Noise Reduction in images.

[iampartha]

In images taken with cameras of higher ISO speed, the noise is relatively high.
There are many tools to reduce this noise, using YCrCb colour scope and using Threshold values. I want to find if this process can be automated.

That is finding the value of threshold automatically so that the resulting image is ideal.

[Black Pearl]

Don't bother.

Expose correctly, don't muck about with your images and print them out instead of looking at the pixels.

[Colin Southern]

Expose correctly, don't muck about with your images and print them out instead of looking at the pixels.

+1

Noise becomes visible due to two reasons:

1. Under-exposure. Effective dynamic range decreases with increasing ISO thus moving the target data closer to the noise floor. With a normal dynamic range scene this still produces acceptable results - however - if the shot is under-exposed to any significant degree (with under-exposure in this case being highlights not using the full remaining capacity of the sensor) then one loses ones safety margin, thus revealing noise when the photo is subsequently adjusted in post-processing. So as Robin says, "Expose Correctly" (meaning ETTR in this case).

2. Excessive cropping. The random noise is very small - too small to be seen on an average size print or at normal screen sizes / resolutions, but are effectively magnified by excessive cropping or by magnifying on-screen.

So 99 times out of 100 "an ounce of prevention" ("correct exposure & focal length") is better than "a pound of cure" ("noise reduction")

[kdoc856]

To reinforce Colin and Robin: as someone who often as a matter of personal taste prefers images a tad on the dark side, I used to intentionally under-expose. But that completely locks you in, and if you subsequently decide you want the exposure lifted- welcome to a noisy world. I still play at times on the dark side, but I now am very careful NOT to under-expose, as I can effectively reduce it in PP if desired.

I do have Dfin 2, which I occasionally apply, sometimes with really nice outcomes and sometimes the excessive sacrifice of sharpness.

Good luck,

Kevin

[iampartha]

I accept prevention is better than cure, but as of now i am talking about pictures that have already been taken, and nothing can be done about their exposure. Even those pictures deserve another chance ( :-P ).

So, any concepts about this guys?

[Colin Southern]

I accept prevention is better than cure, but as of now i am talking about pictures that have already been taken, and nothing can be done about their exposure. Even those pictures deserve another chance ( :-P ).

So, any concepts about this guys?

Programs like NoiseNinja have profiles for specific cameras (off memory, haven't used a NR program for many years) -- but I'd still just apply NR in ACR on an image by image basis to get the best possible result.

[Dave Humphries]

I accept prevention is better than cure, but as of now i am talking about pictures that have already been taken, and nothing can be done about their exposure. Even those pictures deserve another chance ( :-P ).

So, any concepts about this guys?

I use Neat Image, it has Camera (+ISO) profiles, but can also sample from the image, which is my default way of working.

You can try it for a while free I believe, works great (IMO), so I bought it, but beware which version you buy; they have 8 bit "Home" version, all you'll need if the pics are jpg. They also have a 16 bit version, I upgraded to this so I didn't need to convert my RAW based images to 8 bit to treat them. These are image bit depths I am talking about.

Now, with 64 bit PCs, they have a separate version for them, which seems a bit odd since Adobe give you both 32 and 64 bit versions of CS5 and Neat Image works on one, but not the other.

Cheers,

[Degsy]

Try Topaz DeNoise. A trial can be downloaded. I use it on sports images taken under poor floodlights which most non league grounds have.

[Glenn NK]

There are situations where over/under-exposure is impossible to avoid because our digital cameras have a limited dynamic range. Actually our eyes have a limited DR too - our pupils contract in high light levels to protect the retina, and open in low light levels (the cones and rods respectively of the retina).

Shooting a sunset is problematic; if one exposes to not blow out too much of the sky (sun), then there will be areas in the foreground that are under-exposed, and these will be noisy. I expose to the right (ETTR), pushing the RGB histogram as close to the right side (clipping side) as possible, but in the case of a sunset, there will still be noise aplenty in the shadows. (notice that I didn't mention sunrises - I simply cannot get up that early )

I've come to the realization that to capture all the detail of the entire scene, several exposures levels are required so they can be blended. Perhaps this is one of the major advantages that digital has over film. The trick here is to get something that still looks "real".

Glenn

[Photon Hacker]

In images taken with cameras of higher ISO speed, the noise is relatively high.
There are many tools to reduce this noise, using YCrCb colour scope and using Threshold values. I want to find if this process can be automated.

That is finding the value of threshold automatically so that the resulting image is ideal.

No really. The question itself is flawed because there isn't an ideal threshold. Noise reduction procedures imply a compromise between detail preservation and apparent noise reduction. There is no ideal point because this is subjective. It's just that artists have preferred settings for a given photograph. Your work as the artist includes to choose how much noise reduction to apply (Possibly none); just like other aspects of photography.

