Livestack Histogram Autostretch odd behaviour

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zerolatitude
Posts: 156
Joined: Mon Mar 01, 2021 5:24 am

Livestack Histogram Autostretch odd behaviour

#1

Post by zerolatitude »

Title says it all.

Picture 1 is LS with histogram (LS, not small one on right), after Reset.

Picture 2 is after Autostretch. The black, mid and white points all seem to come together.

Something I'm doing wrong, or more info needed?

ASI533MC
Flats and darks
Background subtraction
LS_Hist_Reset_S.png
LS_Hist_Reset_S.png (886.52 KiB) Viewed 766 times
LS_Hist_Auto_S.png
LS_Hist_Auto_S.png (120.37 KiB) Viewed 766 times
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admin
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Re: Livestack Histogram Autostretch odd behaviour

#2

Post by admin »

Hi,

could you share some frame files and the dark/flat frames - I will try the same myself and see if I can work out what is going on.

cheers,

Robin
zerolatitude
Posts: 156
Joined: Mon Mar 01, 2021 5:24 am

Re: Livestack Histogram Autostretch odd behaviour

#3

Post by zerolatitude »

Of course. Give me a day to upload them somewhere.
zerolatitude
Posts: 156
Joined: Mon Mar 01, 2021 5:24 am

Re: Livestack Histogram Autostretch odd behaviour

#4

Post by zerolatitude »

Sent on PM
zerolatitude
Posts: 156
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Re: Livestack Histogram Autostretch odd behaviour

#5

Post by zerolatitude »

Message is stuck in PM outbox for hours. No idea whats happening.

can you pick it up from there?

Thanks
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Re: Livestack Histogram Autostretch odd behaviour

#6

Post by admin »

Hi,

the outbox is a bit odd on the forum software we use - messages stay in it until they are read by the recipient. Basically everything is working as expected - it's just that I hadn't looked at my forum messages yet.

cheers,

Robin
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Re: Livestack Histogram Autostretch odd behaviour

#7

Post by admin »

Hi,

thanks for sharing those files - I have had a look at the cause of the odd auto-stretch and found a combination of two things

1) Once dark, flat and background subtraction is applied, the background is very uniform (narrow histogram peak) and there is little else visible in frame except a bright few stars - in a way this is a sign of how well your dark and flat (and background subtraction) are working together to clean up the dark bits of the image!

2) SharpCap's auto stretch is based on the histogram data - in particular the while level value is based on looking for a histogram level with only a small number of pixels (typically 100) brighter than that level. Another tweak to speed up slow computers is that the live stack histogram is only calculated for every 13th pixel when the image is large (more than about 3.5Mp for colour, 10Mp for mono). The threshold of 100 pixels was not compensated for the 1:13 reduction in counts in the histogram, meaning that the stretch at the white level end was excessive.

I have tweaked the code to account for this and the results are significantly improved.

cheers,

Robin
zerolatitude
Posts: 156
Joined: Mon Mar 01, 2021 5:24 am

Re: Livestack Histogram Autostretch odd behaviour

#8

Post by zerolatitude »

Thanks, will wait for the update.

The histograms for my images are normally thin as I cant expose for more than 15(max 30) sec due to LP. We had a discussion on this in another thread. Also, these particular frames are from a relatively star poor region anyway.

Anyway, from what I gather from your post, the calibrations seem to be working well. Plus, he lights seem to do fine when processed, so all is well :-)
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