Sobel Operator MaskX

Question

Why my image quality after masking x get worse?

public void doMaskX() {
    int[][] maskX = { { -1, -2, -1 }, { 0, 0, 0 }, { 1, 2, 1 } };

    int rgb, alpha = 0;
    int[][] square = new int[3][3];

    for (int y = 0; y < width - 3; y++) {
        for (int x = 0; x < height - 3; x++) {
            int sum = 0;
            for (int i = 0; i < 3; i++) {
                for (int j = 0; j < 3; j++) {
                    rgb = imgx.getRGB(y + i, x + j);

                    alpha = (rgb >> 24) & 0xff;
                    int red = (rgb >> 16) & 0xff;
                    int green = (rgb >> 8) & 0xff;
                    int blue = rgb & 0xff;

                    square[i][j] = (red + green + blue)/3;
                    sum += square[i][j] * maskX[i][j];
                }
            }
            rgb = (alpha << 24) | (sum << 16) | (sum << 8) | sum;
            imgx.setRGB(y, x, rgb);

        }
    }
    writeImg();
}

the quality should be better of second image and why is yellow color appears?

As far as I know, the Sobel operator is for edge detection and not for enhancing image quality, however you define that. The yellow pixels are probably caused by overflows. Are you sure that `sum` is always <256? — Frank Puffer, Apr 08 '16 at 16:21
By the way: This is a somewhat odd way of implementing it. You could also consider to [not reinvent the wheel](https://docs.oracle.com/javase/8/docs/technotes/guides/2d/spec/j2d-image.html#wp63208) — Marco13, Apr 08 '16 at 23:32

gpasch · Accepted Answer · 2016-04-08T19:19:19.710

1

It is important to realize that you are computing the intensity of the gradient here and that is what you are displaying. Therefore the intensity (or magnitude) is a positive number. You have to take the absolute value:

sum=Math.abs(sum);

If you take the y derivative also then you can combine:

sum=Math.sqrt(sumx*sumx+sumy*sumy);

edited Apr 08 '16 at 19:19

answered Apr 08 '16 at 18:43

gpasch

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sum = Math.abs(sum); solved the problem THank you very match. Math.sqrt... is also used in merging imgX and imgY – szufi Apr 08 '16 at 19:36

Sobel Operator MaskX

1 Answers1