Author Topic: Image Integration, Winsorized Clipping, Default  (Read 21154 times)

Offline Niall Saunders

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Re: Image Integration, Winsorized Clipping, Default
« Reply #45 on: 2010 February 10 13:46:49 »
Hi Juan,

Yes, your 'dirty workaround' does work.

My Bias Frames, which originally have a nice Gaussian distribution curve, centred around a typical Mean of about 3970, with a StDev of about 40 and a Minimum of around 3780 and a Maximum of around 4170, now appear as they should when I open them - with the 'peak' way down at the bottom end of the [0,1] range.

Obviously, if I now apply the ReScale process, or if I use the Histo [ClipLow/ClipHigh] process, then I get what I was seeing before - the same very nice Gaussian distribution, but it is now more or less centred on 0.500, and with a Min/Max of 0.000 and 1.000 respectively. Absolutely NON-typical of a Bias frame - and therefore useless for further processing as such.

However, I still cannot use the ImageIntegration process - there is obviously an internal rescale() call that is being applied. I cannot tell whether you call this prior to processing each image, or whether you rescale the final integrated image after 'combining'. In any case, because the rescale call IS made, once again the resulting image has a (now 'very' nice) Gaussian curve - but back to being centred around 0.500

Am I missing some critical point here? Why can the image data not just be left 'as is' after the combination step?

NONE of the four possible combination methods (Average, Median, Maximum, Minimum) are mathemagically capable of generating an output image with 'out of range' values - providing the original images were all within range themselves. OK, sure, at SOME POINT during the 'Average Combine' process, the 'working image' might contain values that can be up to 'n-times' out of range - but as soon as that 'summed' image is then divided by 'n' again (to provide the 'averaged' result) the image MUST return back to 'in range' again.

I just don't see the need for a hard-coded rescale() call. You could, if needed, provide it as an option in exactly the same way as is done in PixelMath.

Which is a case in point - I can use PixMath to 'add' all the images together, and 'divide-by-n', and I get EXACTLY the result I am after (obviously without the Winsorized Clipping that I get from ImageIntegration). But, if I enable the ReScale option in PixMath, I lose the correct 'position' of the Gaussian curve - the Mean of the rescaled image ends up back at 0.500 again.

Don't get me wrong - there IS a possibility that I will NEED to have the image 'rescaled' for it to be useful as a calibration frame. I haven't got that far - my brain seems to be 'hung up' on these 'entry-level' issues.

But, I really do feel that - if you ARE automatically implementing a rescale() call - then I (for one) would like to be able to play with ImageIntegration with at least the OPTION of having the call being made, or not.

I hope I have been able to explain myself clearly - I think my brain is on the brink of shut-down. Perhaps I need to run a defrag on it. Tequila should do the trick :cheesy:

Cheers,
Cheers,
Niall Saunders
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Offline dhalliday

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Re: Image Integration, Winsorized Clipping, Default
« Reply #46 on: 2010 February 15 13:34:49 »
Wow...this is a deep thread... >:D
Just wanted to say that I FINALLY got around to doing a RGB (so far) stack with "winsorized clipping)...
It seemed to filter some crud out,and not a bad result..
I think it does a better job than DSS,but a bit fiddly.
Here is M1 (again..!) (RGB)
http://www.flickr.com/photos/daveh56/4360610524/sizes/l/

cheers all

Dave
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Offline dhalliday

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Re: Image Integration, Winsorized Clipping, Default
« Reply #47 on: 2010 February 16 05:48:50 »
Still talking to myself..?
Here is an EXCELLENT example of the power of this tool.I am loving it more and more,esp the ability to look at the rejected data.
It is the best Pix tool since DBE...!

I tried to stack 2 sets of exposures,different sky conditions,and got THIS nightmare;
http://www.flickr.com/photos/daveh56/4360801543/
But after :"winsorization"...(!!) I got this;
http://www.flickr.com/photos/daveh56/4361077951/
(well this is the end result/RGB,the first was just the R signal.)
(The frames were shot at roughly 180 degrees opposite...)

DSS could not "clean/match" the frames...despite playing with main settings. :'(

A Pix victory !! 8)
I would LOVE to hear more about the "kappa" settings,etc.

Dave
Dave Halliday
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Offline Jack Harvey

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Re: Image Integration, Winsorized Clipping, Default
« Reply #48 on: 2010 February 16 06:01:46 »
M 97 does look nicely integrated
Jack Harvey, PTeam Member
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Offline dhalliday

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Re: Image Integration, Winsorized Clipping, Default
« Reply #49 on: 2010 February 18 07:22:09 »
What I want to know is...how do we interpret the high/low rejection maps..??

I mean its ok to hear Harry say "winsor 5 high and low works for me"...(sorry Harry  >:D)
But it depends on the data..no?
I mean is a LOWER number rejecting MORE data,etc etc..?
Lots of questions remain for me on this tool...I would prefer a wee bit more insight from the Jedii...

Dave
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Offline Harry page

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Re: Image Integration, Winsorized Clipping, Default
« Reply #50 on: 2010 February 18 09:52:33 »
Hi Dave

Yes you are correct when you stack more images Pi can find the outliers more easily so a higher sigma number can be used ( the higher the sigma number the less pixels are rejected)
I start my sigma setting at about 4 ( High and low ) and stack away and out put the rejection maps  O:)

I then inspect the maps , which should only really show them outliers i.e. hot / cold pixels , sat / plane trails , cosmic ray hits and any other one of events

Unless you have really bad subs ( Then you should exclude these) there should not be much in the way of background pixels included in the maps  ;D

then you can decide to lower the sigma ( Reject more if there are still outliers on your result image ) or higher it if to many pixels are being rejected  :laugh:

Remember you want to reject as few as possible or you could be throwing away good info  :'(

I tend to set my low sigma the same as the high, seems to work most of the time


Harry
Harry Page