Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Resized layer is blurry
#11
(11-22-2016, 05:49 PM)kayeng Wrote: Also, what's better, sinc or cubic?  I've read in one website that sinc is better, but another website says cubic is better.

You'll see a lot of misinformation about this, many people hitting a problem with one method and not the other.

First off, Sinc/Lanczos is somewhat more compute-intensive that Cubic, so if it didn't give better results in most cases, it would not even exist. Gimp uses cubic out of the box as a service to under-powered machinesSmile

Now, at least two things come into play:

1) Computation accuracy. Sinc/Lanczos can suffer from round-off errors. Just scale down a pure white layer, and at some scaling ratios you'll find a pattern in the scaled down version. However, this is not noticeable on photography.

2) Spatial frequency folding (a.k.a aliasing). See Wikipedia for some background. The various scaling methods behave like filters and will filter out different frequencies. So they won't produce aliasing on the same pictures. People tend to denigrate the method they used until they hit that very problem, and praise the other method which by chance worked better on their specific picture. You can also so see the stepped-scaling, which is just another way to obtain a different frequency filtering. All this can be avoided by adequately blurring the picture before scaling down.
Reply
#12
Great thread !

(11-22-2016, 06:29 PM)rich2005 Wrote: As Kevin suggested, scale it down in 2 or 3 stages,

So this is really a thing ?
I would have guessed scaling down in several steps degrades the quality more than doing it in one step.
Reply
#13
(11-22-2016, 09:19 PM)Espermaschine Wrote: Great thread !

(11-22-2016, 06:29 PM)rich2005 Wrote: As Kevin suggested, scale it down in 2 or 3 stages,

So this is really a thing ?
I would have guessed scaling down in several steps degrades the quality more than doing it in one step.

See part 2) of my answer Smile
Reply
#14
Been doing some more research/reading... It appears that Cubic (all variants) introduces more blur when scaling down than sinc/Lanczos (in other words, it cuts off more strongly the higher frequencies). In other words, sinc/Lanczos will usually keep more detail, but is more prone to ringing artifacts.
Reply
#15
(11-23-2016, 12:31 AM)Ofnuts Wrote: See part 2) of my answer Smile

Right i missed that slightly because of the wording. Not sure i get the frequency part.
That wikipedia article is way too much for my brain-power.

I did use the pre-blurring for a while, but it didnt seem to make a difference.
Reply
#16
And here's the latest Computerphile video, this time on Bicubic interpolation: https://www.youtube.com/watch?v=poY_nGzEEWM
Reply
#17
Ok, blind test... The following 4 images are the same 1200x800 image resized to 800x533, using the 4 methods available in Gimp (Lanczos,Cubic,Linear,None). Can you tell which is which?

https://www.dropbox.com/s/hm6cpd49zw24ht...d.zip?dl=0

(the files are PNGs to avoid JPEG artefacts... Since the result totals 3.2MB, I put it on my DropBox)
Reply
#18
my estimate

none B
linear C
cubic A
sinc D
Reply


Forum Jump: