Using other keys for the waitKey function of opencv

Question

I m working on a program  python  opencv  in which I use the spacebar to go to the next frame  and Esc to exit the program  These are the only two keys i ve got working  I tried to find out about more keys   tried various codes for them but didnt work  especially arrow keys   I found this about waitkey  but it doesn t work   So my question is  How do I catch other keys besides esc and spacebar to trigger certain functions in my python-opencv program

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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

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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

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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

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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  lt  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

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This works the best for me   http   www asciitable com   Sometimes it s the simple answers that are the best

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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  lt  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  amp   1  lt  lt  18       as the  control key  flag is bit 19  Shift is at bit 17  the state of CapsLock at bit 18  Alt is at bit 20  and NumLock is at bit 21

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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

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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

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You can use ord   function in Python for that   For example  if you want to trigger  a  key press  do as follows    if cv2 waitKey 33     ord  a       print  pressed a    See a sample code here  Drawing Histogram  UPDATE    To find the key value for any key is to print the key value using a simple script as follows    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   With this code  I got following values    Upkey   2490368 DownKey   2621440 LeftKey   2424832 RightKey  2555904 Space   32 Delete   3014656          Continue yourself

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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

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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   - gt   key         10  60        cv2 FONT HERSHEY SIMPLEX       1       255 255 255       2    cv2 imshow  Keypressed   im c    if key    27  break    ESC

[python] Using other keys for the waitKey() function of opencv

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