Using other keys for the waitKey() function of opencv

The keycodes returned by waitKey seem platform dependent.
However, it may be very educative, to see what the keys return
(and by the way, on my platform, Esc does not return 27…)

The integers thay Abid’s answer lists are mosty useless to the human mind
(unless you’re a prodigy savant…). However, if you examine them in hex,
or take a look at the Least Significant Byte, you may notice patterns…

My script for examining the return values from waitKey is below:

#!/usr/bin/env python

import cv2
import sys

cv2.imshow(sys.argv[1], cv2.imread(sys.argv[1]))
res = cv2.waitKey(0)
print('You pressed %d (0x%x), LSB: %d (%s)' % (res, res, res % 256,
    repr(chr(res%256)) if res%256 < 128 else '?'))

You can use it as a minimal, command-line image viewer.

Some results, which I got:

• q letter:

  You pressed 1048689 (0x100071), LSB: 113 (‘q’)

• Escape key (traditionally, ASCII 27):

  You pressed 1048603 (0x10001b), LSB: 27 (‘x1b’)

• Space:

  You pressed 1048608 (0x100020), LSB: 32 (‘ ‘)

This list could go on, however you see the way to go, when you get ‘strange’ results.

BTW, if you want to put it in a loop, you can just waitKey(0) (wait forever), instead of ignoring the -1 return value.

EDIT: There’s more to these high bits than meets the eye – please see Andrew C’s answer (hint: it has to do with keyboard modifiers like all the “Locks” e.g. NumLock).

My recent experience shows however, that there is a platform dependence – e.g. OpenCV 4.1.0 from Anaconda on Python 3.6 on Windows doesn’t produce these bits, and for some (important) keys is returns 0 from waitKey() (arrows, Home, End, PageDn, PageUp, even Del and Ins). At least Backspace returns 8 (but… why not Del?).

So, for a cross platform UI you’re probably restricted to W, A, S, D, letters, digits, Esc, Space and Backspace 😉

For C++:

In case of using keyboard characters/numbers, an easier solution would be:

int key = cvWaitKey();

switch(key)
{
   case ((int)('a')):
   // do something if button 'a' is pressed
   break;
   case ((int)('h')):
   // do something if button 'h' is pressed
   break;
}

The answers which have already been posted suggest that some of the unusual values obtained by waitKey are due to platform differences. Below, I propose that (at least on some platforms) the apparently odd behaviour of waitKey is due to keyboard modifiers. This post looks similar to Tomasz’s answer because I initially wrote this as an edit, which was rejected.

The keycodes returned by waitKey change depending on which modifiers are enabled. NumLock, CapsLock, and the Shift, Ctrl, and Alt keys all modify the keycode returned by waitKey by enabling certain bits above the two Least Significant Bytes. The smallest of these flags is Shift at 0x10000.

A modified version of the script Tomasz posted is given below:

#!/usr/bin/env python

import cv2
import sys

cv2.imshow(sys.argv[1], cv2.imread(sys.argv[1]))
res = cv2.waitKey(0)
print 'You pressed %d (0x%x), 2LSB: %d (%s)' % (res, res, res % 2**16,
    repr(chr(res%256)) if res%256 < 128 else '?')

Which give the following results:

• q letter with NumLock:

  You pressed 1048689 (0x100071), 2LSB: 113 (‘q’)

• Escape key with CapsLock but not NumLock:

  You pressed 131099 (0x2001b), 2LSB: 27 (‘x1b’)

• Space with Shift and NumLock:

  You pressed 1114144 (0x110020), 2LSB: 32 (‘ ‘)

• Right Arrow Key with Control, NumLock off:

  You pressed 327507 (0x4ff53), 2LSB: 65363 (‘S’)

I hope that helps to explain the unusual behaviour of waitKey and how to get the actual key pressed regardless of the state of NumLock and CapLock. From here it’s relatively simple to do something like:

ctrlPressed = 0 != res & (1 << 18)

…as the "control key" flag is (counting the least significant bit as bit 0) bit 18. Shift is at bit 16, the state of CapsLock at bit 17, Alt is at bit 19, and NumLock is at bit 20. As Tomasz was kind enough to point out, just pressing Shift on its own also returns a value, with distinct values for LShift and RShift (still with all these modifiers just described). I encourage you to double-check all of these modifiers and values on your own platform before relying on them. 🙂

With Ubuntu and C++ I had problems with the Character/Integer cast. I needed to use cv::waitKey()%256 to obtain the correct ASCII value.

