Efficient difference of gaussians

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Hello!

I am trying to implement difference of guassians (DoG), for a specific case of edge detection. As the name of the algorithm suggests, it is actually fairly straightforward:

Mat g1, g2, result;
Mat img = imread("test.png", CV_LOAD_IMAGE_COLOR); 
GaussianBlur(img, g1, Size(1,1), 0);
GaussianBlur(img, g2, Size(3,3), 0);
result = g1 - g2;

However, I have the feeling that this can be done more efficiently. Can it perhaps be done in less passes over the data? I have encountered the concept of seperable filters, but I'm too much of an image processing noob to understand how to apply them in this case.