[Image-SIG] What is the best way to anti-alias a very large resolution image in PIL

[Craig Coleman (C)]

Hi,

I have a really thorny problem.

I need to downsample a raster map that is 23622x23622 pixels to
7874x7874 pixels using the ANTIALIAS filter.

I have the following Python code:

>>> import Image
>>> img = Image.open(r"C:\temp\24bit_nd.tif")
>>> nimg = img.resize((7874,7874),Image.ANTIALIAS)

As soon as the resize method is called, python crashes instantly.  I
presume this is a memory allocation issue.

Is there another way of performing anti-aliasing on such a large image
(its 2.7GB uncompressed although I'm using LZW for storage).

I have tried converting the file to 8bit with a palette and this
successfully downsamples but the ANTIALIAS is not performed.  What am I
doing wrong?

Craig Coleman
Technical Lead
Information Systems, Ordnance Survey 
L0F2, Adanac Drive, SOUTHAMPTON, United Kingdom, SO16 0AS
Phone: +44 (0) 2380 054641
www.ordnancesurvey.co.uk <http://www.ordnancesurvey.co.uk/>  |
craig.coleman at ordnancesurvey.co.uk
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