open cv error: (-215) scn == 3 || scn == 4 in function cvtColor

Please Set As Below

img = cv2.imread('2015-05-27-191152.jpg',1)     // Change Flag As 1 For Color Image
                                                //or O for Gray Image So It image is 
                                                //already gray

Give the full path of image with forward slash. It solved the error for me.

E.g.

import numpy as np
import cv2

img = cv2.imread('C:/Python34/images/2015-05-27-191152.jpg')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

Also, if you give 0 in second parameter while loading image using cv2.imread than no need to convert image using cvtColor, it is already loaded as grayscale image eg.

import numpy as np
import cv2

gray = cv2.imread('C:/Python34/images/2015-05-27-191152.jpg',0)