Working with transparent background using OpenCV

Question

I'm trying to obtain the biggest yellow object in my image. I'm doing the following:

def findYellow(img):
    hsv = cv2.cvtColor(img,cv2.COLOR_BGR2HSV)
    lower_yellow, upper_yellow = np.array([18, 50, 90]), np.array([30, 255, 255])
    mask = cv2.inRange(hsv, lower_yellow, upper_yellow)
    _, contours, _ = cv2.findContours(mask, cv2.RETR_TREE, cv2.CHAIN_APPROX_NONE)
    max_contour = max(contours, key = cv2.contourArea)
    mask = np.zeros(img.shape[:2], dtype = 'uint8')
    cv2.drawContours(mask, [max_contour], -1, (255), -1)
    result = cv2.bitwise_and(img, img, mask=mask)
    cv2.imwrite('result.png', result)
    return result

Up to this point everything is correct.

But now I want to do the same but with transparent background (since now was black, mask = np.zeros()).

I understand that I have to work with the alpha channel, like mask = np.zeros((600,800, 4), dtype = 'uint8') but appears this error.

result = cv2.bitwise_and(img, img, mask=mask) cv2.error: OpenCV(3.4.2) C:\Miniconda3\conda-bld\opencv-suite_1534379934306\work\modules\core\src\arithm.cpp:241: error: (-215:Assertion failed) (mtype == 0 || mtype == 1) && _mask.sameSize(*psrc1) in function 'cv::binary_op'

Therefore my idea was to convert my current BGR image to BGRA like b = cv2.cvtColor(img, cv2.COLOR_BGR2BGRA) but fails again with same error.

def findYellow(img):
    hsv = cv2.cvtColor(img,cv2.COLOR_BGR2HSV)
    lower_yellow, upper_yellow = np.array([18, 50, 90]), np.array([30, 255, 255])
    mask = cv2.inRange(hsv, lower_yellow, upper_yellow)
    _, contours, _ = cv2.findContours(mask, cv2.RETR_TREE, cv2.CHAIN_APPROX_NONE)
    max_contour = max(contours, key = cv2.contourArea)
    mask = np.zeros((600, 800, 4), dtype = 'uint8')
    cv2.drawContours(mask, [max_contour], -1, (255), -1)
    b = cv2.cvtColor(img, cv2.COLOR_BGR2BGRA)
    result = cv2.bitwise_and(b, b, mask=mask)
    cv2.imwrite('result.png', result)
    return result

Can somebody help me? I need to work with transparent background because in later steps black color affects me negatively.

Thank you very much!!

Hihikomori · Accepted Answer · 2020-12-19 12:36:48Z

1

If the first code is correct, do following:

result = cv2.bitwise_and(img, img, mask=mask)
result = cv2.cvtColor(result, cv2.COLOR_BGR2BGRA)
result[:,:,3] = mask #Set mask as alpha channel
cv2.imwrite('result.png', result)
return result

answered Dec 19, 2020 at 12:36

Hihikomori

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