Bart,
I was told by a highly respected landscape photographer and processing guru who works closely with Art Printers Of Nevada that the owner of Nevada Printers is always researching & testing the latest cutting edge upscaling techniques for large print making and he says that the Lanczos algorithm in Raw Therapee is the best upscaling method at this time. Granted I used it in Iridient & not Raw Therapee.
I don't know if there is a difference between those 2.
Hi,
Yes, in landscape resampling with a Lanczos windowing filter is quite useful, because the ringing artifacts are harder to spot than in e.g. architecture or product shots.
I've intensively studied the various interpolation algorithms and methods that are around, and I contributed an open source resampler based on ImageMagick
here on LuLa. It uses a mix of upsampling settings at various gamma settings which are then blended together to avoid those resampling artifacts as best possible.
Presumably, some form of this technique found its way into the Capture One Raw converter, a few versions ago. That also benefitted the quality of lens distortion corrections, rotations, and keystone corrections.
But what all these traditional interpolation methods have in common, is that they mathematically create smooth transitions between existing pixels. The Lanczos family of filters performs very good, but with caveats that need to be addressed/corrected (which not all software does because it slows down the procedure).
Topaz A.I. Gigapixel works in a different way, in that it has learned from examples what that detail between existing pixels probably actually looked like at high resolution and uses that to fill in the blanks instead of using a weighted average. This allows it to sometimes create actual detail with higher resolution than the original capture.
Cheers,
Bart