OpenCV cvtColor issue in python

Question

I'm attempting to follow the instructional exercise here and I continue getting the accompanying mistake. 

The issue is on the grounds that cvtColor just takes 8 or 16-bit images

My picture is a png, from plt.savefig() in matplotlib and I think it is a 24-bit picture

1) Is my diagnosis of the problem correct or am I completely wrong?

2) Assuming 1) is true, I have been looking for ways to turn the 24-bit image into a 8/16 bit, but I haven't found anything so far. My matplotlib plot was a picture saved from a numpy array that is saved as array.npy file. Should I try to modify the array.npy file instead?

Code for reference:

import cv2 as cv

im = cv.imread("binary_graph_8.png")

imgray = cv.cvtColor(im, cv.COLOR_BGR2GRAY)

ret, thresh = cv.threshold(imgray, 127, 255, 0)

im2, contours, hierarchy = cv.findContours(thresh, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)

Error message:

File "C:/Users/ek247/.PyCharm2018.3/config/scratches/scratch_1.py", line 5, in <module>

    imgray = cv.cvtColor(im, cv.COLOR_BGR2GRAY)

cv2.error: OpenCV(4.0.0) C:\projects\opencv-python\opencv\modules\imgproc\src\color.cpp:181: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'

Answer 1

Please check the answer below:

This error happened because the image didn't load properly . So you have problem with the previous line cv2.imread my suggestion is :

- check if the images exist in the path you give

- check the count variable if he have valid number

I hope, this helps you

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