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How do I replace the alpha channel of any image (png, jpg, rgb, rbga) with specified background color? It must also work with images that do not have an alpha channel.

Humphrey
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3 Answers3

39

This can be done by checking if the image is transparent

def remove_transparency(im, bg_colour=(255, 255, 255)):

    # Only process if image has transparency (http://stackoverflow.com/a/1963146)
    if im.mode in ('RGBA', 'LA') or (im.mode == 'P' and 'transparency' in im.info):

        # Need to convert to RGBA if LA format due to a bug in PIL (http://stackoverflow.com/a/1963146)
        alpha = im.convert('RGBA').split()[-1]

        # Create a new background image of our matt color.
        # Must be RGBA because paste requires both images have the same format
        # (http://stackoverflow.com/a/8720632  and  http://stackoverflow.com/a/9459208)
        bg = Image.new("RGBA", im.size, bg_colour + (255,))
        bg.paste(im, mask=alpha)
        return bg

    else:
        return im
Humphrey
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    In modern versions of Pillow you can probably use `.getchannel('A')` instead of `.split()[-1]`. – gps Jun 09 '18 at 18:52
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    After pasting the image onto the background, I had to also convert to RGB using `bg.convert('RGB')` – tutuDajuju Jan 04 '19 at 14:03
22

I'd suggest using Image.alpha_composite.
This code can avoid a tuple index out of range error if png has no alpha channel. 

from PIL import Image

png = Image.open(img_path).convert('RGBA')
background = Image.new('RGBA', png.size, (255,255,255))

alpha_composite = Image.alpha_composite(background, png)
alpha_composite.save('foo.jpg', 'JPEG', quality=80)

I also recommend you inspect both results with a image.show().

Credits for this answer goes to shuuji3 and others who helped build a vast answers repertoire in this other question.

Vinícius M
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  • If the source image is always a PNG, this should be fine, but if it may encounter an image type that doesn't have transparency, e.g. a .jpg, this produces an `OSError: cannot write mode RGBA as JPEG`. I added a check when opening the image to resolve this. – alstr May 23 '22 at 14:03
4

I updated Vincius M's code with the user, tutuDajuju, suggestion to make [height, width, 3] matrix (not [height, width, 4]):

from PIL import Image
import numpy as np

png = Image.open(img_path).convert('RGBA')
background = Image.new('RGBA', png.size, (255,255,255))

alpha_composite = Image.alpha_composite(background, png)
# if you check the matrix dimension, channel, it would be still 4.  
h, w, channel = np.asarray(alpha_composite).shape

alpha_composite_3 = alpha_composite.convert('RGB')
# if you check the matrix dimension, channel, it would be 3.  
h, w, channel = np.asarray(alpha_composite_3).shape
Cloud Cho
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