can't capture keyboard strokes with cvWaitKey() - keyboard

Question

I have a problem here and need your help. I've been trying to capture keyboard strokes using cvWaitKey() function.
cvWaitKey(10) should return the keyboard stroke pressed every 10ms.
But in my case, I'm getting every 18 seconds, all the keys that I've pressed during this period of 18 seconds, at once.
And the window that should normally close when I press 'esc' (see the code) is still open.
In console I get this:
VIDIOC_QUERYMENU:Invalid argument
VIDIOC_QUERYMENU:Invalid argument
VIDIOC_QUERYMENU:Invalid argument
-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-11536870939-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-111536870939-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1
With:
1536870939 corresponds to when I pressed 'esc'
-1 correspond to when no key was pressed
Thanks for help.
#include "opencv/cv.h"
#include "opencv/highgui.h"
#include "iostream"
using namespace std;
int main()
{
IplImage *src=cvCreateImage(cvSize(640,480), 8, 3);
CvCapture* capture =cvCaptureFromCAM(CV_CAP_ANY);
int key;
while(1){
src = cvRetrieveFrame( capture );
cvNamedWindow( "out", CV_WINDOW_AUTOSIZE );
cvShowImage( "out", src );
key = cvWaitKey(10);
cout<<key;
if( key == 1536870939 ) break; //if 'esc' is pressed (in ubuntu 10.04)
cvGrabFrame( capture );
}
cvDestroyAllWindows();
cvReleaseCapture( &capture );
return 0;
}
I'm using: Opencv 2.2.0 , ubuntu 10.04 LTS , CodeBlocks 8.02

Answer 1

try using a 0xff mask to return 27 on an ESC key press:
if( (cvWaitKey(10)&0xff) == 27 ) break;
Alternatively, you can cast to a character:
if( (char)cvWaitKey(10) == 27 ) break;

Answer 2

The cause of the problem is the way cout works: it buffers characters and only sends them to the console if you send a newline character or if you explicitly ask it to do so. (Third case: the buffer gets full... but that's what you have to wait 18 seconds for.)
So put either cout << '\n'; or cout.flush(); after your current cout statement.
(By the way: VIDIOC_QUERYMENU:Invalid argument is from the v4l (webcam) driver... and I have no idea where 1536870939 comes from, ESC should be 27 :))