How can I identify image noise, and is there software that highlights noisy areas?

Asked 3/15/2012

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I was told one of my photos looks noisy and muddy, but I have trouble recognizing noise unless it is very obvious. Is there any software that can automatically detect or highlight noisy parts of an image, or is this something I need to evaluate manually? If manual, what is the best way to inspect a photo for noise?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

14y ago

2 Answers

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I'm not aware of any software whose purpose is to simply alert you to noise. There are noise removal tools to remove noise though, and thatsoftware will show before and after the noise reduction.

Your image doesn't immediately strike me as noisy. However it's a dark image, and shadows is where you usually find noise.

To see the noise, it's best to zoom in to 100-200%, since noise is at the pixel level and you want a lot of magnification to see individual pixels.

Below is a small bit of your original image. You can see that the transition from light to dark isn't really smooth. You can see spots. I think they look "blocky" which means they may be more due to JPG compression rather than sensor noise. It's hard to see, but these spots are not just a pixel or two, but blocks of pixels as far as I can tell.

enter image description here

Applying a small amount of noise reduction to the image gives the following result, which is a lot smoother. It's removed the spots, whether they were JPG artifacts or actual noise. Whether you can tell the difference looking at the full image at normal resolution is debatable. If you can't easily see the noise, and have to magnify to 200% to find it, in my opinion it doesn't really matter.

enter image description here

Originally by user4191. Source · Licensed CC BY-SA 4.0

user4191

14y ago

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AI Answer

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There isn’t commonly a dedicated tool whose main job is just to flag noisy areas for you. In practice, noise is usually judged visually.

A good way to inspect it is:

  • View the image at 100–200% zoom, since noise exists at the pixel level.
  • Look especially in dark tones and shadow areas, where noise is most noticeable.
  • Increase contrast temporarily (for example with Curves) to make banding, blotchiness, or uneven tonal transitions easier to see.

Also note that what looks like “noise” may actually be JPEG compression artifacts. If the texture appears blocky or occurs in larger square-ish patches rather than fine random grain, compression may be the bigger issue.

So the practical answer is: use your editor to zoom in and, if needed, exaggerate contrast to reveal problem areas. Noise-reduction software can help you compare before/after results, but it usually doesn’t act as a simple automatic noise detector.

UniqueBot

AI

14y ago

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