Finding contours from a thresholded image

Question

I'm currently reading through the example code on the OpenCV website tyring to find contours in an image.

I first read an image and convert to gray-scale:

img = cv2.imread('/.../.../four.png')
imgray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

I then convert the image into binary by applying a threshold:

thresh = cv2.threshold(imgray, 127, 255, 0, cv2.THRESH_BINARY)

According to the tutorials.. I should then be able to call findContours() on the thresholded image:

contours = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

When trying to execute this code, for some reason i'm getting a type error:

contours = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE) TypeError: image is not a numerical tuple

Unsure why?

Here's the full code for easier readability:

img = cv2.imread('/Users/samtozer/Desktop/four.png')
imgray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
thresh = cv2.threshold(imgray, 127, 255, 0, cv2.THRESH_BINARY)
contours = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

cv2.drawContours(img, contours, -1, (0,255,0), 3)

Wondering if anyone has experienced this problem before? And if so, what is going wrong xD

Thanks in advance

Answer 1

As mentioned in the comments there are two issues that you have to look out for:

Return type of cv2.findContours()

There are two return values in cv2.findContours():

• Contours present in the image
• The hierarchy of these contours

Return type of cv2.threshold()

There are two return values in cv2.threshold():

• Return value. (It returns float value of the threshold value which is used to classify the pixel values)
• Thresholded image