Using OpenCV: Developed a web app to convert images to manga style

# Add a background effect
def back_filter(src, manga, effect, th):
    # Grayscale conversion
    img = cv2.bitwise_not(src)
    img_gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
    effect = cv2.cvtColor(effect, cv2.COLOR_BGR2GRAY)

    # Resize the screen tone image to the same size as the input image.
    effect = cv2.resize(effect,(img_gray.shape[1],img_gray.shape[0]))

    # binarization
    ret, img_binary = cv2.threshold(img_gray, th, 255,cv2.THRESH_BINARY)

    # Contour extraction
    contours, _ = cv2.findContours(img_binary, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

    # Get the contour with the largest area
    contour = max(contours, key=lambda x: cv2.contourArea(x))

    mask = np.zeros_like(img_binary)
    cv2.drawContours(mask, [contour], -1, color=255, thickness=-1)
    effect = np.where(mask == 0, effect, manga)

    # Combine tri-level image and contour image
    return effect

# front effect
def front_filter(manga, effect):

    effect = cv2.resize(effect,(manga.shape[1], manga.shape[0]))
    # Mask Image Generator
    mask = effect[:,:,3]

    # Grayscale the effect
    effect = cv2.cvtColor(effect, cv2.COLOR_BGR2GRAY)

    # Combine tri-level image and contour image
    manga = np.where(mask == 0, manga, effect)
    return manga