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cv::fastNlMeansDenoising does not return


I have some problems with cv::findCirclesGrid. Some of my image have very poor contrast and cv::findCircleGrid sometimes takes forever (several minutes) before it returns false.

Therefore, I want to make a better contrast using cv::equalizeHist. The problem with this is, that the image gets really noisy. I tried to denoise it with cv::fastNlMeansDenoising.

I waited several minutes, but the function did not return and I have no idea what to do now.

Has anyone an idea how I could denoise (or generally prepare) my images before using cv::findCirclesGrid? I use Visual Studio 2013 Pro and OpenCv 2.4.9 (x86).

I attach one sample image bevor and after cv::equalizeHist. And here is my code:

for (auto &img : images)
    cv::Mat gray;

    cv::cvtColor(img, gray, CV_BGRA2GRAY);

    cv::Mat grayFiltered(gray.rows, gray.cols, gray.type());

    //cv::Ptr<cv::CLAHE> clahe = cv::createCLAHE();

    //clahe->apply(gray, gray);
    cv::equalizeHist(gray, gray);

    cv::fastNlMeansDenoising(gray, grayFiltered);
    //cv::threshold(gray, gray, 160, 255, CV_THRESH_BINARY); // Needs a different threshold, depending on the actual image