New Feature : Background Subtraction

Discussions, Bug Reports and Issues related to Beta versions of SharpCap
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Djibi
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Re: New Feature : Background Subtraction

#21

Post by Djibi »

Hello Robin,
I made the test and I confirm that the behaviour is as you describe as long as I stay in "one frame mode": my mean background ADU is at 5% of max ADU when I activate the background substraction

But, and this was the point of my previous post, as soon as I activate the "LIve Stacking" mode, the "mean gackground ADU" goes up to 25% with the consequence that I described on the stretch that is needed to enjoy the stack.

from my point of view, the interest of this feature is mainly for "Live Stacking", there are plenty other way to get a colour balanced sky background with post processing softwares.

I was thinking that this behavior during live stacking is a bug, sorry if I mis-reported !

cheers

Jean-Baptiste
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Re: New Feature : Background Subtraction

#22

Post by admin »

Hi,

right, I understand now - it's a consequence of the 5% offset with dark subtraction and the weak stretch that live stacking applies to the image by default - this is the default stretch for live stacking :
Capture.JPG
Capture.JPG (53.63 KiB) Viewed 2242 times
Having that as the default stretch helps pull out a bit more of the faint detail as soon as you turn live stacking on. As you can see it will increase the ~5% background considerably in the on screen image.

The initial stretch is just designed as a starting point for further adjustment - after all every imaging setup will have different background levels, with or without dark frames, etc. Usually pressing the 'autostretch' button will pick a fairly good stretch based on the histogram levels that will give a black background and pull out a good amount of detail.

cheers,

Robin
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Djibi
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Re: New Feature : Background Subtraction

#23

Post by Djibi »

I understand also, it is not a bug, it is a feature and this behaviour is normal.
- the unitary frame background substraction puts the background level at 5% max ADU when the calibrated frame background is below 5%
- there is a defaut stretch when you go to live stack mode, this default stretch transform the pixel at 5% max ADU to 25% max ADU

I played a little with some unitary frames I was keeping. It was not esay to play with the colour because I think that my use cases are not the one you intended: I read you explanation and I understand that you built this feature for frame with high light polution. In that cases, the background level after calibration are quite high and the background substraction feature is lowering the background pixel values.


I am trying to use it when I use specific filters that alter the colour balance (Idas NGS and Idas NBZ).
With a Dual Narrowband filter, my pixel values are very low, I can not reach the "background level". And even if the "colour balance of a HOO capture does not mean a lot", it is interesting trying to get something with the livestack histogram.
- in that case, I never touch the dark level on the histogram (if I move it accidentaly, I can not bring it back manually, I must reset the livestack settings !)
- the 5% translation on the histogram is very disturbing beacuse they are like a kind of Offset that has been added to the stack, it is very difficult to position properly the "dark level" and the autostrech does not help)
- in this very specific case, I would have prefer not to have this 5% shift of the stack, even if I know that some pixels may be clipped

this is just a feedbcak on a specific use case, thank you a lot for your comments, understanding that the colour adjustment are made on values before the stretch will be very userful !
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Harmonious
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Re: New Feature : Background Subtraction

#24

Post by Harmonious »

I tried the blended background subtraction and it is I think going to be an excellent tool. I do see two aspects that might improve usability.

Background subtraction considerably steepens the right side of the peak curve in the live stacking histogram to the point where it is difficult, very difficult to see the separate color lines.

As expected the auto color button in the histogram window did not bring red, blue and green into perfect alignment nor did it bring them to match the white line. But with the narrow peak and close spacing of the black, mid and white lines the histogram was very difficult to use.

The image was made using an ASI2600 on a 120mm refractor at f/7, a few miles inland from San Francisco Bay, under scattered low coastal clouds and higher thin clouds- meaning unstable seeing and sky brightness. It was a night for tuning and testing various things, but not for good images.
SC Histo.jpg
SC Histo.jpg (914.34 KiB) Viewed 2162 times
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Re: New Feature : Background Subtraction

#25

Post by admin »

Hi,

thanks for the feedback - I certainly was not expecting to see a histogram stretch like that! I must test to see if I can make something similar happen myself so that I can understand the cause and possible ways out.

cheers,

Robin
clouzot
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Re: New Feature : Background Subtraction

#26

Post by clouzot »

I tested this feature with a recent 7928 beta (not the final release I'm afraid, so it may have been fixed in the mean time) and can report the same behavior with a histogram peak that gets highly "compressed" in dynamics. I was using quite short subs (8s) because of the severe LP from my location.

Without background subtraction
bgsub1.jpg
bgsub1.jpg (815.82 KiB) Viewed 1805 times
With background subtraction
bgsub2.jpg
bgsub2.jpg (766.37 KiB) Viewed 1805 times
Color balancing is then almost impossible to achieve as even minute adjustments of the RVB slider will make the respective curve jump in the final image
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Re: New Feature : Background Subtraction

#27

Post by admin »

Hi,

do you have any individual frames along with the dark/flat so I can test this?

cheers,

Robin
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Djibi
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Re: New Feature : Background Subtraction

#28

Post by Djibi »

Hello,
This is exactly what I described and what I experienced when I made some test,

From the exchange I had, I would investigate the “5% minimum” of the background that is implemented

In that case, the stretch which is required with black and mid level is so “big” that the colour balancing becomes impossible
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clouzot
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Re: New Feature : Background Subtraction

#29

Post by clouzot »

admin wrote: Fri Jul 09, 2021 3:40 pm Hi,

do you have any individual frames along with the dark/flat so I can test this?

cheers,

Robin
Thanks Robin,

Not for this session, but I'll save individual frames tonight and will post them here as soon as possible.
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Re: New Feature : Background Subtraction

#30

Post by admin »

Hi,

thanks - the more data I can collect on situations where this happens, the more chance I have of understanding the cause and finidng the right solution.

cheers,

Robin
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