How can I draw a rectangle from the points I want, using ROI?

Question

Hello I am beginner in OpenCv. I have a maze image. I wrote maze solver code. I need to get the photo like the picture for this code to work. I want to choose the contours of the white area using ROI but I could not

When I try the ROI method I get a smooth rectangle with a black area selected.

https://i.stack.imgur.com/Ty5BX.png -----> this is my code result

https://i.stack.imgur.com/S7zuJ.png --------> I want to this result

import cv2
import numpy as np

#import image
image = cv2.imread('rt4.png')

#grayscaleqq
gray = cv2.cvtColor(image,cv2.COLOR_BGR2GRAY)
#cv2.imshow('gray', gray)
#qcv2.waitKey(0)

#binary
#ret,thresh = cv2.threshold(gray,127,255,cv2.THRESH_BINARY_INV)
threshold = 150
thresh = cv2.threshold(gray, threshold, 255, cv2.THRESH_BINARY)[1]
cv2.namedWindow('second', cv2.WINDOW_NORMAL)
cv2.imshow('second', thresh)
cv2.waitKey(0)
cv2.destroyAllWindows()

#dilation
kernel = np.ones((1,1), np.uint8)
img_dilation = cv2.dilate(thresh, kernel, iterations=1)
cv2.namedWindow('dilated', cv2.WINDOW_NORMAL)
cv2.imshow('dilated', img_dilation)
cv2.waitKey(0)
cv2.destroyAllWindows()




#find contours
im2,ctrs, hier = cv2.findContours(img_dilation.copy(), 
cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)


#sort contours
sorted_ctrs = sorted(ctrs, key=lambda ctr: cv2.boundingRect(ctr) 
[0])

list = []

for i, ctr in enumerate(sorted_ctrs):
# Get bounding box
x, y, w, h = cv2.boundingRect(ctr)

# Getting ROI
roi = image[y:y+h, x:x+w]

a = w-x
b = h-y
list.append((a,b,x,y,w,h))


# show ROI
#cv2.imshow('segment no:'+str(i),roi)
cv2.rectangle(image,(x,y),( x + w, y + h ),(0,255,0),2)
#cv2.waitKey(0)

if w > 15 and h > 15:
    cv2.imwrite('home/Desktop/output/{}.png'.format(i), roi)

cv2.namedWindow('marked areas', cv2.WINDOW_NORMAL)
cv2.imshow('marked areas',image)
cv2.waitKey(0)
cv2.destroyAllWindows()




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

gray = np.float32(gray)
dst = cv2.cornerHarris(gray,2,3,0.04)

#result is dilated for marking the corners, not important
dst = cv2.dilate(dst,None)


image[dst>0.01*dst.max()]=[0,0,255]

cv2.imshow('dst',image)
if cv2.waitKey(0) & 0xff == 27:
cv2.destroyAllWindows()

list.sort()
print(list[len(list)-1])

If you threshold the image and findContours on it, then drawContours, you should get the second one. — Kinght 金, Sep 15 '18 at 03:17

score 0 · Answer 1 · answered Sep 14 '18 at 17:15

A simple solution to just draw a slanted rectangle would be to use cv2.polylines. Based on your result, I'm assuming you have the coordinates of the vertices of the area already, lets call them [x1,y1], [x2,y2], [x3,y3], [x4,y4]. The polylines function draws a line from vertex to vertex to create a closed polygon.

import cv2
import numpy as np

#List coordinates of vertices as an array
pts = np.array([[x1,y1],[x2,y2],[x3,y3],[x4,y4]], np.int32)
pts = pts.reshape((-1,1,2))

#Draw lines from vertex to vertex
cv2.polylines(image, [pts], True, (255,0,0))

hi, First of all thank you for your reply. I do not have corner points. I just have ROI rectangle points and I do not know how to get the corner points. I am a beginner opencv. Do you know how to get ? — ömer ali erdemir, Sep 15 '18 at 06:44

Eshita Shukla · Accepted Answer · 2018-09-16T09:08:23.060

0

I misunderstood your question earlier. So, I'm rewriting.

As @Silencer has already stated, you could use the drawContours method. You can do it as follows:

import cv2
import numpy as np

#import image
im = cv2.imread('Maze2.png')
gaus = cv2.GaussianBlur(im, (5, 5), 1)
# mask1 = cv2.dilate(gaus, np.ones((15, 15), np.uint8, 3))
mask2 = cv2.erode(gaus, np.ones((5, 5), np.uint8, 1))
imgray = cv2.cvtColor(mask2, cv2.COLOR_BGR2GRAY)
ret, thresh = cv2.threshold(imgray, 127, 255, 0)
im2, contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

maxArea1=0
maxI1=0

for i in range(len(contours)):
    area = cv2.contourArea(contours[i])
    epsilon = 0.01 * cv2.arcLength(contours[i], True)
    approx = cv2.approxPolyDP(contours[i], epsilon, True)
    if area > maxArea1 :
        maxArea1 = area

print(maxArea1)
print(maxI1)
cv2.drawContours(im, contours, maxI1, (0,255,255), 3)

cv2.imshow("yay",im)
cv2.imshow("gray",imgray)
cv2.waitKey(0)

cv2.destroyAllWindows()

I used it on the following image: And I got the right answer. You can add additional filters, or you could decrease the area using an ROI, to decrese the discrepancy, but it wasn't required

Hope it helps!

edited Sep 16 '18 at 09:08

answered Sep 14 '18 at 17:39

Eshita Shukla

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thanks for your help but I can not use cv2.selectROI because code automatically selected rectangle – ömer ali erdemir Sep 15 '18 at 06:46
My fault! I didn't understand the question properly. I will update my answer in sometime.. Apologies! – Eshita Shukla Sep 15 '18 at 07:12
my bad english, sorry :( – ömer ali erdemir Sep 15 '18 at 08:16
thanks for your help, I have quastion. I need full white contours. Your code gives me a little outside of the white is drawing the frame – ömer ali erdemir Sep 16 '18 at 08:16
I figured out the problem, just run the erosion on `gaus` rather than on `mask1`, i.e. don't perform dilation at all. It works perfectly now. Run it and let me know if it works :) – Eshita Shukla Sep 16 '18 at 09:07
worked perfectly ! Thank you. I have one more question :) how can i crop the detected area and rotate it. – ömer ali erdemir Sep 16 '18 at 11:20
I searched a lot, there is no documentation on cutting out slant rectangles, but look here: https://stackoverflow.com/questions/52355175/cut-out-a-slant-rectangle-from-an-image-python3, might get some answers.Answer if you get to know how to do it. – Eshita Shukla Sep 16 '18 at 17:04

How can I draw a rectangle from the points I want, using ROI?

2 Answers2