How to draw a rounded rectangle (rectangle with rounded corners) with OpenCV?

Question

How can I draw a rectangle with rounded corners in OpenCV? I know that the functions ellipse() and line() can be simply put together to draw it. I just wonder if someone has done it before and has put it in a proper function so I can use it? Ideally the corner radius is to calibrate in a parameter.

I searched a lot for that, but it seems no one had that problem before. If no one has such I function I will probably post my own solution here in a few days.

Answer 1

I realized, this is much easier that I thought. Here is my function. I hope it is helpful for someone.

/**
 * Draws a rectangle with rounded corners, the parameters are the same as in the OpenCV function @see rectangle();
 * @param cornerRadius A positive int value defining the radius of the round corners.
 * @author K
 */
void rounded_rectangle( Mat& src, Point topLeft, Point bottomRight, const Scalar lineColor, const int thickness, const int lineType , const int cornerRadius)
{
    /* corners:
     * p1 - p2
     * |     |
     * p4 - p3
     */
    Point p1 = topLeft;
    Point p2 = Point (bottomRight.x, topLeft.y);
    Point p3 = bottomRight;
    Point p4 = Point (topLeft.x, bottomRight.y);

    // draw straight lines
    line(src, Point (p1.x+cornerRadius,p1.y), Point (p2.x-cornerRadius,p2.y), lineColor, thickness, lineType);
    line(src, Point (p2.x,p2.y+cornerRadius), Point (p3.x,p3.y-cornerRadius), lineColor, thickness, lineType);
    line(src, Point (p4.x+cornerRadius,p4.y), Point (p3.x-cornerRadius,p3.y), lineColor, thickness, lineType);
    line(src, Point (p1.x,p1.y+cornerRadius), Point (p4.x,p4.y-cornerRadius), lineColor, thickness, lineType);

    // draw arcs
    ellipse( src, p1+Point(cornerRadius, cornerRadius), Size( cornerRadius, cornerRadius ), 180.0, 0, 90, lineColor, thickness, lineType );
    ellipse( src, p2+Point(-cornerRadius, cornerRadius), Size( cornerRadius, cornerRadius ), 270.0, 0, 90, lineColor, thickness, lineType );
    ellipse( src, p3+Point(-cornerRadius, -cornerRadius), Size( cornerRadius, cornerRadius ), 0.0, 0, 90, lineColor, thickness, lineType );
    ellipse( src, p4+Point(cornerRadius, -cornerRadius), Size( cornerRadius, cornerRadius ), 90.0, 0, 90, lineColor, thickness, lineType );
}

Answer 2

Here is the python version with filled/not filled feature and corner_radius automatically calculated based on the image's height.

import cv2
import numpy as np

def rounded_rectangle(src, top_left, bottom_right, radius=1, color=255, thickness=1, line_type=cv2.LINE_AA):

    #  corners:
    #  p1 - p2
    #  |     |
    #  p4 - p3

    p1 = top_left
    p2 = (bottom_right[1], top_left[1])
    p3 = (bottom_right[1], bottom_right[0])
    p4 = (top_left[0], bottom_right[0])

    height = abs(bottom_right[0] - top_left[1])

    if radius > 1:
        radius = 1

    corner_radius = int(radius * (height/2))

    if thickness < 0:

        #big rect
        top_left_main_rect = (int(p1[0] + corner_radius), int(p1[1]))
        bottom_right_main_rect = (int(p3[0] - corner_radius), int(p3[1]))

        top_left_rect_left = (p1[0], p1[1] + corner_radius)
        bottom_right_rect_left = (p4[0] + corner_radius, p4[1] - corner_radius)

        top_left_rect_right = (p2[0] - corner_radius, p2[1] + corner_radius)
        bottom_right_rect_right = (p3[0], p3[1] - corner_radius)

        all_rects = [
        [top_left_main_rect, bottom_right_main_rect], 
        [top_left_rect_left, bottom_right_rect_left], 
        [top_left_rect_right, bottom_right_rect_right]]

        [cv2.rectangle(src, rect[0], rect[1], color, thickness) for rect in all_rects]

    # draw straight lines
    cv2.line(src, (p1[0] + corner_radius, p1[1]), (p2[0] - corner_radius, p2[1]), color, abs(thickness), line_type)
    cv2.line(src, (p2[0], p2[1] + corner_radius), (p3[0], p3[1] - corner_radius), color, abs(thickness), line_type)
    cv2.line(src, (p3[0] - corner_radius, p4[1]), (p4[0] + corner_radius, p3[1]), color, abs(thickness), line_type)
    cv2.line(src, (p4[0], p4[1] - corner_radius), (p1[0], p1[1] + corner_radius), color, abs(thickness), line_type)

    # draw arcs
    cv2.ellipse(src, (p1[0] + corner_radius, p1[1] + corner_radius), (corner_radius, corner_radius), 180.0, 0, 90, color ,thickness, line_type)
    cv2.ellipse(src, (p2[0] - corner_radius, p2[1] + corner_radius), (corner_radius, corner_radius), 270.0, 0, 90, color , thickness, line_type)
    cv2.ellipse(src, (p3[0] - corner_radius, p3[1] - corner_radius), (corner_radius, corner_radius), 0.0, 0, 90,   color , thickness, line_type)
    cv2.ellipse(src, (p4[0] + corner_radius, p4[1] - corner_radius), (corner_radius, corner_radius), 90.0, 0, 90,  color , thickness, line_type)

    return src

Usage:

top_left = (0, 0)
bottom_right = (500, 800)
color = (255, 255, 255)
image_size = (500, 800, 3)
img = np.zeros(image_size)
img = rounded_rectangle(img, top_left, bottom_right, color=color, radius=0.5, thickness=-1)

cv2.imshow('rounded_rect', img)
cv2.waitKey(0)