[Image-SIG] PNG PIL compression questions

[Svensson, Stefan]

Hello,

I have some qustions concerning compression behaviour and compression ratio
when saving images in PNG-format with PIL.

I have a scanned image that I have saved in Paintshop Pro:
1. First saved as TIFF LZW (38 kb) in Paintshop Pro and then opened in the
Python interpretor and saved as PNG. The PNG file size become 38kb.

Import Image
im = Image.open("c:/image.tif")
im.save("c:/image.png")

2. The same imaged saved in Paintshop Pro as a PNG (21 kb!) and when
openened and saved in the Python interpretor as an PNG the file size become
55 kb!

As an alternative to PIL and Python I have also tryed the Java Advaced
Imaging API (JAI) and the resulting PNG file size is always 21 kb. But PIL
is so much easier to use...

I wonder why does this happen and are there any ways to improve the
compression ratio and get a consistent behaviour when using PIL together
with PNG?

BTW, any plans for TIFF G4 support?

I am using Windows NT4, Python 1.5.2 (binary release got from python.org)
and PIL 1.0 (pil-win32-991101.zip binary release got from pythonware.com). 

Best regards,
/Stefan Svensson
stsvs@wmdata.com