How to save OpenCV image with contour

Question

I want to save image with contour

Here is my code:

img = cv2.imread('123.png')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
ret, binary = cv2.threshold(gray, 150, 255, cv2.THRESH_BINARY)
image, contours, _ = cv2.findContours(binary, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

for cnt in contours:
    # some code in here
    cv2.imwrite('234.jpg', cnt)

Thanks a lot.

Answer 1

What you want to do is to create a mask that you draw the contours on to, then use that to snip out the rest of the picture, or vice-versa. For instance, based on this tutorial:

(contours, _) = cv2.findContours(binary, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
mask = np.ones(img.shape[:2], dtype="uint8") * 255

# Draw the contours on the mask
cv2.drawContours(mask, contours, -1, 0, -1)

# remove the contours from the image and show the resulting images
img = cv2.bitwise_and(img, img, mask=mask)
cv2.imshow("Mask", mask)
cv2.imshow("After", img)
cv2.waitKey(0)

Answer 2

The easiest way to save the contour as image is taking out its ROI(region of image) and saving it using imwrite() as follows -

First use cv2.boundingRect to get the bounding rectangle for a set of points (i.e. contours):

x, y, width, height = cv2.boundingRect(contours[i])

You can then use NumPy indexing to get your ROI from the image:

roi = img[y:y+height, x:x+width]

And save the ROI to a new file:

cv2.imwrite("roi.png", roi)