Emgu CV视频播放缓慢吗？

Question

i am trying to open a video file from disk and try to play it using openCV wrapper: Emgu CV in visual studio c#. i'm using imagebox provided by Emgu CV to show my video on a form. i calculate waiting time for showing next frame of video using this code:

await Task.Delay(Convert.ToInt32( 1000.0 / FPS));

problem is the video plays slower than calculated formula. i open a 29.97 FPS video file and the formula should return (1000/30)=33. but when i play the video i can immediately see the video is playing slower than 33 FPS. if i go to my imagebox properties by right clicking on the imagebox , it says that the FPS is 21 which i guess is the actual FPS video is playing. what is happening?

this is my code. it's very basic:

public partial class Form1 : Form
{

    VideoCapture videoCapture;
    double FPS;
    double totalFrames;
    int currentFrameNo;

    public Form1()
    {
        InitializeComponent();
    }

    private void btnOpen_Click(object sender, EventArgs e)
    {
        OpenFileDialog ofd = new OpenFileDialog();
        if (ofd.ShowDialog() == DialogResult.OK)
        {
            videoCapture = new VideoCapture(ofd.FileName);
            FPS = videoCapture.GetCaptureProperty(Emgu.CV.CvEnum.CapProp.Fps);
            totalFrames = videoCapture.GetCaptureProperty(Emgu.CV.CvEnum.CapProp.FrameCount);
            currentFrameNo = 0;
        }
    }

    private async void btnPlay_Click(object sender, EventArgs e)
    {
        while (currentFrameNo<totalFrames)
        {
            imageBox1.Image = videoCapture.QueryFrame();
            currentFrameNo += 1;
            await Task.Delay(Convert.ToInt32( 1000.0 / FPS));
        }
    }
}

what i want is a smooth playing. what should change? and sorry for the bad language.

Answer 1

As other comments have suggested - the problem is that a constant delay is being added to whatever the loop processing time is.

A better solution would be to use a Timer (roughly!) as follows:

bool playingVideo = false;
Timer frameTimer = new Timer(this.GrabFrame, null, 0, 1000 / 33);

private void btnPlay_Click(object sender, EventArgs e)
{
    playingVideo = true;
}


private void GrabFrame(object sender, EventArgs e)
{
    if(playingVideo)
    {
        while (currentFrameNo<totalFrames)
        {
            imageBox1.Image = videoCapture.QueryFrame();
            currentFrameNo += 1;
        }
    }
}

This way the frames will be requested every 33ms regardless of the capture time