Resizing and sharpening in GIMP

[Morgan_Hardwood]

@nik

First, the highest quality resizing options are cubic and lanczos. Of these, which should I prefer as my default option, and why?

GIMP-2.10 doesn’t support Lanczos, it has NoHalo and LoHalo. NoHalo is a little sharper than LoHalo.

The nature of a given resizing algorithm will decide whether you need any post-resize sharpening, and how much. I like Lanczos because its downscaled output looks sharp while not suffering from aliasing. GIMP’s Lo/NoHalo are fine too, though a bit softer.

@nik

Second, which sharpening method should I prefer for output sharpening (post-resizing)?

For sharpening I typically use USM with a contrast threshold in RawTherapee. I only use RL Deconvolution when needed, as it can sometimes exaggerate bright points making them look like noise.

Do your own tests. Areas to pay attention to are those with a fine repetitive pattern, diagonals, thin lines, high contrast edges.

See here:

[Solved] Sharpening opinions, please

Compare these images by opening them in tabs and switching back-and-forth. GIMP-2.10.4 NoHalo: [nohalo] RawTherapee-5.5 Lanczos: [rt%20lanczos]

@nik

R-L deconvolution itself may not be suitable for me because it takes forever to run

It’s just the implementation you’re using which is slow - find a different implementation, e.g. RL in RT is fast. GIMP-2.10 is currently slow in general - the plan I read is to perform optimization last, once the dev team are done implementing new features.