opencv waitkey，不精确，无法正常工作吗？

Question

I'm developing a video player with C++ and OpenCV, and need high precision, but when I make a cv::waitKey, the function never waits the specified miliseconds:

    tWait.resetAndRestart();
    cv::waitKey((int)(30));
    realDelay = tWait.getElapsedMsec();
    cout << "realDelay :: " << realDelay.count() << "\n";

This is the awesone output

...
realDelay :: 25
realDelay :: 24
realDelay :: 25
realDelay :: 21
realDelay :: 21
realDelay :: 24
realDelay :: 24
realDelay :: 20
realDelay :: 25
....

Any idea?

Edit: This is the timer initialization, but I think that the problem it's not here.

void timer::start() {
    //Start the timer
    started = true;

    //Unpause the timer
    paused = false;

    //Get the current clock time    
    //startMs = std::chrono::duration_cast<milliseconds>(high_resolution_clock::now() - startTime);
    startTime = std::chrono::high_resolution_clock::now();  
}

void timer::reset()
{
    paused = false;
    started = false;
    startTime = std::chrono::high_resolution_clock::now(); 
    pauseMs = std::chrono::milliseconds(0);
}

inline std::chrono::milliseconds timer::getElapsedMsec( void )
{
//  return std::chrono::duration_cast<std::chrono::milliseconds>(high_resolution_clock::now());
    //If the timer is running
    if( started == true )
    {
        //If the timer is paused
        if( paused == true )
        {
            //Return the number of ticks when the timer was paused
            return pauseMs;
        }
        else
        {
            //Return the current time minus the start time
            return std::chrono::duration_cast<std::chrono::milliseconds>( std::chrono::high_resolution_clock::now() - startTime);
        }    
    }

    //If the timer isn't running
    return std::chrono::milliseconds(0);   
}

Answer 1

Finally, I found a good solution: First, I compute the desired delay, then use the minimum value in cvWaitKey and measure the real delay, and then use std::this_thread::sleep_for (who is really exact)with the corrected value:

    delayMeasure0 = std::chrono::high_resolution_clock::now(); 
    cv::waitKey((int)(1));          
    delayMeasure1 = std::chrono::high_resolution_clock::now(); 
    delta = std::chrono::duration_cast<std::chrono::milliseconds>(delayMeasure1-delayMeasure0);
    computedDelay -= delta;

    std::this_thread::sleep_for(computedDelay);