Reference system in SimpleBlobDetector

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integer pixels are used here, no subpixel accuracy, so there is no "upper left corner" of a pixel

In most cases, the integer value of a pixel represents the center of the pixel. For example

Mat tst(1,3,CV_32F);
tst.setTo( 0 );
tst.at<float>( 0 ) = 1;
tst.at<float>( 1 ) = 2;
tst.at<float>( 2 ) = 1;
Moments m = moments(tst);
cout<<m.m01/m.m00<<" "<<m.m10/m.m00<<"\n";

prints "0 1"

The x property of the SimpleBlobDetector keypoint is of type float. Therefore, when keypoint.x = 1.5, do it indicate a point between a pair of pixels?

1.5 would be exactly on the dividing line between two pixels. For example, if you make that 1 2 2 1 instead, you'll get 1.5, ie: the middle of the four pixels, or right between the middle two.