OpenCV, cv2.approxPolyDP() Draws double lines on closed contour

问题

I want to create some polygons out of this mask:

image 1 - Mask

So i created these contours with openCV findcontours():

image 2 - Contours

When creating polygons I get these polygons:

image 3 - Polygons

As you can see some polygons are drawn using double lines. How do I prevent this?

See my code:

import glob
from PIL import Image
import cv2
import numpy as np


# Let's load
image = cv2.imread(path + "BigOneEnhanced.tif") 

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

# Find Canny edges 
edged = cv2.Canny(gray, 30, 200) 

# Finding Contours 
contours, hierarchy = cv2.findContours(edged,  
    cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_TC89_L1) 

canvas = np.zeros(image.shape, np.uint8)

# creating polygons from contours

polygonelist = []

for cnt in contours:

    # define contour approx
    perimeter = cv2.arcLength(cnt,True)
    epsilon = 0.005*cv2.arcLength(cnt,True)
    approx = cv2.approxPolyDP(cnt,epsilon,True)


    polygonelist.append(approx)

cv2.drawContours(canvas, polygonelist, -1, (255, 255, 255), 3)


imgB = Image.fromarray(canvas)
imgB.save(path + "TEST4.png")

回答1:

The problem source is the Canny edges detection:

After applying edge detection you are getting two contours for every original contour - one outside the edge and one inside the edge (and other weird stuff).

You may solve it by applying findContours without using Canny.

Here is the code:

import glob
from PIL import Image
import cv2
import numpy as np

path = ''

# Let's load
#image = cv2.imread(path + "BigOneEnhanced.tif") 
image = cv2.imread("BigOneEnhanced.png") 

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

# Apply threshold (just in case gray is not binary image).
ret, thresh_gray = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY+cv2.THRESH_OTSU)

# Find Canny edges 
#edged = cv2.Canny(gray, 30, 200)

# Finding Contours cv2.CHAIN_APPROX_TC89_L1
#contours, hierarchy = cv2.findContours(edged, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)

contours, hierarchy = cv2.findContours(thresh_gray, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)

canvas = np.zeros(image.shape, np.uint8)

# creating polygons from contours
polygonelist = []

for cnt in contours:
    # define contour approx
    perimeter = cv2.arcLength(cnt, True)
    epsilon = 0.005*perimeter #0.005*cv2.arcLength(cnt, True)
    approx = cv2.approxPolyDP(cnt, epsilon, True)

    polygonelist.append(approx)

cv2.drawContours(canvas, polygonelist, -1, (255, 255, 255), 3)

imgB = Image.fromarray(canvas)
imgB.save(path + "TEST4.png")

Result:

来源：https://stackoverflow.com/questions/60654186/opencv-cv2-approxpolydp-draws-double-lines-on-closed-contour

标签

python

OpenCV

polygon

mask

opencv-contour