Goodbye noisy skies with "distorted pixels"!


posted on 14th of march, 2012

Hi Dreamers!

I guess I am not alone tackling the issue of denoising the skies in outdoor shots,

especially when it comes to night photography. Even with photos taken at nominal ISO values (ISO-200 for my camera) I found it often frustratingly noise-prone to make the sky brighter and richer by boosting the levels/contrast.

Here are a couple of examples, where my first attempts to "improve" the skies led to refusals initially. That time I knew little about the origin of "distorted pixels" and possible noise at nominal IS0. Simply by the method of a "reasonable guess", I decided to redo the editing with less contrast and saturation boost.

Later, however, I have discovered for myself a pretty simple method of noise reduction especially suitable for the skies. Now I hope it might be useful for you too. Well, what I do (in GIMP) is really simple (and it is no blur at all), but the results are very satisfying and no deterioration of detail occurs!

So, I introduce another layer over the original picture, then in this new transparent layer create a gradient over the sky area with the two utmost colors picked up from where I like in the original photo (normally the darkest and lightest spots in the sky). Finally, I vary the transparency of the top layer to my liking, while checking for the noise and "distorted pixel" artifacts at 100%. The two edge colors for the gradient can be made slightly darker/brighter, so as to make the sky look more "dramatic". Of course, before merging the layers into the final picture, one has to erase from the created gradient layer those parts where it overlays with the non-sky part of the photo (for ex. by using some smart selection tool or simply brushing out in the mask mode and then cutting out; alternatively, one could of course pre-select the sky area beforehand and apply the gradient only to the selection).

An important tip: be careful about not making the two ends of your gradient too different from each other. Because any gradient is a set of stepwise levels, so if you are unlucky you may end-up with annoying clearly seen color banding in the skies, instead of the desired smoothness.

As a matter of fact, the resulting skies can be further made richer and brighter (the brightness is somewhat lost under the gradient, obviously). The final touch is simple: within the pre-selected sky decrease slightly the level for the white point in the levels dialog, say by setting the value between 240 and 250 (but better don't do anything with the black point, as that might introduce too much of contrast resulting in "distorted pixels" in the gradiented area).

Well, it might be an obvious method for some, but also might be helpful for others.

Happy shooting, editing and selling! :)

PS: I did not mention above, but first of all, one should probably consider using neutral gradient or polarizing filters when shooting in daylight, just to minimize the editing needed...

Comments (18)

Posted by Ferdie2551 on April 08, 2012
TFS Andromantic. Very informative post.
Posted by Androniques on April 06, 2012
By the way, here is the last night photo I complained about: ID 23924972 - everything worked out nicely eventually. :)
Posted by Androniques on March 26, 2012
@Aandersonb
yep, GIMP is a great package that comes with great functionality by itself and enhanced by lots of third party enthusiastic filters... mere sharpening has at least four different options (in my version) including selective and smart sharpening, plus content-aware selection removal which is great for getting rid of small artifacts and wires/cables on the sky. GIMP also seem to allow saving jpeg's at much better quality than Photoshop's 12th grade (at least before it was the case) - if I save at GIMP's 100% the jpeg is around 10-12 Mb - similar size as Nikon's RAW/NEF file (well, it also tells you about the efficiency and compactness of Nikon's RAW format).
Posted by Aandersonb on March 21, 2012
Its good to know that others on this Photography site also use Gimp as an editing tool. Great article.
Posted by Androniques on March 19, 2012
arrrgh... I just encountered another night shot where I could not fix the problem of noise by merely overlaying an appropriate gradient over the sky, even though I tried several times varying the gradient direction, length and colours... This is *very_frustrating* because the picture looks great unless you check the sky at 100%. No matter what I did, I got really bad banding with those color levels and rough edges between them. So beware!

Well, I also made several attempts to vary the noise reduction parameters in RAW processing, which invariably resulted in patchy-spotty sky. Finally, the G'MIC after some 20 mins of its "optimized" smoothing (not the fastest method, rather the opposite!) once again produced gradient-like colour banding...

Seems there have to be yet another method for me to discover/learn, effectively dealing with such really tricky cases. For now I have to stop tweaking that picture and keep it for myself (or maybe I could simply try to introduce the skies from another...(More)
Posted by Androniques on March 17, 2012
@Sadman0026,

Sorry, I noticed your comments only now. Yes, I know about G'MIC... rather powerful piece of software!
However, it does affect the detail, making the picture softer as a result.
My understanding is, G'MIC is intended for high ISO values, when nothing else helps.
It is also quite resource and time demanding, at least some tens of mins per picture at full CPU speed.

Anyways, I appreciate your concern to add some extra info to my blog. I also liked you examples! :)

PS2: regarding the overlaid gradient method, I forgot to mention that one can combine a set of gradients, as well as use colour-to-transparent gradients etc.
Posted by Androniques on March 16, 2012
@Parkinsonsniper:

"So many wonders to discover
Are yet with the enlightenment spirit,
Experience, the son of painful errors,
And genius, the paradoxes' friend,
And accident, inventive god..."

Alexander Pushkin (this is my favourite translation, done by another scientist)
Posted by Serjedi on March 16, 2012
Thanks for sharing.. very helpful blog!
Posted by Parkinsonsniper on March 16, 2012
Great discovery! :) as expected from a scientist...
Posted by Sadman0026 on March 15, 2012
Thanks for sharing!

Anyway, I am using the in Gimp. it fits also for clouds area. [imgr]http://www.dreamstime.com/winter-landscape-thumb21582877.jpg[/imgr]

[imgl]http://www.dreamstime.com/corfu-fortress-thumb17946432.jpg[/imgl] also for no clouds in the sky!

Sometimes i am using, in Gimp, the GMIC noise remover functionality only for sky area.
Posted by Androniques on March 15, 2012
Wow, I did not realize that one gets credit points when readers vote for the blog-article by clicking on the green word "useful" in the top-right corner! I thought writing a blog will simply increase the exposure of my port...

Again, thanks to all! :)
Posted by smartview27 on March 15, 2012
Thank you for sharing !
Posted by FabioConcetta on March 15, 2012
Thanks for info!
Posted by Androniques on March 14, 2012
Thank you, all who cared!
It would be also great to hear one or two tips of yours regarding the noisy skies issue. I've been especially confused and disappointed by the fact that the RAW editors (well, those available for Linux) did not help me much with the night shots at the nominal ISO, and I still have to apply the described approach in those tricky cases.
Posted by Egomezta on March 14, 2012
Great, thanks for sharing.
Posted by M4rio1979 on March 14, 2012
useful blog, thanks!
Posted by Jdanne on March 14, 2012
Thanks for sharing! Nice photos!
Posted by TMarchev on March 14, 2012
Good!



Comments (18)

This article has been read 2583 times. 11 readers have found this article useful.
Photo credits: Androniques.

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