CODEWITHSUNDEEP
c#.netopencvcomputer-visionemgucv
Aishwar
Aishwar

Reputation: 9724

EmguCV/OpenCV QueryFrame slow/buffers

We have an application, where we get a message from an external system and then we take a picture, do some processing and return something back to the external system. Doing some performance testing, I found two problems (they are somewhat related). I was hoping someone will be able to explain this to me.

1) Does _capture.QueryFrame() buffer frames? What we see is, if there is a gap between the query for two frames from a web camera, the second frame is often an older picture and not the one when the queryFrame was called.

We were able to mitigate this problem to some extent by discarding some frames, i.e. calling _capture.QueryFrame() 2-3 times and discarding the results.

2) The second issue is when we timed different parts of the application, we found that clearing the buffer (calling QueryFrame() 2-3 times and not using the results) takes about 65ms and then this line: Image<Bgr, Byte> source = _capture.QueryFrame() takes about 80ms. These two parts take the biggest chunk of processing time, our actual processing takes just about 20-30ms more.

Is there a faster way (a) to clear the buffer (b) to capture the frame?

If you have experience with OpenCV and know of something related, please do let me know.

Upvotes: 2

Views: 7938

Answers (3)

Jay Dubal
Jay Dubal

Reputation: 253

You could call _capture.Grab(); during the idling time. Or inside the code of a timer. Keep the timer interval matching your camera frames capture rate.

Then when you need call the _capture.QueryFrame();

What Grab() will do is flush out the buffer and get you the latest frame when calling _capture.QueryFrame();

Upvotes: 0

Chris
Chris

Reputation: 3482

I answered a similar question System.TypeInitializationException using Emgu.CV in C# and having tested the various possibilities to acquire an up to date frame I found the bellow the bes method.

1) yes when you set up a Capture from a webcam a ring buffer is created to store the images in this allows effcient allocation of memory.

2) yes there is a faster way, Set your Capture device up globally and set it of recording and calling ProcessFrame to get an image from the buffer whenever it can. Now change your QueryFrame simply to copy whatever frames its just acquired. This will hopefully stop your problem of getting the previous frame and you will now have the most recent frame out of the buffer.

private Capture cap;
Image<Bgr, Byte> frame;

public CameraCapture()
{
    InitializeComponent();
    cap= new Capture();
    cap.SetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_FRAME_HEIGHT, height);
    cap.SetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_FRAME_WIDTH, width);

    Application.Idle += ProcessFrame;
}

private void ProcessFrame(object sender, EventArgs arg)
{
    frame = _capture.QueryFrame();
    grayFrame = frame.Convert<Gray, Byte>();
}

public Image<Bgr,byte> QueryFrame()
{
    return frame.Copy();
}

I hope this helps if not let me know and I'll try and tailor a solution to your requirements. Don't forget you can always have your acquisition running on a different thread and invoke the new QueryFrame method.

Cheers

Chris

Upvotes: 2

Shivam
Shivam

Reputation: 144

This could also be due to the refresh rate of the webcamera you are using. My camera works at 60Hz so I have a timer that takes captures a frame every 15 milliseconds.

Upvotes: 0

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