Bear in mind noise (Or Random noise, depending on the definitions) implies uncertainty, lack of information. The so called noise reduction algorithms don't recover this information; they just generate a more pleasing image based on the available information (The image, the algorithm itself and its settings). Same goes for any procedure. It's remarkable that some algorithms like deconvolution and deblurring ones make the contained information visible, when otherwise it was invisible, for well defined problems. Sometimes it is not clear where the information is contained, or whether the question makes sense. Read Gödel, Escher, Bach: An Eternal Golden Braid by Douglas Hofstadter if the topic grabs your interest.

[OhShoot]

Colin,
Do you have a suggestion as to which new DSLR camera to buy for a person just beginning to enter the advanced photographer category? I'm beginning to think that relying on auto-exposure capability of a camera takes the artist out of the picture, the camera is doing all the hard work. I guess that's why some people buy very expensive cameras that do all the thinking for the person pressing down on the shutter release button. How difficult is that to do?
Best regards,
Ohshoot.

[OhShoot]

Glen,
Interesting analysis. I am considering buying a Sony SLT a55 which has a setting which blends several exposures, all taken within a couple thousandth of a second, then produces a blended final image. What new DSLR camera with an articulated LCD screen, with what lens would you suggest for someone just beginning to enter the advanced photographer category?
OhShoot

[jeeperman]

Ohshoot, many of the newer cameras can do this. The issue is that mode is done in Jpeg and not raw. I would not buy based in anyway on that as you can blend several images from any digital cam in PP. The A55 is a nice camera but some don't really like the electronic viewfinder. Now if you were to step up to the A65 of A77, they have an OLED viewfinder, less the issues of the other series. Also it is 100% view. I can atest to how well it works as I shoot the A77. They are a larger body than the A55 though.

I am not pushing Sony here, really just have to figure your bottom/top dollar and once you have a few in your sights, go in and get hands on. There really is no going wrong with one brand over the other but the layout of controls and access to a deverse # of lenses is something to weigh. Sony has less of a selection of lenses, even when considering the Minolta lenses that can be used. However they are bringing new lenses out all the time and really there is a better selection than most give credit.

[Colin Southern]

Colin,
Do you have a suggestion as to which new DSLR camera to buy for a person just beginning to enter the advanced photographer category? I'm beginning to think that relying on auto-exposure capability of a camera takes the artist out of the picture, the camera is doing all the hard work. I guess that's why some people buy very expensive cameras that do all the thinking for the person pressing down on the shutter release button. How difficult is that to do?
Best regards,
Ohshoot.

Hi Ya,

What's your first name by the way?

I'm sure we can make some great suggestions, but there's an old saying "prescription without diagnosis is malpractice" - so we really need to know a little more about your budget and shooting requirements.

With regards to "cameras doing all the thinking etc" - nah - not really. In terms of exposure the camera is very capable of coming up with a starting point, but I'm constantly over-riding that with EC (Exposure Compensation). I can honestly say that never once have I felt "out of the picture" due to the automation; Getting a good exposure is easy with or without the automation -- harder choices are things like aperture (for Depth of Field control) (which the camera can't help with) -- but the hardest things of all are the likes of composition / viewpoint / filter selection / posing / lighting -- all things that the camera and all of it's automation don't help at all with.

In my mind, not using the automation to make life easier is a lot like doing a long division by hand instead of using a calculator -- if you really want to then go for it, but personally, I've got far better things to do with myself than to make life hard just for the heck of it. Automation is our assistant -- not our master

[mrme]

I don't have a professional SLR and often have to reduce noise in my pictures.
Reducing noise will smooth out the detail in a picture. It is a trade off that is up to your personal taste to decide where to leave the threshold at.
I'm surprised no one has mentioned this. Try G'MIC for noise reduction. G'MIC is a plugin for GIMP. It is completely free. Under the enhancement section there is an anisotropic and patch based filter. They are really useful, expecially the anisotropic filter. There is also a bilateral filter that you can use on the blue/red chrominance channels to reduce color noise.
Other programs like Raw Therapee and UFRaw will let you reduce noise before processing the RAW picture. Whatever program you use to convert your RAW images probably let's you reduce color noise, but I find it better to use G'MIC's anisotropic filter.

[tclune]

I don't have a professional SLR and often have to reduce noise in my pictures.
Reducing noise will smooth out the detail in a picture. It is a trade off that is up to your personal taste to decide where to leave the threshold at.

One of the things that seems to get lost as people get used to processing packages is an understanding of what the processing does. A traditional approach to salt-and-pepper noise reduction back when people wrote their own image processing routines was something called a "median filter." This is an example of a filter that uses a selection kernel instead of a convolution kernel. While explaining the difference would probably put everyone to sleep, the important point is that a selection kernel is statistical while a convolution kernel is linear. If you combine a linear filter for noise reduction (like using a low-pass filter) with a sharpening filter (I am not familiar with any sharpening approach that is not linear), you are working at directly cross-purposes. However, if you apply a median filter for noise reduction, there will be substantially less interference between the noise reduction and the sharpening. As with the rest of life, the more you need to apply the tool, the less satisfactory the results will be. FWIW