How to resize ROI?

Question

I want to resize my ROI accordingly a scale factor. I'm trying this:

std::cout << roi.x << ", " << roi.y << std::endl;
std::cout << roi.x + roi.width << ", " << roi.y + roi.height << std::endl;

std::cout << img.size().width << ", " << img.size().height << std::endl << std::endl;

scale = 0.1; // 10%

cv::Rect new_size(
        roi.x*(1-scale),
        roi.y*(1-scale),
        (roi.x + roi.width)*(1+scale),
        (roi.y + roi.height)*(1+scale));
cv::Mat tmp = img(new_size);

It prints:

828, 142
892, 206
2048, 1150

281, 173
356, 248
2048, 1150

1025, 168
1100, 243
2048, 1150

And:

OpenCV Error: Assertion failed (0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows) in Mat, file /tmp/opencv-TqF1/opencv-2.4.7.1/modules/core/src/matrix.cpp, line 323
libc++abi.dylib: terminating with uncaught exception of type cv::Exception: /tmp/opencv-TqF1/opencv-2.4.7.1/modules/core/src/matrix.cpp:323: error: (-215) 0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows in function Mat

Well, I think I'm trying to set a ROI that overlaps my image boundaries. My question is: do I have to manually do this? If yes, how to scale until it touches the boundary? OpenCV should a method for this, right? I've been searching and I could find adjustROI, but I'm not sure how to use it.

Answer 1

You can either manually check/constrain the ROI boundaries or use Rect overlap, like this:

cv::Rect new_size(/*Construct rect here*/);
cv::Rect imgBounds(0,0,img.cols,img.rows);
new_size = new_size & imageBounds;
// Now you can do the following without worrying (except in the case that new_size is empty!!)
cv::Mat tmp = img(new_size);