is a bit mask which sets the left 24 bits to zero, because ord() returns a value betwen 0 and 255, since your keyboard only has a limited character set
• Therefore, once the mask is applied, it is then possible to check if it is the corresponding key.

Answer 2

As per the cv2.waitkey docs :

It returns the code of the pressed key or -1 if no key was pressed before the specified time had elapsed.

Generally in the OpenCV tutorials and blogs, it is a general convention to use " q " key for halting any indefinite operation such as capturing frames from camera in your case. In your case, the program indefinitely checks at each iteration if " q " key is being pressed using cv2.waitKey(1) & 0xFF == ord('q') statement. If True then it simply brakes the infinite while loop. You can set it to any key of your choice.

Answer 3

First, cv2.waitKey(1) & 0xFF will be executed.

• Doing wait 1 ms for user press.
• If user press, for example, q then waitKey return DECIMAL VALUE of q is 113. In Binary , It is expressed as 0b01110001 .
• Next, AND operator is excuted with two inputs are 0b01110001 and 0xFF ( 0b11111111 ).

0b01110001 AND 0b11111111 = 0b01110001 . Exactly result is DECIMAL VALUE of q

Second, compare value of left expression 0b01110001 with ord('q') . Clearly, these values is the same as another value. And final result is break is invoked.