Python Opencv drawContour error

Question

It work, but my contour's color is black. How to change it to red or green?

    import numpy as np
    import cv2
    from matplotlib import pyplot as plt
    img = cv2.imread('1.jpg',0)
    img1 = cv2.imread('5.jpg',0)
    dest = cv2.subtract(img, img1)
    res = cv2.bitwise_not(dest)
    ret , threshold = cv2.threshold(res,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU)          
    cv2.namedWindow('thresimage', cv2.WINDOW_NORMAL)
    cv2.imshow('thresimage',threshold)
    _, contours, hierarchy = cv2.findContours(threshold,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
    print "Number of contours detected %d -> "%len(contours)
    cv2.drawContours(threshold,contours,-1,(0,255,0),3)
    cv2.namedWindow('contour', cv2.WINDOW_NORMAL)
    cv2.imshow('contour',threshold)
    cv2.waitKey(0)
    cv2.destroyAllWindows()

enter image description here

Kinght 金 · Accepted Answer · 2018-12-07 04:50:08Z

2

First:

contours, hierarchy = cv2.findContours(threshold,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)[-2:]

Second

ret , threshold = cv2.threshold(res,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU) 
# ...
cv2.drawContours(threshold,contours,-1,(0,255,0),3)

You draw in color (0,255,0) on binary threshed image, then it will always be the first element 0, that's black. You should convert gray to BGR first, then draw color.

canvas = cv2.cvtColor(threshold, cv2.COLOR_GRAY2BGR)
cv2.drawContours(canvas,contours,-1,(0,255,0),3)

edited Dec 7, 2018 at 4:50

answered Dec 17, 2017 at 12:36

Kinght 金

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2 Comments

Bùi Chí Thanh Over a year ago

Thank you very much! It work, but i can't see the contours after draw it.

Kinght 金 Over a year ago

@BùiChíThanh Post your result, otherwise, we cannot diagnose it.

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