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I was editing a JPG image taken by Nikon D5300, a mid-range DSLR. Original picture was somewhat washed out. After some color-correction, I exported the image both in JPG and PNG format.

Surprisingly, both the exported JPG as well as PNG images contain fairly visible color-grains/noise IF viewed in 'GNOME Image Viewer' on Ubuntu 14.04. The noise is NOT visible IF viewed in 'Shotwell Photo Viewer'. Such noise is not visible either inside GIMP environment or in the original image. What can the reason be?

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    Your first link is broken. – Rafael Mar 2 '16 at 18:10
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If you view these images at 1:1 in Gimp (or any other software), they are quite noisy. It's probably disguised in some viewers when scaled down — for example, smaller versions on the site you link look just fine to me, and Firefox's native scaling also hides the noise.

This will also be true if you print these images at a reasonable size, so I wouldn't worry.

If you are worried, though, I think the next steps reduce down to What is noise in a digital photograph? and Are there any good open source noise removal tools?.

I also see the noise in your Original JPG image, but note that it's 4898×3265 instead of 6000×4000 in the Exported versions. That means when viewed at the same size, there's a different amount of scaling going on, and that may explain why it appears different to you.

  • It's probably the scaling algorithm. Nearest-neighbour vs something better like a bicubic. – James Snell Mar 2 '16 at 21:02
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When you did "...some color correction..." you also increased contrast and saturation, both of which will make noise present in the image more noticeable. It appears a considerable amount of banding was also introduced in the editing/exporting process. Banding in dark areas can lead to blocky looking splotches of black.

As others have pointed out, the JPEG and PNG both look about the same when viewed at 100%. When sized to fit the screen the differences in scaling algorithms from one image viewing application to the next comes into play and may affect how perceptible the noise in the images are.

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    "...increased contrast and saturation" - is the key. I did both. – Holmes.Sherlock Mar 3 '16 at 10:41

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