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How do I reduce the dimensions of large .png images? I've tried GIMP, Paint.NET, and IrfanView, and they work well for smaller images but won't even open larger ones.

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what do you mean by large (dimensions and file size)? how much RAM do you have? –  Mat Jun 8 '11 at 20:39
    
I haven't yet found a file IrfanView can't handle. I'd imagine your issue is not with the software. The RAM question is good. Also, how large is "large"? What is the file size? What are the dimensions in pixels? What is the bit depth? –  music2myear Jun 8 '11 at 21:29

4 Answers 4

Grab a trial of Photoshop from adobe.com, it should work well for what you're looking to do. Otherwise you could use something like Picasa that can export images at a specific resolution.

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Have you looked at Imagemagick? It is a command-line tool. Docs talk specifically about large image support. And it is open source.

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+1, that'd been my suggestion too. Especially for batch processing having the command line can be useful. –  slhck Jun 8 '11 at 20:54

Have you looked at nip2/vips, it's an open source image manipulation program which can deal with large images easily.

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I tried benchmarking ImageMagick vs. nip2 (as mentioned by Phil above) on a 23,000 by 26,000 RGB PNG, with the default compression level 6. This file is about 600MB on disk, and around 1.6GB when uncompressed. I used a 5 year old HP workstation (2 x Opteron 254s at 2.7GHz, 4GB memory).

nip2 took 35s to load the image. I resized to 80% of the original and selected bicubic interpolation using Image / Transform / Resize / Scale. I pressed 'save' and selected compression level 6 again. The save operation took around 200s. Peak memory use was about 250MB (peak rss as seen by top).

I did the same operation in ImageMagick with the command:

convert -define registry:temporary-path=/tmp -limit memory 250mb big8.png -resize 80% big8c.png

Memory use reached 2.1GB initially, I imagine as the image was decompressed to memory, then dropped away to about 250MB as processing began. Unfortunately during this processing phase CPU utilisation never rose above about 3%; it seemed to be spending almost all its time swapping to disk. I let it run for 30 minutes with no result, then killed it.

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