If you want to pause the program to take screenshots of the progress

(shown in let’s say cv2.imshow)

cv2.waitKey(0) would continue after pressing “Scr” button (or its combination), but you can try this

cv2.waitKey(0)
input('')

cv2.waitkey(0) to give the program enough time to process everything you want to see in the imshow and input(”)

to make it wait for you to press Enter in the console window

this works on python 3

As to me, the below code does’t work, when it runs,the image will step to the next quickly without your press:

import cv2
img = cv2.imread('sof.jpg') # load a dummy image
while(1):
    cv2.imshow('img',img)
    k = cv2.waitKey(33)
    if k==27:    # Esc key to stop
        break
    elif k==-1:  # normally -1 returned,so don't print it
        continue
    else:
        print k # else print its value

But this works:

def test_wait_key():
    lst_img_path = [
        '/home/xy/yy_face_head/face_det_test/111.png',
        '/home/xy/yy_face_head/face_det_test/222.png'
        #.....more path ...
    ]

    for f_path in lst_img_path:
        img = cv2.imread(f_path)
        cv2.imshow('tmp', img)
        c = cv2.waitKey(0) % 256

        if c == ord('a'):
            print "pressed a"
        else:
            print 'you press %s' % chr(c)

Output as below:

I too found this perplexing.
I’m running Ubuntu 18 and found the following:
If the cv.imshow window has focus, you’ll get one set of values in the terminal – like the ASCII values discussed above.

If the Terminal has focus, you’ll see different values. IE- you’ll see “a” when you press the a key (instead of ASCII value 97) and “^]” instead of “27” when you press Escape.

I didn’t see the 6 digit numbers mentioned above in either case and I used similar code. It seems the value for waitKey is the polling period in mS. The dots illustrate this.

Run this snippet and press keys while focus is on the test image, then click on the terminal window and press the same keys.

    import cv2
    img = cv2.imread('test.jpg') 
    cv2.imshow('Your test image', img)

    while(1):
      k = cv2.waitKey(300)
      if k == 27:
        break
      elif k==-1:
       print "."
       continue
      else:
        print k 

This prints the key combination directly to the image:

The first window shows 'z' pressed, the second shows 'ctrl' + 'z' pressed. When a key combination is used, a question mark appear.

Don’t mess up with the question mark code, which is 63.

import numpy as np
import cv2

im = np.zeros((100, 300), np.uint8)
cv2.imshow('Keypressed', im)
while True:
  key = cv2.waitKey(0)
  im_c = im.copy()
  cv2.putText(
    im_c,
    f'{chr(key)} -> {key}',
    (10, 60), 
    cv2.FONT_HERSHEY_SIMPLEX, 
    1,
    (255,255,255),
    2)
  cv2.imshow('Keypressed', im_c)
  if key == 27: break # 'ESC'

The answer that works on Ubuntu18, python3, opencv 3.2.0 is similar to the one above. But with the change in line cv2.waitKey(0). that means the program waits until a button is pressed.

With this code I found the key value for the arrow buttons: arrow up (82), down (84), arrow left(81) and Enter(10) and etc..

import cv2
img = cv2.imread('sof.jpg') # load a dummy image
while(1):
    cv2.imshow('img',img)
    k = cv2.waitKey(0)
    if k==27:    # Esc key to stop
        break
    elif k==-1:  # normally -1 returned,so don't print it
        continue
    else:
        print k # else print its value

Interesting nobody has mentioned cv2.waitKeyEx() as described in this answer of another Stack Overflow thread. OpenCV’s documentation on cv2.waitKeyEx() reads as follows:

Similar to waitKey, but returns full key code.

Note

Key code is implementation specific and depends on used backend:
QT/GTK/Win32/etc

So, some attention may be required for cross-platform implementations. However, for me this was by far the easiest and most straight forward solution to get arrow keys etc. working on Windows.

flag = True
while flag:
    cv2.imshow("result", image_to_show)
    key = cv2.waitKey(0)
    if key == ord('b'):     # pressed a key
        <Do something>
    elif key == ord('b'):   # pressed b key
        <Do something>
    elif key == 27:         # pressed Esc key
        flag = False