OpenCV - How to find rectangle contour of a rectangle with round corner?

Question

I'm trying to find the contour of a rectangle object with round corner in a image. I tried HoughLinesP and findContours, but did not achieve the desired result.

I want to find the rectangle like this:

Code:

import cv2
import matplotlib.pyplot as plt
import util

image = cv2.imread("./img/findrect0.png", 1)
gray = util.grayImage(image)

edges = cv2.Canny(image, 50, 200)
lines = cv2.HoughLinesP(edges, 1, cv2.cv.CV_PI/180, 50, minLineLength=50, maxLineGap=10)[0]
linesImage = image.copy()
util.drawLines(linesImage, lines, thickness=10)

contoursImage = image.copy()
(contours, hierarchy) = cv2.findContours(gray.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

util.drawContours(contoursImage, contours, thickness=10)

util.showOpenCVImagesGrid([image, edges, linesImage, contoursImage], 2, 2, titles=["original image", "canny image", "lines image", "contours image"])

util:

import cv2
import math
import matplotlib.pyplot as plt

def showOpenCVImagesGrid(images, x, y, titles=None, axis="on"):
    fig = plt.figure()
    i = 1
    for image in images:
        copy = image.copy()
        channel = len(copy.shape)
        cmap = None
        if channel == 2:
            cmap = "gray"
        elif channel == 3:
            copy = cv2.cvtColor(copy, cv2.COLOR_BGR2RGB)
        elif channel == 4:
            copy = cv2.cvtColor(copy, cv2.COLOR_BGRA2RGBA)

        fig.add_subplot(x, y, i)
        if titles is not None:
            plt.title(titles[i-1])
        plt.axis(axis)
        plt.imshow(copy, cmap=cmap)
        i += 1
    plt.show()


def grayImage(image):
    gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
    return gray


def drawLines(image, lines, thickness=1):
    for line in lines:
        # print("line="+str(line))
        cv2.line(image, (line[0], line[1]), (line[2], line[3]),
                (0, 0, 255), thickness)


def drawContours(image, contours, thickness=1):
    i = 0
    for contour in contours:
        cv2.drawContours(image, [contours[i]], i, (0, 255, 0), thickness)
        area = cv2.contourArea(contour)
        i += 1

I'm using Python 2.7.13 and OpenCV 2.4.13.3.

I've been thinking to extend these lines and get intersection points of lines. Finally, I will get four coordinates of rectangle. But if the image is more complex, I don't know how to deal with.

Answer 1

You need to find the bounding rectangle of the found contours.

img = cv2.imread("image.png", -1)

gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

binary = cv2.bitwise_not(gray)

(_,contours,_) = cv2.findContours(binary, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)

for contour in contours:
    (x,y,w,h) = cv2.boundingRect(contour)
    cv2.rectangle(img, (x,y), (x+w,y+h), (0,255,0), 2)