CODEWITHSUNDEEP
numpyopencv
Mustafa Magdy
Mustafa Magdy

Reputation: 1330

Grayscale image using opencv from numpy array failed

I use the following numpy array that hold an image which is black and white image with the following shape

print(img.shape)

(28, 112)

when I try to grayscale the image, to use it to get contours using opencv with following steps

#grayscale the image
 grayed = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

 #thredshold image
 thresh = cv2.threshold(grayed, 0, 255, cv2.THRESH_BINARY_INV | cv2.THRESH_OTSU)[1]

I got the following error

<ipython-input-178-7ebff17d1c18> in get_digits(img)
      6 
      7     #grayscale the image
----> 8     grayed = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
      9 
     10 

error: C:\projects\opencv-python\opencv\modules\imgproc\src\color.cpp:11073: error: (-215) depth == 0 || depth == 2 || depth == 5 in function cv::cvtColor

the opencv errors have no information in it to be able to get what is wrong

Upvotes: 2

Views: 7694

Answers (1)

Zev
Zev

Reputation: 3491

Here is the working code for how you were trying it:

img = np.stack((img,) * 3,-1)
img = img.astype(np.uint8)
grayed = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
thresh = cv2.threshold(grayed, 0, 255, cv2.THRESH_BINARY_INV | cv2.THRESH_OTSU)[1]

A simpler way of getting the same result is to invert the image yourself:

img = (255-img)
thresh = cv2.threshold(img, 0, 255, cv2.THRESH_OTSU)[1]

As you discovered, as you perform different operations on images, the image is required to be in different formats.

cv2.THRESH_BINARY_INV and cv2.THRESH_BINARY are designed to take a color image (and convert it to grayscale) so you need a three channel representation.

cv2.THRESH_OTSU works with grayscale images so one channel is okay for that.

Since your image was already grayscale from the start, you weren't able to convert it from color to grayscale nor did you really need to. I assume you were trying to invert the image but that's easy enough on your own (255-img).

At one point you tried to do an cv2.THRESH_OTSU with floating point values but cv2.THRESH_OTSU requires integers between 0 and 255.

If openCV had more user-friendly error messages it would really help with issues like these.

Upvotes: 3

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