Question

I’m new with C++, and try to figuring out what this line of code means:

cur_rect = cv::Rect(cur_rect) & cv::Rect(0, 0, mat->cols, mat->rows); // here
if( cv::Rect(cur_rect) == cv::Rect() )  //here
{
.......
}

Answer 1

The Rect & Rect part intersects two rectangles and gives a non-empty rectangle back when the two inputs overlap.

So you can compare the result to Rect() to see whether there was an intersection. Your code crops cur_rect to (0, 0, mat->cols, mat->rows) and then checks whether it is empty or not.

Sources:

http://opencv.willowgarage.com/documentation/cpp/core_basic_structures.html?highlight=rect

How can one easily detect whether 2 ROIs intersects in OpenCv?

Edit

An alternative implementation, a bit cleaner:

// crop cur_rect to rectangle with matrix 'mat' size:
cur_rect &= cv::Rect(0, 0, mat->cols, mat->rows);
if (cur_rect.area() == 0) {
    // result is empty
    ...
}