python opencv threshold red image - python-3.x

Question

I am trying to threshold a BGR image after I separate the red channel, but
my code always return "Segmentation fault".
import numpy as np
import cv2
def mostrarVentana (titulo, imagen):
print('Mostrando imagen')
cv2.imshow(titulo, imagen)
k = cv2.waitKey(0)
if k == 27: # wait for ESC key to exit
cv2.destroyAllWindows()
img = cv2.imread('RepoImagenes/640x480/P5.jpg', 1) # loading image in BGR
redImg = img[:, :, 2] # extracting red channel
rbin, threshImg = cv2.threshold(redImg, 58, 255, cv2.THRESH_BINARY) # thresholding
mostrarVentana('Binary image', threshImg)
I have read the documentation on how to use the threshold() function and I can not figure out what's wrong. I only need to work on the red channel, how can I get this done?
I am using python 3.4 and opencv 3.1.0

Answer 1

First of all opencv provides a simple API to split n-channel image, using cv2.split() which would return a list of various channels in the image.
There is also a bug in your mostrarVentana method, you have never created a cv2.namedWindow() and you are directly referencing to cv2.imshow(), but you cannot simply cv2.imshow(), without creating a cv2.namedWindow().
Also you must be sure that the image is properly loaded and then access the desired channel, otherwise it would lead to weird errors. Your code with some scenario handling would look like this:
import numpy as np
import cv2
def mostrarVentana (titulo, imagen):
print('Mostrando imagen')
cv2.namedWindow(titulo, cv2.WINDOW_NORMAL)
cv2.imshow(titulo,imagen)
k = cv2.waitKey(0)
if k == 27: # wait for ESC key to exit
cv2.destroyAllWindows()
img = cv2.imread('RepoImagenes/640x480/P5.jpg', 1) # loading image in BGR
print img.shape #This should not print error response
if not img is None and len(img.shape) == 3 and img.shape[2] == 3:
blue_img, green_img, red_img = cv2.split(img) # extracting red channel
rbin, threshImg = cv2.threshold(red_img, 58, 255, cv2.THRESH_BINARY) # thresholding
mostrarVentana('Binary image', threshImg)
else:
if img is None:
print ("Sorry the image path was not valid")
else:
print ("Sorry the Image was not loaded in BGR; 3-channel format")