(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 machines
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.