Set rectangle style in OpenCV

Question

I wish to know how many styles does OpenCV have for drawing detections. I wish know how to draw the rectangle like in this image:

Answer 1

OpenCV doesn't provide styles. You can only draw a rectangle with a given color, with 4/8 connected or with anti-aliasing points, with a given thickness.

You can, however, simply draw 8 lines recovering the coordinates from the rectangle:

The code is pretty straightforward:

#include <opencv2/opencv.hpp>
using namespace cv;

void drawDetection(Mat3b& img, const Rect& r, Scalar color = Scalar(0,255,0), int thickness = 3)
{
    int hor = r.width / 7;
    int ver = r.height / 7;

    // Top left corner
    line(img, r.tl(), Point(r.x, r.y + ver), color, thickness);
    line(img, r.tl(), Point(r.x + hor, r.y), color, thickness);

    // Top right corner
    line(img, Point(r.br().x - hor, r.y), Point(r.br().x, r.y), color, thickness);
    line(img, Point(r.br().x, r.y + ver), Point(r.br().x, r.y), color, thickness);

    // Bottom right corner
    line(img, Point(r.br().x, r.br().y - ver), r.br(), color, thickness);
    line(img, Point(r.br().x - hor, r.br().y), r.br(), color, thickness);

    // Bottom left corner
    line(img, Point(r.x, r.br().y - ver), Point(r.x, r.br().y), color, thickness);
    line(img, Point(r.x + hor, r.br().y), Point(r.x, r.br().y), color, thickness);
}

int main()
{
    // Load image
    Mat3b img = imread("path_to_image");

    // Your detection
    Rect detection(180, 160, 220, 240);

    // Custom draw
    drawDetection(img, detection);

    imshow("Detection", img);
    waitKey();

    return 0;
}