How to add in modification function in captured image?

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C#

Emgu-CV

Hi all, I have a captureButton_Click function whereby it will start capture image using webcam and stop when user click on stop button. Where should I add the modification function in or after the captureButton_Click function? Which method is more easier, array or 3*3 matrix? I just want to do some modification on the center part of the image. I also have a processFrame function to change the RGB into grayscale.

private void captureButton_Click(object sender, EventArgs e)
        {
            #region if capture is not created, create it now
            if (_capture == null)
            {
                try
                {
                    _capture = new Capture();
                }
                catch (NullReferenceException excpt)
                {
                    MessageBox.Show(excpt.Message);
                }
            }
            #endregion

            if (_capture != null)
            {
                if (_captureInProgress)
                {  //stop the capture
                    captureButton.Text = "Start Capture";
                    Application.Idle -= ProcessFrame;
                }
                else
                {
                    //start the capture
                    captureButton.Text = "Stop";
                    Application.Idle += ProcessFrame;
                }

                _captureInProgress = !_captureInProgress;
            }
        }

I'm using VS2010 64bit with emgu library. Still new in C#. Anyone can me? Thanks in advance.