Multiple results in OpenCVSharp3 MatchTemplate

Question

I am trying to find image occurences inside an image. I have written the following code to get a single match using OpenCVSharp3:

Mat src = OpenCvSharp.Extensions.BitmapConverter.ToMat(Resources.all);
Mat template = OpenCvSharp.Extensions.BitmapConverter.ToMat(Resources.img);
Mat result = src.MatchTemplate(template, TemplateMatchModes.CCoeffNormed);

double minVal, maxVal;
OpenCvSharp.Point minLoc, maxLoc;
result.MinMaxLoc(out minVal, out maxVal, out minLoc, out maxLoc);
Console.WriteLine("maxLoc: {0}, maxVal: {1}", maxLoc, maxVal);

How can I get more matches based on a threshold value?

Thank you!

Answer 1

Here's a port of the C++ code. Good luck!

using OpenCvSharp;
using OpenCvSharp.Util;
static void RunTemplateMatch(string reference, string template)
{
    using (Mat refMat = new Mat(reference))
    using (Mat tplMat = new Mat(template))
    using (Mat res = new Mat(refMat.Rows - tplMat.Rows + 1, refMat.Cols - tplMat.Cols + 1, MatType.CV_32FC1))
    {
        //Convert input images to gray
        Mat gref = refMat.CvtColor(ColorConversionCodes.BGR2GRAY);
        Mat gtpl = tplMat.CvtColor(ColorConversionCodes.BGR2GRAY);

        Cv2.MatchTemplate(gref, gtpl, res, TemplateMatchModes.CCoeffNormed);
        Cv2.Threshold(res, res, 0.8, 1.0, ThresholdTypes.Tozero);

        while (true)
        {
            double minval, maxval, threshold = 0.8;
            Point minloc, maxloc;
            Cv2.MinMaxLoc(res, out minval, out maxval, out minloc, out maxloc);

            if (maxval >= threshold)
            {
                //Setup the rectangle to draw
                Rect r = new Rect(new Point(maxloc.X, maxloc.Y), new Size(tplMat.Width, tplMat.Height));

                //Draw a rectangle of the matching area
                Cv2.Rectangle(refMat, r, Scalar.LimeGreen, 2);

                //Fill in the res Mat so you don't find the same area again in the MinMaxLoc
                Rect outRect;
                Cv2.FloodFill(res, maxloc, new Scalar(0), out outRect, new Scalar(0.1), new Scalar(1.0));
            }
            else
                break;
        }

        Cv2.ImShow("Matches", refMat);
        Cv2.WaitKey();
    }
}

Answer 2

I think that code should help you: http://opencv-code.com/quick-tips/how-to-handle-template-matching-with-multiple-occurences/

It's C++, but conversion to C# shouldn't be a big deal.

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

int main() 
{
    cv::Mat ref = cv::imread("reference.png");
    cv::Mat tpl = cv::imread("template.png");
    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;
}