Python-OpenCV cv2 OpenCV Error: Assertion failed

Question

I'm trying to create an image in python using openCV. I make a list of lists, each list having 16 numbers, from 0 to 255 (16 lists). Then I convert the big list in a numpy ndarray, and try to write that into an image using cv2.imwrite(). This is my code:

import cv2
import numpy as np
colours = []
numbers = []
a=0
for i in range(256):
 numbers.append(a)
 a+=1
for x in range(16):
 new_list = [numbers[16*x:16*x+16]]
 colours.append(new_list)
col = np.asarray(colours)
new_image = cv2.imwrite("rainbow.png",col)

It runs well until the last line. Then it gives me this error:

OpenCV Error: Assertion failed (image.channels() == 1 || image.channels() == 3 || image.channels() == 4) in cv::imwrite_, file C:\projects\opencv-python\opencv\modules\imgcodecs\src\loadsave.cpp, line 600
Traceback (most recent call last):
 File "kormou.py", line 16, in <module>
 new_image = cv2.imwrite("rainbow.png",col)
cv2.error: C:\projects\opencv-python\opencv\modules\imgcodecs\src\loadsave.cpp:600: error: (-215) image.channels() == 1 || image.channels() == 3 || image.channels() == 4 in function cv::imwrite_

In generally I'm a beginner so it may be something very obvious which I'm missing but I haven't been able to find similar error question here.

• Lamar Latrell: It's saying that the assertion that "image.channels() == 1 || image.channels() == 3 || image.channels() == 4" has failed in the imwrite function. So, how many channels does the image have?
• Dan Mašek: You're feeding a 16 channel "image" to imwrite. As the docs state, it only supports 1,3 or 4 channels. Also, it doesn't return an image, but a boolean, so the variable name new_image is rather misleading.

Answer 1

There is this very minute mistake you are doing. Figure out from the following working code -

import cv2
import numpy as np
colours = []
numbers = []
a=0
for i in range(256):
 numbers.append(a)
 a+=1
for x in range(16):
 new_list = numbers[16*x:16*x+16]
 colours.append(new_list)
print colours
col = np.asarray(colours)
new_image_flag = cv2.imwrite("rain.png",col)

Check edit for hint.