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I'm suspicious about how the Sobel filter treats non-continuous images.

Two identical images -- one made by changing the ROI of a larger image and the other by cloning -- produce different results when run through the filter:

#include <cassert>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

int main(int argc, char** argv)
{
    // Create a matrix that is all-zeros other than the last column
    Mat orig(100, 100, CV_8U, cv::Scalar(0)), copy;
    orig.col(orig.cols-1).setTo(255);

    // Crop the last column out of the matrix in two ways:
    //  a) Using colRange, which only creates a new header
    //  b) Using copyTo, which creates a new header and data
    orig = orig.colRange(0, orig.cols - 1);
    orig.copyTo(copy);

    // Assert that the matrices are the same
    assert(countNonZero(orig != copy) == 0);

    // Run a Sobel filter on both matrices
    Sobel(orig, orig, CV_16S, 1, 0);
    Sobel(copy, copy, CV_16S, 1, 0);

    // Assert that the Sobel filter results are the same
    assert(countNonZero(orig != copy) == 0);
}

This bit of sample code fails on the second assert statement.

(OpenCV 2.4.6.1 on Mac OS 10.6, 64-bit)