Orpington Astronomical Society

Astronomy => Technical => Topic started by: MarkS on Oct 12, 2017, 11:31:06

Title: PixInsight ArcsinhStretch
Post by: MarkS on Oct 12, 2017, 11:31:06
I have some great news.  From today my ArcsinhStretch module is available as an official PixInsight update if you are running version 1.8.5.   It appears in the IntensityTransformations category and it has proper documentation (unlike most modules). 

I'm a real PixInsight developer now  ;)

Mabula, the developer of AstroPixelProcessor has contacted me and he intends to work on functionality for colour preserving stretches.

All my hard work behind the scenes is beginning to pay off.  Colour preserving stretches are about to go mainstream - no more washed out star colours unless you are one of those poor unfortunates still using Photoshop curves :)

Mark

 
Title: Re: PixInsight ArcsinhStretch
Post by: Carole on Oct 12, 2017, 11:44:41
Congratulations Mark, you have done well.  Hmmm looks as though I am going to have to bite the bullet and get Pixinsight, though I don't do a lot of broadband, even if I just use it to retain star colour.

Can you do the same for PS please.  Lol

Having said that, of late, I have started to stretch in layers and increase colour in the lower layer and letting the star colour through which seems to work to some extent, though nothing like your wonderful star colours.

Also need to get some good data on which to use it. No use around here.

Carole
Title: Re: PixInsight ArcsinhStretch
Post by: julian on Oct 12, 2017, 15:06:24
Congratulations Mark
May have to start imaging.
PS can you sort out a mirror testing program. I have pixInsight  :D
Title: Re: PixInsight ArcsinhStretch
Post by: JohnP on Oct 12, 2017, 16:14:20
Top marks Mark :-) excellent job - I guess the question we all need an answer to though is if you could only buy one would it be PI or APP & if so why?

Well done again - John.
Title: Re: PixInsight ArcsinhStretch
Post by: MarkS on Oct 12, 2017, 17:42:27
Quote from: JohnP
Top marks Mark :-) excellent job - I guess the question we all need an answer to though is if you could only buy one would it be PI or APP & if so why?

Definitely PixInsight because of its power and versatility.  It's a fantastic "laboratory" to experiment and invent new techniques.


Quote from: Julian
PS can you sort out a mirror testing program. I have pixInsight  :D

I wish!


Quote from: Carole
Can you do the same for PS please.  Lol

Sure!  I can already do it but I've kept it very quiet up until now. 
I'm not joking!

Mark
Title: Re: PixInsight ArcsinhStretch
Post by: MarkS on Oct 12, 2017, 20:57:08
Quote from: MarkS
Quote from: Carole
Can you do the same for PS please.  Lol

Sure!  I can already do it but I've kept it very quiet up until now. 
I'm not joking!

Here you go then:  https://stargazerslounge.com/topic/301646-colour-preserving-stretch-in-photoshop/

Try it out and let me know how you get on.

Mark
Title: Re: PixInsight ArcsinhStretch
Post by: Carole on Oct 13, 2017, 00:53:08
That's brilliant Mark, I have tried it both on your sample image and also on my Cave Nebula LRGB image and there is definitely a lot more star colour in the final result.

I tried to make an action for it so I didn't have to keep repeating all the steps, but it would not work, but by the time I had done the process a few times, I got used to it anyway.  (Maybe it just didn't apply the curve function, I might try that again).

My version of PS is C3 and so I applied gamma 2.5. 

The only thing I wasn't quite sure about and I am sure this is just terminology is your statement:
Quote
stacked linear data (i.e. data with no stretches performed on it) and that you have applied white balance and background subtraction.
i.e. applying white balance and subtraction, I have no idea what action that is, but I just did a little levels bringing the slider in to the histogram each side, and that seemed to work OK.

Bonus: I don't have to bother about getting my head (and purse) around Pixinsight.

Carole

Title: Re: PixInsight ArcsinhStretch
Post by: MarkS on Oct 13, 2017, 06:33:14
Hi Carole,

I knew you would like it!

As for terminology:
White balance is just getting the colours right in the first place i.e. the multipliers for the RGB channels so the star colours are correct at the start.
Background subtraction is just removing the light pollution - as you say, the levels sliders do this well.

I've no idea about how to make actions but it sounds like a good idea if you can make it work.  I wonder why Photoshop changed the way gamma works?  It makes it quite confusing when migrating onto a later version.

Maybe you should demo this at your imaging evening ;)

Mark
Title: Re: PixInsight ArcsinhStretch
Post by: Carole on Oct 13, 2017, 09:47:13
Ah, got it.  I just call white balance, colour balance. 
I did have to do a colour balance at the end as it ended up very Magenta, so will do it at the beginning next time. 

In the past I would always start to process RGB data as having to combine them as mono filters I needed to see the stars more clearly to check I had lined them all up properly, but this did seem to work oK, I guess I could slide the stars around afterwards if necessary. 

I have never created an action before either, but worked it out.  It only appeared to do part of the action, so will give it another try and see if I can work out where it stops.  If it does 3/4 of the action it will save some time.

Thanks so much Mark, yes a good idea for an imaging session demo, thanks for that.

Carole
Title: Re: PixInsight ArcsinhStretch
Post by: doug on Oct 13, 2017, 09:50:43
     Well done Mark. Another feather in your cap.....

    Doug.
Title: Re: PixInsight ArcsinhStretch
Post by: Carole on Oct 13, 2017, 11:05:52
OK Mark, well I have managed to write some actions for this. 
I have done it in two parts.

Part one (up to number 6 in the instructions) works well on both your sample image and on mine, but part two (the rest of the instructions) only seems to work on my RGB filter image, but not on your sample OSC image.

Even 1 - 6  Cuts out a lot of the fiddle though. 

Should be able to E Mail a link for actions, but have yet to find out how to do that.  If I find out will send you the link/s.

Carole
Title: Re: PixInsight ArcsinhStretch
Post by: Carole on Oct 13, 2017, 12:06:19
OK, I have uploaded my Cave Nebula comparing my process with Mark's Star colour process as above.

My processed image:
(http://cdn.astrobin.com/images/thumbs/0d5720c5be76c0c58596fb2d75f06eee.1824x0_q100_watermark.jpg)

My image but after using Mark's PS Star colour preserving process (above):
(http://cdn.astrobin.com/images/thumbs/78423a30770e751633fe9690f8a36029.1824x0_q100_watermark.jpg)
Title: Re: PixInsight ArcsinhStretch
Post by: MarkS on Oct 13, 2017, 19:50:35
That's a nice result!

The process appears to have worked well.

Mark
Title: Re: PixInsight ArcsinhStretch
Post by: Carole on Oct 13, 2017, 22:42:04
Thanks Mark
Title: Re: PixInsight ArcsinhStretch
Post by: MarkS on Oct 19, 2017, 08:57:44
I've now managed to improve the Photoshop approach.  It is actually possible to use Arcsinh (Hyperbolic Arcsine) curves in Photoshop instead of the gamma stretch I previously suggested.  I know this, because I designed and built them in a spreadsheet  ;)

If you want to know more and if you want to download and use the curves, the info is here:
https://stargazerslounge.com/topic/301646-colour-preserving-stretch-in-photoshop/?page=&do=findComment&comment=3304684

Carole, I would be interested to know if you find the preset curves easy to use.

Mark