SimpleBlobDetector in opencv does not detect anything

Question

I'm using OpenCV 3.1 to do some blob detection using SimpleBlobDetector but I'm having no luck and no tutorial has been able to solve this. My environment is XCode on x64.

I'm starting out with this image:

Then I'm turning it into greyscale:

Finally I turn it into a binary image and doing the blob detection on this:

I've included "iostream" and "opencv2/opencv.hpp".

using namespace cv;
using namespace std;

Mat img_rgb;
Mat img_gray;
Mat img_keypoints;    
Ptr<SimpleBlobDetector> detector = SimpleBlobDetector::create();

vector<KeyPoint> keypoints;

img_rgb = imread("summertriangle.jpg");

//Convert to greyscale
cvtColor(img_rgb, img_gray, CV_RGB2GRAY);

imshow("Grey Scale", img_gray);

// Start by creating the matrix that will allocate the new image
Mat img_bw(img_gray.size(), img_gray.type());

// Apply threshhold to convert into binary image and save to new matrix
threshold(img_gray, img_bw, 100, 255, THRESH_BINARY);

// Extract cordinates of blobs at their centroids, save to keypoints variable.
detector->detect(img_bw, keypoints);
cout << "The size of keypoints vector is: " << keypoints.size();

The keypoints vector is always empty. Nothing I've tried works.

Answer 1

So I solved this, did not read the fine print on the docs. Thanks Dai for the heads up on the Params, made me give the docs a closer look.

Default values of parameters are tuned to extract dark circular blobs.

I had to simply do this when creating the SimpleBlobDetector object:

SimpleBlobDetector::Params params;

params.filterByArea = true;
params.minArea = 1;
params.maxArea = 1000;
params.filterByColor = true;
params.blobColor = 255;

Ptr<SimpleBlobDetector> detector = SimpleBlobDetector::create(params);

This did it.