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Normalizing Sobel Filter Noise

asked 2016-01-24 21:33:19 -0500

MartinTarrou gravatar image

I'm writing my own sobel filter and am having trouble getting it to run properly. I'm applying a vertical and horizontal convolution to a grayscale image, which appear to have some result when applied separately, but not together. I only get a slightly darkened image from the original.

As you can tell from my code, I've had some trouble with exactly how to normalize the results to be within 0-255. I believe 255/1442 is correct for grayscale, though if I change the multiplier, I get white spaces around corners. Though if I do this, I also get a lot of noise that seems to be extend towards the lower left corner. In the image, I've only applied the filter to a section of the image.

Am I using the right multiplier? Is there another problem I'm encountering?

Hopefully I'm not making a pixel coordinate mistake or something, I've gone through a lot of the code in the debugger and all seems to work okay from the algorithm descriptions I've read.

Thanks in advance for any help or advice.

image description

void sobel_filter(Mat im1, Mat im2, int x_size, int y_size){

int hweight[3][3] = { { -1, 0, 1 },
{ -2, 0, 2 },
{ -1, 0, 1 } };
int vweight[3][3] = { { -1, -2, -1 },
{ 0, 0, 0 },
{ 1, 2, 1 } };


//Apply filter
float gradx;
float grady;
for (int x = 0; x < x_size; x++){
    for (int y = 0; y < y_size; y++){
        gradx = 0;
        grady = 0;
        for (int cx = -1; cx < 2; cx++){
            for (int cy = -1; cy < 2; cy++){
                if (x + cx>0 && y + cy > 0){
                    gradx = gradx + hweight[cx + 1][cy + 1] * (int) im1.at<uchar>(y + cy, x + cx);
                    grady = grady + vweight[cx + 1][cy + 1] * (int) im1.at<uchar>(y + cy, x + cx);
                }
            }
        }
                   //Use pythagorean theorem to combine both directions.
        float pyth;
        pyth = sqrt(pow(gradx, 2) + pow(grady, 2));
        //pyth = pyth / 3;
        pyth = pyth * 255/1442;
        //pyth = pyth * 255 / (1442 * 3);
        //pyth = gradx * 255.0 / 1020.0;
        im2.at<uchar>(y, x) = (int) pyth;
    }
}

}

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answered 2016-01-24 21:50:55 -0500

berak gravatar image

updated 2016-01-24 21:53:11 -0500

typical noob errors:

  • reinventing the wheel (don't!)
  • reinventing the wheel (don't!)
  • reinventing the wheel (don't!)
  • buffer overruns (need to spare a 1 pixel border for x,y when iterating)
  • getting the types wrong (the result won't fit into a uchar, you need something larger, like CV_16S or CV_32F)
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Asked: 2016-01-24 21:33:19 -0500

Seen: 822 times

Last updated: Jan 24 '16