What does OpenCV s cvWaitKey function do

Question

What happens during the execution of cvWaitKey    What are some typical use cases  I saw it in OpenCV reference but the documentation isn t clear on its exact purpose

User · Answer

Assuming this is a while loop - gt  e g  video stream where img is obtained from say web camera        cvShowImage  Window  img       A small interval of 10 milliseconds  This may be necessary to display the image correctly    cvWaitKey 10         to wait until user feeds keyboard input replace with cvWaitKey 0

User · Answer

waits milliseconds to check if the key is pressed  if pressed in that interval return its ascii value  otherwise it still -1

User · Answer

cvWaitKey 0  stops your program until you press a button   cvWaitKey 10  doesn t stop your program but wake up and alert to end your program when you press a button  Its used into loops because cvWaitkey doesn t stop loop   Normal use  char k   k cvWaitKey 0    if k     ESC     with k you can see what key was pressed

User · Answer

The cvWaitKey simply provides something of a delay  For example   char c   cvWaitKey 33   if  c    27   break    Tis was apart of my code in which a video was loaded into openCV and the frames outputted  The 33 number in the code means that after 33ms  a new frame would be shown  Hence  the was a dely or time interval of 33ms between each frame being shown on the screen  Hope this helps

User · Answer

cvWaitKey milliseconds  simply wait for milliseconds provided as a parameter for a next key stroke of keyboard    Human eyes not able to see the thing moving in less than 1 10 second  so we use this to hold same image frame for some time on screen  As soon as the key of keyboard is pressed the next operation will be perform   In short cvWaitKey milliseconds  wait either for key press or millisecond time provided

User · Answer

argument of 0 is interpreted as infinite    in order to drag the highGUI windows  you need to continually call the cv  waitKey   function  eg for static images   cv  imshow  winname   img    while cv  waitKey 1     27      27   ascii value of ESC

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Note for anybody who may have had problems with the cvWaitKey    function  If you are finding that cvWaitKey x  is not waiting at all  make sure you actually have a window open  i e  cvNamedWindow        Put the cvNamedWindow      declaration BEFORE any cvWaitKey   function calls

User · Answer

Plain simply  cvWaitKey   sleeps for X miliseconds  waiting for any key to be pressed   int cvWaitKey int X     If a key is pressed  this function returns the ASCII code of key  Or returns -1 if no keys were pressed during that time

User · Answer

cvWaitKey x    cv  waitKey x  does two things     It waits for x milliseconds for a key press on a OpenCV window  i e  created from cv  imshow     Note that it does not listen on stdin for console input  If a key was pressed during that time  it returns the key s ASCII code  Otherwise  it returns -1   If x is zero  it waits indefinitely for the key press   It handles any windowing events  such as creating windows with cv  namedWindow    or showing images with cv  imshow      A common mistake for opencv newcomers is to call cv  imshow   in a loop through video frames  without following up each draw with cv  waitKey 30   In this case  nothing appears on screen  because highgui is never given time to process the draw requests from cv  imshow

[c++] What does OpenCV's cvWaitKey( ) function do?

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