Does a simple percentage based center crop.

centerCropMat

Mat centerCropMat(const cv::Mat& image, double cropPercentage)
{
	cropPercentage = min(cropPercentage, 1.0);

	int newRows = (int)floor(image.rows * cropPercentage + 0.5);
	int newCols = (int)floor(image.cols * cropPercentage + 0.5);
	Point center(image.cols / 2, image.rows / 2);

	Rect roi(
		center.x - (newCols / 2),
		center.y - (newRows / 2),
		newCols,
		newRows
	);

	return image(roi).clone();
}