How can I use Blob Detector
<p>I'm trying to use SimpleBlobDetector in OpenCV 3 to detect blobs of heat in thermal images, for example people. Any simple code or example will be appreciated.</p>

<p>i tried`   </p>

<pre><code>#include &lt;opencv2/core/core.hpp&gt;

#include &lt;opencv2/highgui/highgui.hpp&gt;

#include &lt;iostream&gt;

#include "opencv2\features2d.hpp"

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
if (argc != 2)
{
    cout &lt;&lt; " Usage: display_image ImageToLoadAndDisplay" &lt;&lt; endl;
    return 0;
}

Mat image;
image = imread(argv[1], CV_LOAD_IMAGE_GRAYSCALE);   // Read the file

if (!image.data)                              // Check for invalid input
{
    cout &lt;&lt; "Could not open or find the image" &lt;&lt; std::endl;
    return 0;
}



// Set up the detector with default parameters.
//SimpleBlobDetector detector;
// Setup SimpleBlobDetector parameters.
SimpleBlobDetector::Params params;

// Change thresholds
params.minThreshold = 50;
params.maxThreshold = 200;

// Filter by Area.
params.filterByArea = true;
params.minArea = 1500;

// Filter by Circularity
params.filterByCircularity = true;
params.minCircularity = 0.1;

// Filter by Convexity
params.filterByConvexity = true;
params.minConvexity = 0.87;

// Filter by Inertia
params.filterByInertia = true;
params.minInertiaRatio = 0.01;

// Detect blobs.
std::vector&lt;KeyPoint&gt; keypoints;
cv::Ptr&lt;cv::SimpleBlobDetector&gt; detector = cv::SimpleBlobDetector::create(params);
//detector-&gt;detect(img, keypoints);
detector-&gt;detect(image, keypoints);
//params.detect(image, keypoints);

// Draw detected blobs as red circles.
//DrawMatchesFlags::DRAW_RICH_KEYPOINTS flag ensures the size of the circle corresponds to the size of blob
Mat im_with_keypoints;
drawKeypoints(image, keypoints, im_with_keypoints, Scalar(0, 0, 255), DrawMatchesFlags::DRAW_RICH_KEYPOINTS);

// Show blobs
imshow("keypoints", im_with_keypoints);
waitKey(0);

//namedWindow("Display window", WINDOW_AUTOSIZE);// Create a window for display.
//imshow("Display window", image);                   // Show our image inside it.

//waitKey(0);                                          // Wait for a keystroke in the window
//return 0;
</code></pre>

<p>}
` but it just returns the gray image unchanged.</p>