Python-Opencv:how to draw not complete rectangle with four corners on image

Question

After searching in the net, I could not find a way to draw a bounding box like the one in this image using OpenCV in Python. It has two features first four corners which are not connected with each other and second one the transparent bounding box.

I know I should use Polygon, but I can not go further that this point.

Answer 1

The following function draws an incomplete rectangle around the region of interest. I made use of cv2.line() twice for each point provided. Additionally I also made use of cv2.circle() to mark the 4 points.

There is one constraint. You will have to provide the 4 points in the following order: top-left, bottom-left, top-right, bottom-right.

Also there is an option to vary the length of the line you want to draw line_length.

Code:

def draw_border(img, point1, point2, point3, point4, line_length):

    x1, y1 = point1
    x2, y2 = point2
    x3, y3 = point3
    x4, y4 = point4    

    cv2.circle(img, (x1, y1), 3, (255, 0, 255), -1)    #-- top_left
    cv2.circle(img, (x2, y2), 3, (255, 0, 255), -1)    #-- bottom-left
    cv2.circle(img, (x3, y3), 3, (255, 0, 255), -1)    #-- top-right
    cv2.circle(img, (x4, y4), 3, (255, 0, 255), -1)    #-- bottom-right

    cv2.line(img, (x1, y1), (x1 , y1 + line_length), (0, 255, 0), 2)  #-- top-left
    cv2.line(img, (x1, y1), (x1 + line_length , y1), (0, 255, 0), 2)

    cv2.line(img, (x2, y2), (x2 , y2 - line_length), (0, 255, 0), 2)  #-- bottom-left
    cv2.line(img, (x2, y2), (x2 + line_length , y2), (0, 255, 0), 2)

    cv2.line(img, (x3, y3), (x3 - line_length, y3), (0, 255, 0), 2)  #-- top-right
    cv2.line(img, (x3, y3), (x3, y3 + line_length), (0, 255, 0), 2)

    cv2.line(img, (x4, y4), (x4 , y4 - line_length), (0, 255, 0), 2)  #-- bottom-right
    cv2.line(img, (x4, y4), (x4 - line_length , y4), (0, 255, 0), 2)

    return img

line_length = 15

img = np.zeros((512,512,3), np.uint8)
cv2.imshow('img', img)

point1, point2, point3, point4 = (280,330), (280,390), (340,330), (340,390)
fin_img = draw_border(img, point1, point2, point3, point4, line_length)

cv2.imshow('fin_img', fin_img)

cv2.waitKey()
cv2.destroyAllWindows() 

Result: