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

[Craig Coleman (C)]

Hi all,
 
Thanks for all of the response.  In the end, I used a combination of
GDAL and Image Magick which are fast, reliable, extremely flexible and
free.  Subsequently, we now use Python to call out GDAL and Image Magick
as needed.
 
I changed my process to export to high resolution PDF in order to
preserve the quality as PDF is vector-based rather than use a very high
resolution TIFF.  I use gdal_translate with the --config
GDAL_PDF_DPI=400 option to reduce the resolution to 7874 which also
performs anti-aliasing on the fly as it happens.  After that, I use
Image Magick to map the 24-bit raster to an 8-bit raster using a colour
palette in another image - GDALs rgc2pct script can do this but it
always applies a dodgy Floyd Steinberg ditther which produces horrible
results, Image Magick can avoid the dither altogether somehow (clever
stuff).  Finally, I use gdal_translate again to ensure the GeoTIFF tags
are in place.  Voila - 8bit, 400dpi tiff is produced.
 
Thanks,

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 
Please consider your environmental responsibility before printing this
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________________________________

From: andrew.k.wilson at gmail.com [mailto:andrew.k.wilson at gmail.com] On
Behalf Of Andrew Wilson
Sent: 13 September 2011 17:25
To: Craig Coleman (C)
Cc: image-sig at python.org
Subject: Re: [Image-SIG] What is the best way to anti-alias a very large
resolution image in PIL


Hello,
I've had similar problems with PIL, you may want to take a look at
OpenCV it can open rather large images and resize them.
  Thanks
  Andrew


On Thu, Sep 8, 2011 at 7:07 AM, Craig Coleman (C)
<Craig.Coleman at ordnancesurvey.co.uk> wrote:


	

	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 <tel:%2B44%20%280%29%202380%20054641>

	www.ordnancesurvey.co.uk <http://www.ordnancesurvey.co.uk/>  |
craig.coleman at ordnancesurvey.co.uk 
	Please consider your environmental responsibility before
printing this email. 

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	Ordnance Survey
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