Why does noise reduction seem to disappear when I zoom out in RawTherapee?
Asked 7/30/2014
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When editing a high-ISO RAW file in RawTherapee, strong chroma noise reduction looks effective at 100% or higher zoom, but at 50% zoom the image appears noisy again. Is this just a display/rendering artifact? How should noise reduction be evaluated in RawTherapee on images like a Nikon D90 ISO 2000 shot?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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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.
Originally by user2237. Source · Licensed CC BY-SA 4.0
user2237
12y ago
0
Generated from our catalog & community — verify before relying on it.
Yes — this is mainly a preview/rendering issue, not the noise actually “coming back.” In RawTherapee, zoom levels below 100% often use a faster, lower-quality preview mode. In that mode, some processing such as noise reduction, sharpening, and even full-quality demosaicing may be simplified or skipped to keep the program responsive.
At 100% or higher, RawTherapee renders a smaller area with higher quality, so you see a more accurate result. At 50%, it would need to process the whole image and then scale it, which is much more computationally expensive, so the preview is reduced in quality.
For judging noise reduction and fine detail, evaluate the image at 100% (or higher), and rely on the final export for the true result. At lower zoom levels, use the preview mainly for overall composition and color, not for pixel-level detail or noise assessment.
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