Draw contours correctly on the image (OpenCV/Python)

Question

I am trying to draw contours around my test image. I am using canny edge detection in the background.

The findContours method works fine for my image but when I try to do drawContours method on that image. It does not show anything.

Here is what I have tried

import cv2
import numpy as np

image = cv2.imread('/path/to/image.jpg')

gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
blurred = cv2.GaussianBlur(gray, (11, 11), 0)
cv2.imshow("blurred", blurred)

canny = cv2.Canny(blurred, 30, 150)

(_, cnts, _) = cv2.findContours(canny.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

print "Contours in the image, %d" % (len(cnts))

shape = image.copy()
cv2.drawContours(shape.copy(), cnts, -1, (0, 255, 0), 2)
cv2.imshow("Edges", shape)

From what I gather from the docs, the fourth argument to the method drawContours would be used to specify the colour of the edge to be drawn over. But it does not show anything for instead of the green edge that I am expecting.

len(cnts) returns 2 for me

Here is the image I am trying it out with

I am using opencv version 3.0.0

Relevant SO question

EDIT: After changing the 3rd argument for cv2.findContours() to cv2.CHAIN_APPROX_NONE, it is still not showing the final green edges(or any colour for that matter) on the final cv2.imshow("Edges", shape) image. Here is what I get from the canny edge image

Try with cv2.CHAIN_APPROX_NONE

ZdaR
– ZdaR

2017-03-29 06:13:26 +00:00
Commented Mar 29, 2017 at 6:13 — ZdaR
– ZdaR, Commented Mar 29, 2017 at 6:13
@ZdaR please check the edited question

Tasdik Rahman
– Tasdik Rahman

2017-03-29 06:25:35 +00:00
Commented Mar 29, 2017 at 6:25 — Tasdik Rahman
– Tasdik Rahman, Commented Mar 29, 2017 at 6:25

Jeru Luke · Accepted Answer · 2017-03-29 06:27:56Z

1

You have to modify the last to lines of your code to:

You are already storing a copy of image in shape

shape = image.copy()

So why do you use shape.copy() in cv2.drawContours() again?

Replace it as follows:

cv2.drawContours(shape, cnts, -1, (0, 255, 0), 2)
cv2.imshow("Edges", shape)

NOTE: You already have copied the image once, so use it to draw the contours. You don't have to use a copied version of the copied image.

This is what you get as a result:

answered Mar 29, 2017 at 6:27

Jeru Luke

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