How to load png images with 4 channels?

Question

I have been trying to load .png files with transparency channel (RGB and Alph) with no luck. It appears that openCV strips the 4th channel out of the image. Is there any met

Answer 1

The right way to read a transparent PNG is to use the 4th channel as alpha channel. Most of the times one wants a white background, if that is the case then below code can be used for alpha compositing.

def read_transparent_png(filename):
    image_4channel = cv2.imread(filename, cv2.IMREAD_UNCHANGED)
    alpha_channel = image_4channel[:,:,3]
    rgb_channels = image_4channel[:,:,:3]

    # White Background Image
    white_background_image = np.ones_like(rgb_channels, dtype=np.uint8) * 255

    # Alpha factor
    alpha_factor = alpha_channel[:,:,np.newaxis].astype(np.float32) / 255.0
    alpha_factor = np.concatenate((alpha_factor,alpha_factor,alpha_factor), axis=2)

    # Transparent Image Rendered on White Background
    base = rgb_channels.astype(np.float32) * alpha_factor
    white = white_background_image.astype(np.float32) * (1 - alpha_factor)
    final_image = base + white
    return final_image.astype(np.uint8)

A detailed blog on this is here here.