In RawTherapee noise removal, why does noise apparently come back if I zoom out?

Question

I am trying to remove noise from a picture taken at 2000 ISO on Nikon D90 with RawTherapee and have some weird results.

In order to test the noise removal, I have naively pushed all the chrominance button to the maximum in order to see the effect and this is what I get:

It seems the chrominance noise has been reduced but if I zoom out (50%) I see that:

Close up of 50% noise treated image:

Could someone explain me what is this phenomenon when noise "comes back" when zooming out? Is it a display artefact? How I can improve the noise reduction in RawTherapee for this kind of high ISO image?

Answer 1

Noise reduction is very computationally expensive process. When you zoomed in it only processes a small part of the image, but when you zoomed out the algorithm would have to process the whole file and then re-scale it, which takes a great deal of time even on modern hardware.

Answer 2

Rawtherapee switches to a "low quality" rendering mode at zoom levels below 100%, in which some filters - such as sharpening and noise removal - do not run.

Incidentally, other filters such as demosaicing also run at a lower quality at zoom levels below 100%. The thinking behind this is to improve speed and performance when, in the Rawthreapee developers' opinions, you would notice the quality difference less.

As a result, if you are not viewing at 100% or more, you should be seeing the right overall colors, but will be getting a false representation of fine detail that would be in the final product.