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DBE Pleiades Astrophoto PixInsight · dkamen · ... · 6 · 1040 · 0

dkamen 7.44
dkamen
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Hi guys,

I did DBE with subtractive correction on a linear image with signal level ~0.002 and noticed the result had signal levels around 0.30, practically stretched. How is that mathematically possible?

Thanks,
Dimitris
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mxpwr 7.29
D. Jung
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Mathematically speaking? By subtracting a negative background smile
Joke side, did you enable the rescaling/normalization option in the DBE process? That might be a the root cause for your observation.
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andreatax 9.89
andrea tasselli
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As far as I remember PI only deals with positive numbers so that value seems to be odd.
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CWTauri 22.98
Adam Block
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I think you are taking the semantic meaning of the method "Subtraction" too literally. A gradient correction will fit the background both adding and subtracting values based on the samples and other configured parameters. An additive undesired gradient, yes, will generally be subtracted which is why the method was given this name (I think) because of its practical benefit. However, the correction of a gradient will add or subtract as necessary to make it uniform. Furthermore, if you place samples on dark regions.. DBE will attempt to add values there… and if they are very small and you have not constrained DBE..the result can be quite bright. 

In fact, as I demonstrate in my videos on DBE, you can actually change the value that is added or subtracted in an area by changing the calculate brightness value at any sample. 
Did you ever notice that DBE will sometimes make bright objects have a dark background near them? So people avoid putting samples there..but that isn't enough sometimes… so you can manually adjust the brightness of model (add or subtract) to clean this up.

If you are interested in explanations and demonstrations as I have described above… please become a member of my site at AdamBlockStudios.com . You will find it very informative.

-adam
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jrista 11.18
Jon Rista
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Hi guys,

I did DBE with subtractive correction on a linear image with signal level ~0.002 and noticed the result had signal levels around 0.30, practically stretched. How is that mathematically possible?

Thanks,
Dimitris

What settings did you use? DBE has a normalization option that will attempt to restore the original offset level after extraction. This is disabled by default, which can result in unexpected results sometimes. Usually it results in the image becoming much darker, requiring re-application of STF, however as Adam mentioned, it depends a lot on exactly how you have sampled the image. Sometimes the nature of your sampling and the exact modelization of your gradient will shift the signal when the Normalize option is NOT enabled. 

Try enabling normalization, and see if this helps.
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dkamen 7.44
dkamen
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Update: I cannot replicate it. I mean I opened the same image, did DBE again, this time the result is normal. 

I am 100% certain I didn't have normalize selected, or division, or anything like that. Because I never use those things. Maybe Pixinsight just threw a fit, sometimes it happens, especially with dynamic processes. 

Thank you for your replies.

Cheers,
D.
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ajitheverester 0.00
Ajith Everester
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andrea tasselli:
As far as I remember PI only deals with positive numbers so that value seems to be odd.

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