OpenCV template matching parameters

Question

In the following template matching code I don't understand what the following line is doing:

cv::Mat res(ref.rows-tpl.rows+1, ref.cols-tpl.cols+1, CV_32FC1);

The full code is:

#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

int main()
{
    cv::Mat ref = cv::imread("ref.jpg");
    cv::Mat tpl = cv::imread("temp.jpg");
    if (ref.empty() || tpl.empty())
        return -1;

    cv::Mat gref, gtpl;
    cv::cvtColor(ref, gref, CV_BGR2GRAY);
    cv::cvtColor(tpl, gtpl, CV_BGR2GRAY);

    cv::Mat res(ref.rows - tpl.rows + 1, ref.cols - tpl.cols + 1, CV_32FC1);
    cv::matchTemplate(gref, gtpl, res, CV_TM_CCOEFF_NORMED);
    cv::threshold(res, res, 0.8, 1., CV_THRESH_TOZERO);

    while (true)
    {
        double minval, maxval, threshold = 0.8;
        cv::Point minloc, maxloc;
        cv::minMaxLoc(res, &minval, &maxval, &minloc, &maxloc);

        if (maxval >= threshold)
        {
            cv::rectangle(
                ref,
                maxloc,
                cv::Point(maxloc.x + tpl.cols, maxloc.y + tpl.rows),
                CV_RGB(0, 255, 0), 2
                );
            cv::floodFill(res, maxloc, cv::Scalar(0),
                0, cv::Scalar(.1), cv::Scalar(1.));
        }
        else
            break;
    }

    cv::imshow("reference", ref);
    cv::waitKey();
    return 0;
}

Answer 1

It's creating the matrix that will contain the result of the matching.

According to the documentation for matchTemplate

• result – Map of comparison results. It must be single-channel 32-bit floating-point. If image is W x H and template is wxh , then result is (W-w+1) x (H-h+1).

With reference to your code:

image    -> ref
template -> tpl
W        -> ref.cols
H        -> ref.rows
w        -> tpl.cols
h        -> tpl.rows 

single-channel 32-bit floating-point -> CV_32FC1

However, since matchTemplate will create the result image, you don't need to create it. You can simply:

...
cv::Mat res;
cv::matchTemplate(gref, gtpl, res, CV_TM_CCOEFF_NORMED);
...