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.cole...@ordnancesurvey.co.uk Please consider your environmental responsibility before printing this email. ________________________________ From: andrew.k.wil...@gmail.com [mailto:andrew.k.wil...@gmail.com] On Behalf Of Andrew Wilson Sent: 13 September 2011 17:25 To: Craig Coleman (C) Cc: image-sig@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.cole...@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.cole...@ordnancesurvey.co.uk Please consider your environmental responsibility before printing this email. This email is only intended for the person to whom it is addressed and may contain confidential information. If you have received this email in error, please notify the sender and delete this email which must not be copied, distributed or disclosed to any other person. Unless stated otherwise, the contents of this email are personal to the writer and do not represent the official view of Ordnance Survey. Nor can any contract be formed on Ordnance Survey's behalf via email. We reserve the right to monitor emails and attachments without prior notice. Thank you for your cooperation. Ordnance Survey Adanac Drive Southampton SO16 0AS Tel: 08456 050505 http://www.ordnancesurvey.co.uk _______________________________________________ Image-SIG maillist - Image-SIG@python.org http://mail.python.org/mailman/listinfo/image-sig This email is only intended for the person to whom it is addressed and may contain confidential information. If you have received this email in error, please notify the sender and delete this email which must not be copied, distributed or disclosed to any other person. Unless stated otherwise, the contents of this email are personal to the writer and do not represent the official view of Ordnance Survey. Nor can any contract be formed on Ordnance Survey's behalf via email. We reserve the right to monitor emails and attachments without prior notice. Thank you for your cooperation. Ordnance Survey Adanac Drive Southampton SO16 0AS Tel: 08456 050505 http://www.ordnancesurvey.co.uk
_______________________________________________ Image-SIG maillist - Image-SIG@python.org http://mail.python.org/mailman/listinfo/image-sig
