Noise Reduction - When?

Edward Smith

I may have a habit I need to break.

When I'm shooting at high ISO with my DSLR, I usually do my noise reduction pretty early in my processing.  Since Noise Reduction inherently softens an image, I want to know how much sharpness I'm going to need to bring back.

I notice that in most of the PixInsight processing, people are doing the noise reduction near the very end of their processing.  Why?  I'm guessing it is because when you do noise reduction, the smoothing algorithms are essentially altering the data and if you do that up front, you are changing data that you might want to preserve for other processes.  It looks like some of the processes also act to reduce noise so if you do your noise reduction at the end, you don't need to be as aggressive about it.

So those are my guesses, what is the real reason?

BlockHead

Well..you know that noise reduction messes up your ability to measure stars and noise.

So BXT is before NXT for example.

Noise reduction also tends to correlate pixels that were not originally related. Then as you increase the contrast or brightness these artifacts are enhanced. 

Stated simply... I can tell. Literally. You can look at an image and tell what kind of processing (and perhaps the ordering) took place. Smoothing the image early on bakes in stuff (and perhaps loses some information) so that it is best to wait.

So here is the real question. You are asking why wait- but the question *why do noise reduction early*. Your comment "I want to know how  much sharpness to bring back" does quite make sense to me. The way I see it in general you make your sharpness and contrast adjustments...and then go after the noise. This is a what you see is what you get. There isn't worry about enhancing smoothed artifacts since near last step.

Of course, you can work in incremental processing as well. You can do a *small* amount of anything (noise reduction). This is actually the way I process. But it is nuanced since no single step does anything fully. You need to wait until all of the incremental processing gets you to where you want to be. 

-the Blockhead

Edward Smith

Thank you for the response.  I think the issue here is really different kinds of images.  When I'm shooting and processing with my DSLR, I'm generally not doing astronomical images (although I do some wide field night images) where all my data is crammed into a very narrow, very faint portion of the left side of the histogram.  I've generally been forced to shoot very high ISO because I'm indoors or low light, can't use a flash or tripod, so I need to keep the shutter speed fairly high.  I treat the noise in those cases the way I used to treat film grain on high ISO films in a previous life.  In those images, there really isn't any "signal" in my noise that I'm trying to tease out and any processing I do tends to amplify the noise and make later removal harder.

From your two responses (this one and the one about Linear/Stretched), I can see why you really want to to wait with the noise reduction for the astronomical images.  The signal to noise ratio for the astronomical images is MUCH lower so I have a much higher risk of losing signal if I do the noise reduction early.  For the vast majority of my DSLR work, the risk of losing signal by early noise reduction is fairly insignificant.

I just need to adjust my thinking here a little.

Thank you for the response.

BlockHead

Great..but also ease into it. Really I think the best approach is the incremental one...but it relies on the fact you never see the full improvement of sharpness or smoothness until near the end. It is easy to get off-track too. So maybe this way takes practice. Otherwise, being consistent in when you apply a process like noise reduction will act like a "control" and it will be easier to understand the limits of that process and how it relates to others.

-the Blockhead

Edward Smith

Thank you for the advice.  I have noticed on several occasions where one process appears to make an image worse, but you are really setting things up for a follow-on process or script to be more effective.  Like you said, it will take some experience and practice (and unlearning my "normal" photo processing habits).