Can template matching in OpenCV deal with two same-sized images?

Question

I want to use template matching in OpenCV to get the similarity of two images. As we all know,template matching is usually used to find smaller image parts in a bigger one. Here is my question. I find when template image and source image are same-sized, the result matrix get from function matchTemplate() is always 0, even if the two images are exactly the same one.

Can template matching in OpenCV deal with two same-sized images?

Answer 1

Perhaps I should apologize first: the value of the matrix is indeed zero after normalization, as long as the two pictures are of the same size. I was wrong about that:)

Check out this page: OpenCV - Normalize

Part of the OpenCV source code:

void cv::normalize( InputArray _src, OutputArray _dst, double a, double b,
                    int norm_type, int rtype, InputArray _mask )
{
    Mat src = _src.getMat(), mask = _mask.getMat();

    double scale = 1, shift = 0;
    if( norm_type == CV_MINMAX )
    {
        double smin = 0, smax = 0;  //Records the maximum and minimum value in the _src matrix
        double dmin = MIN( a, b ), dmax = MAX( a, b );
        minMaxLoc( _src, &smin, &smax, 0, 0, mask );  //Find the minimum and maximum value
        scale = (dmax - dmin)*(smax - smin > DBL_EPSILON ? 1./(smax - smin) : 0);
        shift = dmin - smin*scale;
    }

    //...

    if( !mask.data )
        src.convertTo( dst, rtype, scale, shift );
    else
    {
        //...
    }
}

Since there is only one element in the result array, smin = smax = result[0][0]

scale = (dmax - dmin)*(smax - smin > DBL_EPSILON ? 1./(smax - smin) : 0);
      =  (1 - 0 ) * (0) = 0
shift = dmin - smin*scale
      = 0 - result[0][0] * 0
      = 0

After that, void Mat::convertTo(OutputArray m, int rtype, double alpha, double beta) uses the following formula: (saturate_cast has nothing to do with your problem, so we can ignore it for now.)

When you call normalize( result, result, 0, 1, NORM_MINMAX, -1, Mat() ), whatever the element in the matrix is, it will execute src.convertTo( dst, rtype, scale, shift ); with scale = 0, shift = 0. In this convertTo function,

alpha = 0, beta = 0
result[0][0] = result[0][0] * alpha + beta
             = result[0][0] * 0 + 0
             = 0

So, whatever the value in the result matrix is:

As long as the image and the template are of the same size, size of the result matrix will be 1x1, and after normalization, the result matrix will become a [0].