opencv - plot contours in an image

问题

I am trying to draw contour around an image. I can see that contours being found but I am not able to draw the outline. The color of the contour seem to be either of the two (black and white) colors.

import cv2
import numpy as np
import matplotlib.pyplot as plt
from skimage import io
%matplotlib inline
im = io.imread('http://matlabtricks.com/images/post-35/man.png')
plt.imshow(im)
imgray = cv2.cvtColor(im,cv2.COLOR_BGR2GRAY)
plt.figure()
plt.imshow(imgray)

#Contoured image
ret,thresh = cv2.threshold(imgray, 120,255,cv2.THRESH_BINARY)
image, contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
c_img = cv2.drawContours(image, contours, -1, (0, 255, 0), 1)
plt.figure()
plt.imshow(c_img)

回答1:

You need to draw on the original image, not on the one returned from findContuors(). The following works.

# Contoured image
ret,thresh = cv2.threshold(imgray, 120,255,cv2.THRESH_BINARY)
contours = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)[-2]
for contour in contours:
   cv2.drawContours(im, contour, -1, (0, 255, 0), 3)
plt.figure()
plt.imshow(im)

回答2:

This is my result:

---

## Read and convert 
img = io.imread('http://matlabtricks.com/images/post-35/man.png')
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)

## Find outer contours 
_, cnts, _= cv2.findContours(gray,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

## Draw 
canvas = np.zeros_like(img)
cv2.drawContours(canvas , contours, -1, (0, 255, 0), 1)

plt.imshow(canvas)

来源：https://stackoverflow.com/questions/52212239/opencv-plot-contours-in-an-image