Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

stargazine_ep2_banner.thumb.jpg.e37c929f88100393e885b7befec4c749.jpg

Endolf

GIMP help (Manual stretching/colour management)

Recommended Posts

Hi

I'm new to photo processing and astrophotography in general. I've been playing around with ASTAP for stacking and was able to get something ok. I've now taken a look at Astro Pixel Processor. I've managed to get a passable image of M81/82 and M101, I'm now having a go and reprocessing some of my earlier images I did in ASTAP. I've got an image of M42 that I've processed with it. In ASTAP the colours were correct, it was just very dim in it's native FITS result. I was able to do a couple of sets of level adjustment, one that got the nebula details and one that darker that showed the details in the core. I could then blend the images in GIMP, sort of manual HDR.

I've got APP to process the image now, which seems to show a lot more detail. If I save the auto stretched version in APP I get an image with the detail but a blow out core, if I save the non stretched version I am having troubles in GIMP as the colours are off. I'm not familiar enough with photo processing or GIMP to be able to adjust the colours, I've tried adjusting the levels, but can never get a neutral look. I've attached some photos that show the issue, any advice or links to tutorials would be greatly appreciated.

Thanks

Screenshot from 2020-03-24 17-35-17.png

Screenshot from 2020-03-24 17-35-23.png

Screenshot from 2020-03-24 17-35-39.png

  • Like 1

Share this post


Link to post
Share on other sites

Which version of GIMP do you have installed?  Could be that you really need a later version to get the full 32-bit support.

James

  • Like 1

Share this post


Link to post
Share on other sites

2.10, it does support the 32 bit FITS images.

  • Like 1

Share this post


Link to post
Share on other sites

@alacant is good with GIMP perhaps they'll see this, as does another member but I can't get their name right.

  • Like 1

Share this post


Link to post
Share on other sites

Hi

You can process 32 bit in gimp [1] but I'm not sure why you'd want to do that. Make big paper prints maybe? 

The best way to import into gimp for post calibration is to save your stack as 32 (or preferably 16) bit tiff without compression. 

Colour balance is the single most difficult -for me at least- part of post processing, so stick at it; do the colour last of all. Perhaps use the CMYK colour balance module before resorting to curves?

But hey, don't sweat. What used to have to be e.g. red, can now be the once forbidden green. Or violet. Or whatever you choose. Fashion.

Good luck and HTH.

[1] Works for the Linux version. I haven't tested any other platform.

  • Like 1

Share this post


Link to post
Share on other sites

You probably are better off  by stretching in APP or ASTAP.  In GIMP you could stretch using curves, but most likely like in Photoshop you will loose colour of the faint objects:

See e.g.:

http://allthesky.com/articles/colorpreserve.html

or here

https://www.markshelley.co.uk/Astronomy/Processing/Colour_Preserving_Stretch/colour_preserving_stretch.html

 

And explained in this little graph:

image_99770.png.86becbf9f39c907f69fe473dbd48ed58.png

Input:

blue=5
red=10


The input colour ratio  is then 10/2 is 2.

The curve gives these values after stretching:

Output:

blue=0.26
red= 0.38




De output colour ratio is then 0.38/0.26, about 1.5. De result is less colorful.

 

A better method is colour preserved stretching as implemented in ASTAP:

in de range of 0..1:

luminance:=(red+green+blue)/3

luminance_stretched:=gamma_curve(luminance)

red_output :=luminance_stretched * red
greenn_output:=luminance_stretched * green
blue_output:=luminance_stretched * blue


The ratio between red, green and blue so the colour stays the same. Only the luminance is stretched.

 

Han

  • Like 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.