Sobel algorithm

Question

I am trying to implement Sobel algorithm in order to detect the edges from an image.

I wrote the following code:

        bmp = new Bitmap(pictureBox1.Image);

        int[][] sobelx = {new int[] {-1, 0, 1},
                          new int[] {-2, 0, 2},
                          new int[] {-1, 0, 1}};

        int[][] sobely = {new int[] {-1, -2, -1},
                          new int[] { 0, 0, 0},
                          new int[] { 1, 2, 1}};

        for (int i = 1; i < bmp.Width - 1; i++)
        {
            for (int j = 1; j < bmp.Height - 1; j++)
            {int dx = bmp.GetPixel(i - 1, j - 1).R * sobelx[0][0] + bmp.GetPixel(i, j - 1).R * sobelx[0][1] + bmp.GetPixel(i + 1, j - 1).R * sobelx[0][2]
                       + bmp.GetPixel(i - 1, j).R * sobelx[1][0] + bmp.GetPixel(i, j).R * sobelx[1][1] + bmp.GetPixel(i + 1, j).R * sobelx[1][2]
                       + bmp.GetPixel(i - 1, j + 1).R * sobelx[2][0] + bmp.GetPixel(i, j + 1).R * sobelx[2][1] + bmp.GetPixel(i + 1, j + 1).R * sobelx[2][2];

                int dy = bmp.GetPixel(i - 1, j - 1).R * sobely[0][0] + bmp.GetPixel(i, j - 1).R * sobely[0][1] + bmp.GetPixel(i + 1, j - 1).R * sobely[0][2]
                       + bmp.GetPixel(i - 1, j).R * sobely[1][0] + bmp.GetPixel(i, j).R * sobely[1][1] + bmp.GetPixel(i + 1, j).R * sobely[1][2]
                       + bmp.GetPixel(i - 1, j + 1).R * sobely[2][0] + bmp.GetPixel(i, j + 1).R * sobely[2][1] + bmp.GetPixel(i + 1, j + 1).R * sobely[2][2];
                double derivata = Math.Sqrt((dx * dx) + (dy * dy));

                if (derivata > 255)
                {
                    bmp.SetPixel(i, j, Color.White);
                }
                else
                {
                    bmp.SetPixel(i, j, Color.FromArgb(255, (int)derivata, (int)derivata, (int)derivata));
                }
            }
        }

        pictureBox2.Image = bmp;

But the resulted image is mostly white. This is the original image:

This is the transformed image:

I don't know what I am doing wrong. Could anyone help me?

Thank you in advanced!

Václav Struhár · Accepted Answer · 2017-06-03 18:46:49Z

Use another bitmap for saving the output as follows:

            Bitmap res = new Bitmap(bmp.Width, bmp.Height);
            int[][] sobelx = {new int[] {-1, 0, 1},
                          new int[] {-2, 0, 2},
                          new int[] {-1, 0, 1}};

            int[][] sobely = {new int[] {-1, -2, -1},
                          new int[] { 0, 0, 0},
                          new int[] { 1, 2, 1}};

            for (int i = 1; i < bmp.Width - 1; i++)
            {
                for (int j = 1; j < bmp.Height - 1; j++)
                {
                    int dx = bmp.GetPixel(i - 1, j - 1).R * sobelx[0][0] + bmp.GetPixel(i, j - 1).R * sobelx[0][1] + bmp.GetPixel(i + 1, j - 1).R * sobelx[0][2]
                              + bmp.GetPixel(i - 1, j).R * sobelx[1][0] + bmp.GetPixel(i, j).R * sobelx[1][1] + bmp.GetPixel(i + 1, j).R * sobelx[1][2]
                              + bmp.GetPixel(i - 1, j + 1).R * sobelx[2][0] + bmp.GetPixel(i, j + 1).R * sobelx[2][1] + bmp.GetPixel(i + 1, j + 1).R * sobelx[2][2];

                    int dy = bmp.GetPixel(i - 1, j - 1).R * sobely[0][0] + bmp.GetPixel(i, j - 1).R * sobely[0][1] + bmp.GetPixel(i + 1, j - 1).R * sobely[0][2]
                           + bmp.GetPixel(i - 1, j).R * sobely[1][0] + bmp.GetPixel(i, j).R * sobely[1][1] + bmp.GetPixel(i + 1, j).R * sobely[1][2]
                           + bmp.GetPixel(i - 1, j + 1).R * sobely[2][0] + bmp.GetPixel(i, j + 1).R * sobely[2][1] + bmp.GetPixel(i + 1, j + 1).R * sobely[2][2];
                    double derivata = Math.Sqrt((dx * dx) + (dy * dy));

                    if (derivata > 255)
                    {
                        res.SetPixel(i, j, Color.White);
                    }
                    else
                    {
                        res.SetPixel(i, j, Color.FromArgb(255, (int)derivata, (int)derivata, (int)derivata));
                    }
                }
            }

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