On 23/04/15 15:19, wk_ wrote:
So, if you scale down to 51% (which is about 2x) you first do a 2px
Gaussian blur. Result here:

Do you agree that Gimp 2.6 version looks still out much better?




I do agree that this specific implementation/version of the algorithm on this specific part of this specific photo looks better. Period(*)

But the point is, the frequency response of a specific algorithm and its implementation is not linear (especially with potential round-off errors in 8-bit) so you cannot tell in advance if the combination algorithm+image+scaling factor is going to produce artifacts or not. Maybe 2.6 is much worse at another scaling factor... Only the pre-blur will work in all cases.

Most digital cameras have a low-pass filter in front of the sensor to avoid this very phenomenon (hardware blur, so to speak), only a handful of DSLRs work without this (a specific Canon model for astrophotography, and now a Nikon).

(*) An engineer, a physicist, and a mathematician are in car in Scotland, and come across a sheep.
The engineer: in Scotland, sheep have a black head
The physicist; more accurately, it seems that in Scotland, most sheep have a black head, but there can be exceptions The mathematician; sorry chaps, but the only thing we can say is that in Scotland, there is at least one sheep that has at least one side of its head black.

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