[Solved]-Transparent PNGs don't retain transparency after being transformed (Django + PIL)

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')