Transparent PNGs don't retain transparency after being transformed (Django + PIL)

I'm using sorl-thumbnail, PIL, and Django on a webserver to dynamically create thumbnails in templates.

PIL is installed with PNG support, but for some reason the transformations are creating some really bizarre artifacts on the transparent portions of the images.

I used this gist on Github to install the require开发者_JS百科d dependencies: https://raw.github.com/gist/1225180/eb87ceaa7277078f17f76a89a066101ba2254391/patch.sh

Here is the template code that generates the images (I don't think this is where the problem is, but can't hurt to show you):

{% thumbnail project.image "148x108" crop="center" as im %}
  <img src='{{ im.url }}' />
{% endthumbnail %}

Below is an example of what happens. Any help is greatly appreciated!

Before

After

It looks like your resulting image is a JPEG. The JPEG format does not support transparency. Try changing your thumbnail template to this:

{% thumbnail project.image "148x108" crop="center" format="PNG" as im %}

Either:

• add format='PNG'.
• add THUMBNAIL_PRESERVE_FORMAT=True to the settings.
• or use the custom engine as described here:

http://yuji.wordpress.com/2012/02/26/sorl-thumbnail-convert-png-to-jpeg-with-background-color/

"""
Sorl Thumbnail Engine that accepts background color
---------------------------------------------------

Created on Sunday, February 2012 by Yuji Tomita
"""
from PIL import Image, ImageColor
from sorl.thumbnail.engines.pil_engine import Engine


class Engine(Engine):
    def create(self, image, geometry, options):
        thumb = super(Engine, self).create(image, geometry, options)
        if options.get('background'):      
            try:
                background = Image.new('RGB', thumb.size, ImageColor.getcolor(options.get('background'), 'RGB'))
                background.paste(thumb, mask=thumb.split()[3]) # 3 is the alpha of an RGBA image.
                return background
            except Exception, e:
                return thumb
        return thumb

in your settings:

THUMBNAIL_ENGINE = 'path.to.Engine'

You can use the option now:

{% thumbnail my_file "100x100" format="JPEG" background="#333333" as thumb %}
   <img src="{{ thumb.url }}" />
{% endthumbnail %}

I'd suggest you rather look into how sorl's PIL backend handles scaling. I imagine it creates some helper image to apply additional effects on and then tells PIL to scale the original onto that. You need to make sure that the destination is using the RGBA mode to support transparency and that it starts with its alpha set to zero (and not pure white or pitch black or something similar). If your image is using an indexed palette then it's possible it does not get converted to RGBA. In indexed mode PNGs store the transparent color index in their metadata but the process of creating the thumbnail will alter pixels due to antialiasing so you cannot preserve indexed transparency in:

source = Image.open('dead-parrot.png')
source.convert('RGBA')
dest = source.resize((100, 100), resample=Image.ANTIALIAS)
dest.save('ex-parrot.png')