What is meaning of mat in emgucv?

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C#

image-processing

I want to display a histogram of the picture by using emgucv library but I have a problem with the instruction "Mat", the program gives a message that the 'Mat' do not found...how solve that? and by what I can replace it?

What I have tried:

using Emgu.CV;
using Emgu.CV.Structure;

namespace histogram
{
    public partial class Form1 : Form
    {
        Image<Bgr, byte> imgB;
        Image<Gray, byte> imgG;
        public Form1()
        {
            InitializeComponent();
        }
<pre>private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            if(ofd.ShowDialog()==DialogResult.OK)
            {
                imgB = new Image<Bgr, byte>(ofd .FileName );
                imageBox1.Image = imgB;
                imageBox1.FunctionalMode = Emgu.CV.UI.ImageBox.FunctionalModeOption.Minimum;
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            imgG = imgB.Convert<Gray, byte>();
            panAndZoomPictureBox1.Image = imgG.Bitmap;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            DenseHistogram hist = new DenseHistogram(256, new RangeF(0, 255));
            hist.Calculate(new Image<Gray, byte>[] {imgB[0] }, false, null);
            Mat m = new Mat();
            hist.Copy(m);

            histogramBox1.AddHistogram("Blue", Color.Blue, m);
        }
    }