How to merge a transparent png image with another image using PIL

Question

I have a transparent png image "foo.png" and I've opened another image with

im = Image.open("foo2.png");

now what i need is to merge foo.png with foo2.png.

( foo.png contains some text and I want to print that text on foo2.png )

Solution 1

from PIL import Image

background = Image.open("test1.png")
foreground = Image.open("test2.png")

background.paste(foreground, (0, 0), foreground)
background.show()

First parameter to .paste() is the image to paste. Second are coordinates, and the secret sauce is the third parameter. It indicates a mask that will be used to paste the image. If you pass a image with transparency, then the alpha channel is used as mask.

Check the docs.

Solution 2

Image.paste does not work as expected when the background image also contains transparency. You need to use real Alpha Compositing.

Pillow 2.0 contains an alpha_composite function that does this.

background = Image.open("test1.png")
foreground = Image.open("test2.png")

Image.alpha_composite(background, foreground).save("test3.png")

EDIT: Both images need to be of the type RGBA. So you need to call convert('RGBA') if they are paletted, etc.. If the background does not have an alpha channel, then you can use the regular paste method (which should be faster).

Solution 3

As olt already pointed out, Image.paste doesn't work properly, when source and destination both contain alpha.

Consider the following scenario:

Two test images, both contain alpha:

layer1 = Image.open("layer1.png")
layer2 = Image.open("layer2.png")

Compositing image using Image.paste like so:

final1 = Image.new("RGBA", layer1.size)
final1.paste(layer1, (0,0), layer1)
final1.paste(layer2, (0,0), layer2)

produces the following image (the alpha part of the overlayed red pixels is completely taken from the 2nd layer. The pixels are not blended correctly):

Compositing image using Image.alpha_composite like so:

final2 = Image.new("RGBA", layer1.size)
final2 = Image.alpha_composite(final2, layer1)
final2 = Image.alpha_composite(final2, layer2)

produces the following (correct) image:

Solution 4

One can also use blending:

im1 = Image.open("im1.png")
im2 = Image.open("im2.png")
blended = Image.blend(im1, im2, alpha=0.5)
blended.save("blended.png")

Solution 5

def trans_paste(bg_img,fg_img,box=(0,0)):
    fg_img_trans = Image.new("RGBA",bg_img.size)
    fg_img_trans.paste(fg_img,box,mask=fg_img)
    new_img = Image.alpha_composite(bg_img,fg_img_trans)
    return new_img

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Comments

[nosklo]

Don't use ; at the end of your commands in python: It's ugly...

[Arackna]

I'll keep that in my mind , thanks !!

[Mark Ransom]

To make sure the foreground contains transparency in all cases, use foreground.convert('RGBA') for the mask parameter.

[Peter Hansen]

I just used paste() to overlay one semi-transparent image on another, with PIL, and it worked as I expected. In what way doesn't it work as you expected?

[Silouane Gerin]

Thanks. I was too missing the third parameter.

[homm]

@PeterHansen, paste() doesn't work as expected "when the background image also contains transparency".

[homm]

@PeterHansen There is example: github.com/python-pillow/Pillow/issues/…

[Peter Hansen]

@homm thanks. That was so long ago I have no memory of what I tried. It seems likely I did miss the part you quoted about the background image also having transparency.

[physicalattraction]

Wow! This solution did the trick instantly two weeks ago, but now I totally forgot it already, and now I see it again, it again does the trick!

[Deniz Ozger]

I'm getting ValueError: bad transparency mask

[William H. Hooper]

Depending on your version, you may have to install the Python Image Library, and:' from PIL import Image`

[Beatriz Fonseca]

Amazing, you can use image as mask. But can I use vectorial images like svg?

[digitaldavenyc]

I get ValueError: image has wrong made as well @DenizOzger

[Viet]

Thanks for the screenshots! Really helps!

[Lian]

This worked for me whereas paste was punching a "transparent hole" through my background.

[Mithril]

But alpha_composite can not set the offset, would you mind to give an example to completely replace paste function?

[P.Melch]

I guess you would have to create a new empty image with the same size as the garget image, paste the layer at the proper position and use alpha_compositing to blend the new image over the target image.

[Liviu Sosu]

This one ascetically worked for me. The images must have exactly the same size, but it is ok. The paste function did not quite cut it for me ...

[AFP_555]

The secret sauce was tasty

[Schütze]

'ValueError: images do not match'

[Schütze]

TypeError: paste() got an unexpected keyword argument 'alpha'

[nvd]

Possibly, they are of different dimensions. You may need to scale or crop one of those.

[Mingwei Samuel]

@DenizOzger To fix ValueError: bad transparency mask use bg.paste(fg, (0, 0), fg.convert('RGBA'))

[MilkyWay90]

@Schütze see nvd's comment because he/she didn't ping (using @blahblah) you

[jimf]

Hi, can you possibly add a little more context to your answer? Otherwise the requestor is not likely to learn the "why" behind it.

[lllllllllllll]

ValueError: images do not match

[user3613932]

Thank you for indicating the information about the mask and the link to the documentation appreciate it.

[Trect]

OMG!! I was trying some stupid login on cv2 to do this. In fact, spent 3 days on this. Love the secret sause

[Trect]

ValueError: images do not match . Because this method works only when the image sizes are same

[yota]

this does not work as expected, it is not a proper overlay for a transparent image. fading transparency gives ugly results. It transparency is ON/OFF you should get what you expect though

[Luis Jose]

Thank you for sharing. In my case "import Image" doesn't work. I had to use "from PIL import Image" instead.

[Student]

I get: ValueError: images do not match

[P.Melch]

Images need to have the same size

[Jordan The Genius]

@Schütze I had to add these lines to avoid "images do not match": ` cloud_as_img.convert("RGBA") cloud_as_img.putalpha(255) cloud_as_img.save(temp_file_name) `