Cropping Images

Cropping an image is a very simple operation. First, we need to establish a rectangle defining the region which we want to take a section of (referred to as a Region of Interest, or roi).

Defining a Rectangle

To define a rectangle, we need to create a Rect object. There are multiple constructors for the Rect class, one uses two Point objects and the other uses four ints. Both constructors create the same rectangle, but they are used differently. When using the four int method, the first two ints are the x and y coordinates of the top-left corner of the rectangle, and the last two ints are the width and height of the rectangle. When using the two Point method, the first Point is the top-left corner of the rectangle, and the second Point is the bottom-right corner of the rectangle.

Using Points

To use the two Point method, we need to know the top-left and bottom-right corners of the rectangle. We can create two Point objects to represent these corners, then pass them into the Rect constructor.

Point topLeft = new Point(0, 0);
Point bottomRight = new Point(100, 100);
Rect roi = new Rect(topLeft, bottomRight);

This will create a rectangle with the top-left corner at (0, 0) and the bottom-right corner at (100, 100).

Using ints

Using the four int method is different. Instead of using the top-left and bottom-right corners, we use the top-left corner and the width and height of the rectangle.

Rect roi = new Rect(0, 0, 100, 100);

This will create the same rectangle as the previous example.

Extracting the Region of Interest

Once we have our rectangle, we can use it to extract the region of interest from the image. To do this, we will create a new Mat object and use the submat method to extract the region of interest.

Mat cropped = input.submat(roi);

This cropped image will be a reference to the original image, so any changes made to the cropped image will also be made to the original image. Now this is an interesting property, but it can also be a problem. If we don't want this behavior, we can use the clone method to create a copy of the image.

Mat cropped = input.submat(roi).clone();

The clone method creates a copy of the image, so any changes made to the cropped image will not be made to the original image.