sorl-thumbnail: Preserve "progressiveness" of JPEG files

Most of all JPEG files uploaded to my sites a开发者_如何学运维re saved in progressive format, but no thumbnail generated by sorl-thumbnail is generated as progressive when the original image is. This is essential when creating thumbnails of large images, for instance, for display within a carousel/slider.

I have sumbitted an issue on sorl's tracker, but I think maybe creating a custom backend based on the existing ones, could solve the issue. Any ideas?

Thanks!

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from sorl.thumbnail.engines import pil_engine

class ProgressiveBackend(pil_engine.Engine):
    def _get_raw_data(self, image, format_, quality):
        ImageFile.MAXBLOCK = 1024 * 1024
        buf = StringIO()
        try:
            if format_=='JPEG':
                image.save(buf, format=format_, quality=quality, optimize=1, progressive=image.progressive)
            else:
                image.save(buf, format=format_, quality=quality, optimize=1)
        except IOError:
            image.save(buf, format=format_, quality=quality)
        raw_data = buf.getvalue()
        buf.close()
        return raw_data