Pillow: cannot write mode RGBA as JPEG (4.2.0)

[karolpawlowski]

What did you do?

Tried to save JPG file (any JPG file).

What did you expect to happen?

Saving without any exceptions (was working in 4.1.1 version)

What actually happened?

File /usr/lib/python3.6/site-packages/PIL/Image.py, line 1893, in save
save_handler(self, fp, filename)
File /usr/lib/python3.6/site-packages/PIL/JpegImagePlugin.py, line 604, in _save
raise IOError(cannot write mode %s as JPEG % im.mode)
OSError: cannot write mode RGBA as JPEG

What versions of Pillow and Python are you using?

4.2.0 (with 4.1.1 works properly) / Python 3.6

Please include code that reproduces the issue and whenever possible, an image that demonstrates the issue. Please upload images to GitHub, not to third-party file hosting sites. If necessary, add the image to a zip or tar archive.

The best reproductions are self-contained scripts with minimal dependencies. If you are using a framework such as plone, Django, or buildout, try to replicate the issue just using Pillow.

im = Image.open(file_path)
im = im.convert("RGBA")
im.save(hidpi_path, file_type, quality=95)

[wiredfool]

Yep: https://github.com/python-pillow/Pillow/blob/master/docs/releasenotes/4.2.0.rst#removed-deprecated-items

JPEGs can't represent an alpha channel. It's been issuing a warning for a while, removed in this release.

[karolpawlowski]

Thanks @wiredfool ! What is preferred way to make it works with both PNG and JPEG files?

[wiredfool]

If it's got an alpha channel that you want to preserve, PNG is really the only reasonable choice. Alternately you can flatten it to a RGB image and save as a JPEG.

[ghost]

Hi, i am interrested did you find a way to make it works with both PNG and JPEG ?

[radarhere]

@AymericHENRY Hi. While this is just reiterating what has already been said, you should have no problem saving an image in RGBA mode to PNG. To save an image in RGBA mode as JPEG means getting rid of the alpha channel, because JPEG doesn't transparency. So, if you have an image in RGBA mode, then converting it to RGB will let you save it as JPEG.

>>> from PIL import Image
>>> im = Image.new("RGBA",(100,100))
>>> im.save("test.jpg")
>>> Traceback (most recent call last):
  File "PIL/JpegImagePlugin.py", line 602, in _save
KeyError: 'RGBA'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/Image.py", line 1893, in save
  File "PIL/JpegImagePlugin.py", line 604, in _save
OSError: cannot write mode RGBA as JPEG

>>> im = im.convert("RGB")
>>> im.save("test.jpg")
>>> 

[homm]

In most cases the discarding the alpha channel will give you undesirable result, because transparent pixels also have some unpredictable colors. It is much better to fill transparent pixels with certain color:

fill_color = ''  # your background
image = Image.open(file_path)
if image.mode in ('RGBA', 'LA'):
    background = Image.new(image.mode[:-1], image.size, fill_color)
    background.paste(image, image.split()[-1])
    image = background
im.save(hidpi_path, file_type, quality=95)