This is how I managed to use a Sobel Kernel on a GRAYSCALE image.However,I dont actually get how to modify it for a color image.
void Soble()
{
Mat img;
int w = 3;
int k = w / 2;
char fname[MAX_PATH];
openFileDlg(fname);
img = imread(fname, CV_LOAD_IMAGE_GRAYSCALE);
gaussianFiltering(img);
Mat destinationImg = img.clone();
float sobelY[3][3] = { 1, 2, 1, 0, 0, 0, -1, -2, -1 };
float sobelX[3][3] = { -1, 0, 1, -2, 0, 2, -1, 0, 1 };
for (int i = k; i < img.rows - k; i++)
{
for (int j = k; j < img.cols - k; j++)
{
float Gx = 0, Gy = 0;
for (int l = 0; l < w; l++)
{
for (int p = 0; p < w; p++)
{
Gx += img.at<uchar>(i + l - k, j + p - k)*sobelX[l][p];
Gy += img.at<uchar>(i + l - k, j + p - k)*sobelY[l][p];
}
}
destinationImg.at<uchar>(i, j) = sqrt(Gx*Gx + Gy * Gy) / (4 * sqrt(2));
}
}
imshow("Intermediar",destinationImg);
imshow("Initial", img);
waitKey(0);
}
I thought of using each RGB chanel but it does not work and even give some errors.
float GxR = 0, GyR = 0;
float GxG = 0, GyG = 0;
float GxB = 0, GyB = 0;
for (int l = 0; l < w; l++)
{
for (int p = 0; p < w; p++)
{
GxR += img.at<Vec3b>[0](i + l - k, j + p - k)*sobelX[l][p];
GxG += img.at<Vec3b>[1](i + l - k, j + p - k)*sobelX[l][p];
GxB += img.at<Vec3b>[2](i + l - k, j + p - k)*sobelX[l][p];
GyR += img.at<Vec3b>[0](i + l - k, j + p - k)*sobelY[l][p];
GyG += img.at<Vec3b>[1](i + l - k, j + p - k)*sobelY[l][p];
GyB += img.at<Vec3b>[2](i + l - k, j + p - k)*sobelY[l][p];
}
}
destinationImg.at<Vec3b>[0](i, j) = sqrt(GxR*GxR + GyR * GyR) / (4 * sqrt(2));
destinationImg.at<Vec3b>[1](i, j) = sqrt(GxG*GxG + GyB * GyB) / (4 * sqrt(2));
destinationImg.at<Vec3b>[2](i, j) = sqrt(GxG*GxG + GyG * GyG) / (4 * sqrt(2));
Can you please explain how can this code must be rewritten?
You access the image data the wrong way.
destinationImg.at<Vec3b>[0](i, j)
destinationImg is a Mat of type Vec3b. That means it's a 2d array of three dimensional vectors.
You'r [ ] operator is in the wrong place...
The subscript error message tells you that you're using that operator on something that is neither a pointer nor an array which is not possible. You get the other error message because you have that operator where the (i,j) is expected.
First you have to get one of these vectors, then you can get its elements.
destinationImg.at<Vec3b>(i,j) will give you the vector at i,j.
destinationImg.at<Vec3b>(i,j)[0] will give you the first element of that vector.
Example from the OpenCV documentation:
Vec3b intensity = img.at<Vec3b>(y, x);
uchar blue = intensity.val[0];
uchar green = intensity.val[1];
uchar red = intensity.val[2];