Simple difference oriented algorithm for motion detetion in WPF

MotionDetection2Frames.cs

using System;
using System.Windows;
using System.Windows.Media;
using System.Windows.Media.Imaging;
  
  private ImageMatrix create_matrix(string file_name)
  {
      Uri imageUri = new Uri(file_name, UriKind.Relative);
      BitmapImage imageBitmap = new BitmapImage(imageUri);

      int height = imageBitmap.PixelHeight;
      int width = imageBitmap.PixelWidth;

      ImageMatrix res = new ImageMatrix(height, width, file_name);

      int nStride = (imageBitmap.PixelWidth * imageBitmap.Format.BitsPerPixel) / 8;
      byte[] pixelByteArray = new byte[imageBitmap.PixelHeight * nStride];
      imageBitmap.CopyPixels(pixelByteArray, nStride, 0);

      for (int i = 0; i < height; i++)
      {
          for (int j = 0; j < width-4; j++)
          {
              int z = i * width * 4;

              byte alpha = pixelByteArray[z + j*4];
              byte red = pixelByteArray[z + (j*4+1)];
              byte green = pixelByteArray[z + (j*4+2)];
              byte blue = pixelByteArray[z + (j*4+3)];

              Color color = Color.FromArgb(alpha, red, green, blue);
              res.Matrix[i, j] = (byte)((color.R + color.G + color.B) / 3);
          }
      }

      return res;
  }


  private void tsmiDifference_Click()
  {
      ImageMatrix matrix1 = create_matrix("Screenshot-1.jpg");
      ImageMatrix matrix2 = create_matrix("Screenshot-2.jpg");

      ImageMatrix picture = new ImageMatrix(matrix1.Height, matrix1.Width, "");

      for (int i = 0; i < matrix1.Height; i++)
      {
          for (int j = 0; j < matrix1.Width; j++)
          {
              byte light1 = matrix1.Matrix[i, j];
              byte light2 = matrix2.Matrix[i, j];
              byte light = (byte)Math.Abs(light2 - light1);
              if (light < 5) light = 0; else light = 255;
              picture.Matrix[i, j] = light;
          }
      }
      difference_image.Source = picture.GenerateImageFromMatr();

  }

internal class ImageMatrix
{
  public int Height;
  public int Width;
  internal byte[,] Matrix;
  string uri;

  public ImageMatrix(int height, int width, string uri)
  {
      Height = height;
      Width = width;
      this.uri = uri;
      this.Matrix = new byte[Height, Width];
  }

  public WriteableBitmap GenerateImageFromMatr()
  {
      WriteableBitmap wb = new WriteableBitmap(Width,
          Height, 96, 96, PixelFormats.Bgra32, null);

      Int32Rect rect = new Int32Rect(0, 0, Width, Height);

      byte[] pixels = new byte[Width * Height * wb.Format.BitsPerPixel / 8];
      for (int x = 0; x < wb.PixelHeight; x++)
      {
          for (int y = 0; y < wb.PixelWidth; y++)
          {
              byte alpha = 255;
              byte light = Matrix[x,y];

              int pixelOffset = (y + x * wb.PixelWidth) * wb.Format.BitsPerPixel / 8;
              pixels[pixelOffset] = light;
              pixels[pixelOffset+1] = light;
              pixels[pixelOffset+2] = light;
              pixels[pixelOffset+3] = alpha;


          }

          int stride = (wb.PixelWidth * wb.Format.BitsPerPixel) / 8;

          wb.WritePixels(rect, pixels, stride, 0);
      }



      return wb;
  }