I frequently find myself doing retouching and local adjustments of .tiff files from my raw converter. Since these files are often huge (24 MP), they need to be resized for uploading to various websites or emailing to friends. I would like to understand how to resize photos and perform post-resize sharpening (“output sharpening”) for maximum image quality in GIMP. I am sharpening for display on a computer monitor, and the subject matter is – very broadly speaking – street photography.
First, the highest quality resizing options are cubic and lanczos. Of these, which should I prefer as my default option, and why?
Second, which sharpening method should I prefer for output sharpening (post-resizing)? Typically I use some variation of unsharp mask or wavelet sharpening in the original image to increase microcontrast. But since resizing seems to resemble a “blurring” procedure, would an algorithm like Richardson-Lucy deconvolution be appropriate? R-L deconvolution itself may not be suitable for me because it takes forever to run, but if there is something faster and in the same ballpark I would be eager to learn about it. Any suggestions are very welcome.
I did some Google searching and checked here, and while some threads discussed post-resize sharpening, none did so in the context of GIMP. Hence this question.
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.
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:
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.
Thanks @Morgan_Hardwood, that’s great information. Exactly what I was looking for.
Your link about NoHalo versus Lanczos is also extremely informative. Lanczos is clearly much sharper, and perhaps more importantly it avoids the color shifting that NoHalo causes on the shrub to the left of the frame and the bow of the ship.