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Scaling gives bad results

Jan 24, 2014, 16:50 (This post was last modified: Jan 24, 2014 16:58 by Ryo.)
Post: #1
Scaling gives bad results
Hello, I have the problem that scaling images in Gimp sometimes gives me bad results. I don't know why it is like that or if it can be fixed. I tried all the different scaling interpolations, rgb-modus or not, the scaling tool instead of Image -> Scale and all give me bad results with some images.
Photoscape or Xnview on the other hand do fine with the same images, here is one example:

[Image: smqe.png]

Gimp (Cubic)
[Image: lbpp.png]

[Image: uqqn.png]

[Image: 1zg0.png]

Is it possible to fix this? Thank you in advance.
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Jan 24, 2014, 18:33 (This post was last modified: Jan 24, 2014 18:35 by rich2005.)
Post: #2
RE: Scaling gives bad results
That is the technical bit so excuse me for being outspoken, but I can never see the point of this type of post.

If you have found a better way of downscaling an image use it. Nothing is set in stone here, for Gimp to equal the best.

I could also give you a command line for ImageMagick that the experts say is best for downscaling jpegs.

convert in.jpg -colorspace RGB -filter Lanczos -define filter:blur=.9891028367558475 -distort Resize 50% -colorspace sRGB out.jpg

Is it any better? not much.

As per Ofnuts: The rule of thumb, for downscaling in Gimp is to first apply a gaussian blur in proportion to the amount of scaling for 50% then use gaussian blur = 2

Your image is not exact 50% so say use 1.8

Then scale the image, again for this type of image some say cubic is better than lancos. Somewhat subjective, 'in the eye of the beholder'.

I get this: gimp is on the left, a little softer than your XnView. Some might say better.

** now answering questions**
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Jan 25, 2014, 01:41 (This post was last modified: Jan 25, 2014 01:42 by iForStyle007.)
Post: #3
RE: Scaling gives bad results
I thought I'd take a stab at it with some modifications to ofnut's technique, plus another tool.

One or two of my methods use even less steps.

For right picture(less harsh tones):
1. Tools > GEGL Operation... > Box Blur: 1.00

2. Then with scale tool
(lock proportion with link/chain icon in the scale window)
Set interpolation to "none" sometimes when reducing/minimizing, no interpolation can make the image sharper.
scale down to 150 width

3. Unsharp Mask
Radius 0.1
Ammount 0.4
Threshold 0.0

For middle & Left picture, the only the only thing different between the two is interpolation:
1. Filters > Blurs > Selective blur
- Blur radius: 10.00
- Max. delta: 80
notice it preserves the edges and smooths the gradients that get messed up when you scale down, magic! Smile

2. Scale down (interpolation: none) 150w

2. Middle picture: scale ( interpolation: cubic )
(click for full detail)

Goodluck on your pictures

"In order to succeed, your desire for success should be greater than fear of failure." BC
[Image: 29uocaf.png]
iFS007 tut vids on YT ~ auto color B&W landscape tut
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Jan 25, 2014, 12:13 (This post was last modified: Jan 25, 2014 12:14 by Ryo.)
Post: #4
RE: Scaling gives bad results
First of all thank you all for the detailed answers. I have tried it myself now and the result are indeed way better. However I wonder why other programs seem to do all the work automatically in one go while in Gimp I have to do 1 or more additional steps manually to get comparable results.
But whatever, since I really like working with Gimp I am glad there is a way to fix this, thank you.
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