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LRGB and NB workflows: when noise reduction with NoiseXterminator

jeffrey merrilljeffrey merrill
in PixInsight
Hi:  was looking at your LRGB workflow and some NB processing videos.  Your LRGB workflow doesn't mention noise reduction as a step.  

In older videos you mentioned some linear noise reduction techniques and also non-linear.  

If using NoiseXterminator, do you recommend approaching noise reduction only in non-linear state?  
Immediately after stretching Lum and RGB masters? 
Or noise Xterminator on combined LRGB ?  
If you're going to incorporate HDRMT technique to better colorize the final LRGB..  noise reduction after HDRMT use?

jeff

Comments

  • BlockHeadBlockHead
    edited September 2024
    This is a complicated question... and answer depends on some factors. Here are some "rules..."

    1. The profile of noise changes as you combine images. So there is one profile of the "linear" (natural) noise. Processing the image or combining with other images changes the appearance of the noise.

    2. Consider a luminance  (or gray) image. We are concerned with the spatial "graininess" of the image. If you do NXT linear AND NXT non-linear... compare this with only doing NXT non-linear. Then you do whatever it is you are going to do in the linear without any NXT.. you *tend* to mix up this graininess... but not change its "size" ... it just gets scrambled. If you instead do NXT *both* in linear and non-linear it is possible in the linear application to do what you think is normally best. This is the issue... this will make blotchy bits of the diffuse object and then you add some noise doing other processing (HDRMT) in non-linear... so now you muddle smoothed with extra noise. This is a diffrerent result. This is why there is nuance. If I were to do NXT twice... I would *not* apply a full dose of NXT. I would *not* make it look good. This is the part that is hard to teach. You need to do things incrementally.  This isn't a question about order of operations ... it is a question about how much to do (if any at all).

    3. Another rule is that chrominance noise can be treated more aggressively. You are likely to see me apply NXT (or something) in linear and non-linear on noise. See..this is OK for LRGB right? The "noise" in terms of grain will come from the L... colorizing this noise with more color noise is odd. So doing NXT on chrominance in linear and then later again with LRGB makes sense.


    Please consider this when you watch me apply (or not) NXT.

    -the Blockhead
  • jeffrey merrilljeffrey merrill
    still not sure when to introduce noise reduction..noise reduction is a reality that most of us need to do.. so is a bit strange that workflows don't address this to be honest.

     perhaps a video on approaching the actual application of noise reduction to images

    jeff
  • BlockHeadBlockHead
    I guess I disagree. It isn't a step. It depends on the data and what you are doing. 
    I do certainly use it and show using it many times in the demonstration videos. 
    There may be a pattern- but I think it is more nuanced then any simple set of "rules" I can spell out. 

    You need to look at the noise in the image. If the next steps are not good with a smoothed image- then it is best to leave it to later. I think in general I leave it to later when the image has good signal. You will note I did apply NXT sometimes to the Narrowband data because it is so noisy that you need to incrementally keep up with it. 

    So let me reverse the question. You want to know in a workflow when to apply NXT and see it put in there.
    I want to communicate that if I put NXT in a linear workflow step (like a PDF of something)  that isn't always optimal to do so. If I just put it in there... it will be easy to think... yep that is a STEP that needs to be done. How would I communicate the vagaries of when to do NXT for the best result?

    I guess I just see noise reduction as a matter of degree. Kinda similar to Saturation. You can increase the saturation of a color image at any point... but when you do it really depends on what is going on. So I never write saturation as a "step". I do explain in videos in particular cases when I need to do it (or not).

    -the Blockhead
  • Scott Badger
    I agree. Depending how the image is looking, I might apply a small amount of NXT after the first or second stretch iteration (using GHS) and a (sometimes second) application after I'm done stretching, but before further processing. Again depending, maybe also some NXT as a final processing step. But as you said Adam, when and how much is pretty much by feel.

    Cheers,
    Scott
  • Scott Cunningham
    edited November 2024
    It might be helpful here to post some image examples where, in Adam's opinion, it would be good to do some NXT, and some where it should be left alone. As someone with little experience, it's like a heart surgeon telling his student, "you just sort of feel how much voltage you need to apply when restarting the heart." The student says, well, ok... And then a few people die until he gets the "feel" of it.
  • BlockHeadBlockHead
    Well..that last statement is kinda how the real world works to be honest. 
    But I do think his falls in an area that different people really do have different preferences. I am happy to give me opinion of course and explain why I think so... but there is quite of bit of art in this science.

    But please always more screenshots...  much easier to understand questions!

    -the Blockhead
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