GIMP Version: 2.6.11
Operating System: Windows
OS Version: XP - SP3
GIMP Experience: Intermediate Level

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Hi everyone,

I hope everyone's spring is going great. So on to my question. I was working on a project and I usually work with larger image than I need to for details. I noticed that when I reduced my image size by about 50% my details are becoming blurry. I have not had this issue before and kind of at my wits end trying to figure out what is wrong in gimp or my pc that might be causing this. Does anyone have any ideas? Thanks in advance.

Liana

Edit: I forgot to add. I work in layers and save as a xcf file. I can understand losing details in a jpg file but losing them in the xcf files puzzles me.

Someone else may be able to provide a more thorough answer, but whenever you shrink or expand an image, you will lose some detail even if it's not a .jpg file. The only type of file (to my knowledge) that can be grown/shrunk with no loss of detail is a vector file. GIMP is raster-based, so images are subject to detail loss if grown or shrunk.

I can only echo what lacertile said and this very effect is used by a lot of fractal software for antialiasing.

Hopefully ofnuts will be along later to point you to the theory that shows why re-sizing will always cause this, and the maths behind pre-blurring your image before downsizing, but while you're waiting I'll point you to the last time I answered this question: http://www.flickr.com/groups/gimpusers/ ... 9774118473 (to summarise, I do downsizing in multiple steps)

Request to ofnuts. Can you write up a post that explains this process so that someone can make it sticky here or on another forum, because this question keeps coming up.

Kevin

By reducing your picture to 50% you keep only one quarter of the pixels so you lose detail.

If your details are a regular pattern which is close to 2px, you can get hit by "spatial frequency folding". The non-obvious (but mathematically sound) cure is to pre-blur the picture befire scaling it down, by applying a corresponding Gaussian blur (reduce to 50%: apply 2px blur, 33%: 3px, etc...).

In all cases you can recover some sharpness using the "sharpening" filters (Filters/Enhance...)(one of the most efficient, despite its name, is "unsharp mask"), but this sharpening must be the last thing you do to the picture.

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Thankyou for that info ofnuts!
Request a script-fu that can perform those steps automatically with one click please ! I do a lot of resizing. Please and thankyou!

What interpolation are you using? I found that Sinc (Lanczos3) did text better. I don't know the whys and the hows though, I fiddled with the settings and that worked the best.

Try both Cubic and Lanczos3. The type of image is the difference on which one works best. Reduce the image size in steps using the aspect ratio as a guide.

There is already a script that will re size in steps. In fact there are 3.
http://registry.gimp.org/node/9676
http://registry.gimp.org/node/26196

And i have one custom made that keeps the layers.

The stepped technique works because the successive interpolations also blur the image... But that makes too many interpolations...