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Resizing Causes Blurry Images

Jun 6, 2011, 08:44
Post: #1
Resizing Causes Blurry Images
Hello all, fairly new to Gimp and have a question about resizing images. I have a jpg file 1500x1024 pixels. If I resize it down to 400x300 and save it, the image quality is fine in the Gimp window. When I view it outside of Gimp, like in Image Viewer or FSpot its very blurry.

I resized the same image on a Mac using Photoshop and this did not happen, the resized image was perfect. Is there some setting that might cause this that anyone can think of? Is it some sort of limitation in Gimp?

In the Scale Image dialog box the interpolation is set to Cubic.
In the Save As jpeg dialog box I have checked Optimize
Smoothing is set to 1.00
Subsamplingis set to Best Quality

Thank you for any help with this, cheers.
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Jun 6, 2011, 09:01 (This post was last modified: Jun 6, 2011 09:11 by ofnuts.)
Post: #2
RE: Resizing Causes Blurry Images
(Jun 6, 2011 08:44)worldwidejoe Wrote:  Hello all, fairly new to Gimp and have a question about resizing images. I have a jpg file 1500x1024 pixels. If I resize it down to 400x300 and save it, the image quality is fine in the Gimp window. When I view it outside of Gimp, like in Image Viewer or FSpot its very blurry.

I resized the same image on a Mac using Photoshop and this did not happen, the resized image was perfect. Is there some setting that might cause this that anyone can think of? Is it some sort of limitation in Gimp?

In the Scale Image dialog box the interpolation is set to Cubic.
In the Save As jpeg dialog box I have checked Optimize
Smoothing is set to 1.00
Subsamplingis set to Best Quality

Thank you for any help with this, cheers.
The best rescale algorithm is Sinc/Lanczos. You can make it the default in Preferences (it requires more CPU than Cubic, so it's not the default out of the box): Edit/Preferences/Tools options/Scaling.

This said, some pictures can be pathological cases. Since you scale down by a factor of 4 (btw, your scaling isn't keeping the aspect ratio), patterns in the image that have a repeat rate of approximately 4 pixels will create artifacts, and you should first apply a 4x4 pixel gaussian blur (Filters/Blur/Gaussian blur) to your image before scaling it down.

If this doesn't fix your problem, attach the picture here or provide a URL.

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Jun 6, 2011, 09:34
Post: #3
RE: Resizing Causes Blurry Images
I find that by re-sizing in steps (usually 3 steps) I find that I have to do less aggressive (or no) sharpening after re-sizing.

So I'd take your 1500 wide image, resize to 1100 wide, then re-size to 700 then to your final 400. I've made this easy by writing a script for myself to do it automatically.
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Jun 6, 2011, 13:02
Post: #4
RE: Resizing Causes Blurry Images
(Jun 6, 2011 09:34)paynekj Wrote:  I find that by re-sizing in steps (usually 3 steps) I find that I have to do less aggressive (or no) sharpening after re-sizing.

So I'd take your 1500 wide image, resize to 1100 wide, then re-size to 700 then to your final 400. I've made this easy by writing a script for myself to do it automatically.
Do you have an example image for that?

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Jun 6, 2011, 13:50
Post: #5
RE: Resizing Causes Blurry Images
(Jun 6, 2011 13:02)ofnuts Wrote:  
(Jun 6, 2011 09:34)paynekj Wrote:  I find that by re-sizing in steps (usually 3 steps) I find that I have to do less aggressive (or no) sharpening after re-sizing.

So I'd take your 1500 wide image, resize to 1100 wide, then re-size to 700 then to your final 400. I've made this easy by writing a script for myself to do it automatically.
Do you have an example image for that?

We already did that over on GimpChat recently: http://gimpchat.com/viewtopic.php?f=8&t=1866
It's just my personal preference for the end result, and I can't see enough difference to bother changing my habits Smile

I think you've made the important point to the original poster, a straight re-size gets you a fuzzy result.
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Jun 6, 2011, 14:18
Post: #6
RE: Resizing Causes Blurry Images
Aw, that one, yes... pretty drastic rescale, though (:25 instead of :4). Note that an independent judge didn't see much difference between your technique and "mine" (which isn't mine at all), while mine has some theoretical justification Smile

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