Sobel edge implementation issue

Question

For a class I have to implement the sobel edge detector in java, and do I did. But something isnt right, and after 3 hours trying to figure it out I still havent.

public static Color[][] sobelBorder (Color[][] img) {
        Color[][] original = ImageUtils.cloneArray(img);
        Color[][] tmp = grayscale(original);


        int[][] kernel1 = {
                {1,0,-1},
                {2,0,-2},
                {1,0,-1}
        };

        int[][] kernel2 = {
                {1,2,1},
                {0,0,0},
                {-1,-2,-1}
        };

        for( int i = 1; i < tmp.length -1; i++) {
            for( int j = 1; j < tmp[0].length -1; j++) {

                int r1 = 0;
                int r2 = 0;

                int g1 = 0;
                int g2 = 0;

                int b1 = 0;
                int b2 = 0;

                for( int x = 0; x < 3; x++) {
                    for( int y = 0; y < 3; y++) {

                        int aR = (tmp[i + (x - 1)][j + (y - 1)].getRed());
                        int aG = (tmp[i + (x - 1)][j + (y - 1)].getGreen());
                        int aB = (tmp[i + (x - 1)][j + (y - 1)].getBlue());

                        r1 = r1 + (aR * kernel1[x][y]);
                        r2 = r2 + (aR * kernel2[x][y]);

                        g1 = g1 + (aG * kernel1[x][y]);
                        g2 = g2 + (aG * kernel2[x][y]);

                        b1 = b1 + (aB * kernel1[x][y]);
                        b2 = b2 + (aB * kernel2[x][y]);

                    }
                }

                int finalR = (int) Math.sqrt((r1 * r1) + (r2 * r2));
                int finalG = (int) Math.sqrt((g1 * g1) + (g2 * g2));
                int finalB = (int) Math.sqrt((b1 * b1) + (b2 * b2));

                finalR = threshold(finalR, 100);
                finalG = threshold(finalG, 100);
                finalB = threshold(finalB, 100);

                tmp[i][j] = new Color(finalR, finalG, finalB);
            }
        }

        return tmp;
    }

The final RBG values are coming way to high, and the result always come mostly white.

What I am doing wrong?

*Edit: Here are the threshold and greyscale funcs:

public static Color[][]grayscale(Color[][] img) {
        Color[][] tmp = ImageUtils.cloneArray(img);

        for( int i = 0; i < tmp.length; i++)
        {
            for( int j = 0; j< tmp[i].length;j++) {

                Color pixel = tmp[i][j];
                int r = pixel.getRed();
                int g = pixel.getGreen();
                int b = pixel.getBlue();

                int grayNum = (r + g + b)/3;
                tmp[i][j] = new Color (grayNum, grayNum, grayNum);
            }
        }
        return tmp;
    }

    public static int threshold (int pixel, int threshold) {
        if(pixel > threshold){
            return 255;
        }else return 0;

    }

Answer 1

I don't see any issues. Your threshold may be too low. The largest value to be expected is sqrt((5 * 255) ^ 2) = 1275 for a single channel. Assuming that we're right at an edge (horizontal or vertical) between intensities 0 and 255, the weights of 1, 2 and 1 when all added together will be (5 * 255) and so the dynamic range of expected values can be high. 100 is probably too low. You should play around with this threshold and make it higher.