Colour Module

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Melty
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Joined: Fri Dec 22, 2017 1:18 pm

Colour Module

Post by Melty »

Hi,

I've been an avid user of Startools for many a year, and it is indeed the mainstay of my processing workflow (tried others, but keep coming back to ST).

One minor quibble I have is with the defaults in the colour module.

When I choose to process RGB data from my colour cameras, I have to apply SCNR to the linear data at the preprocessing stage. If not there is always some sort of green element/fringing to the stars, after applying colour correction towards the end of my ST process. Wipe, when utilised near the beginning of the process never seems to correct this no matter what I try.

I understand the fact the Bayer matrix consists of 50% green photosites, but I thought Wipe could address this imbalance without outside assistance.

Also, is there any particular reason why the saturation slider defaults to 200%, as I find this unrealistically oversaturates the image. Dialling back to 100% usually remedies this.

Thanks!
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Re: Colour Module

Post by admin »

Hi,
Melty wrote: Thu May 12, 2022 4:45 pm Hi,

I've been an avid user of Startools for many a year, and it is indeed the mainstay of my processing workflow (tried others, but keep coming back to ST).
That's great to hear!
One minor quibble I have is with the defaults in the colour module.

When I choose to process RGB data from my colour cameras, I have to apply SCNR to the linear data at the preprocessing stage. If not there is always some sort of green element/fringing to the stars, after applying colour correction towards the end of my ST process. Wipe, when utilised near the beginning of the process never seems to correct this no matter what I try.
There are a few things to unpack here.
  • Firstly, SCNR (or the ST equivalent "Cap Green") is not meant to be applied at the linear stage. It is in fact a very blunt tool that is best avoided unless something is terribly wrong with the chrominance(!) signal that cannot be corrected during pre-processing.
  • Second, applying SCNR to your dataset before importing it into StarTools will compromise - quite severely - the green channel's luminance signal, wreaking havoc on anything from detail to point spread functions (and thereby severely exacerbating ringing artifacts).
  • Third, it is not Wipe's job to correct color. It sole purpose is to remove any biases (local and/or global) in the individual color channels, luminance and narrowband accents. Wipe's direct/visible output is entirely irrelevant to the final coloring of your image.
Given the above, it would be very helpful if you could share a problematic dataset, as well as an image (or screenshot) showing the symptom you are describing. Green dominance in DSLR or OSC datasets is typically to be expected. Color aberrations around stars, however, are usually a stacker-induced problem (fringes can be any color in that case, including green), or a problem with the optical train (typically purple or blue aberrations; chromatic aberration).
Also, is there any particular reason why the saturation slider defaults to 200%, as I find this unrealistically oversaturates the image. Dialling back to 100% usually remedies this.
There is indeed! The reason why over-saturation and Color Constancy are set by default, is so that it is much easier to see the effects of the current color balance; it is much easier to sanity check the coloring against the known coloring of the various processes/physics that are prevalent in a region.
Ivo Jager
StarTools creator and astronomy enthusiast
Melty
Posts: 29
Joined: Fri Dec 22, 2017 1:18 pm

Re: Colour Module

Post by Melty »

Hi Ivo,

Ah that makes sense now. I had no idea that was the purpose of oversaturating the image...

The main reason I've been using SCNR and Background Extraction lately on linear data is that, at least in my hands, Wipe seems to struggle with complex gradients in my multi session images.

The "Basic" preset, whilst retaining faint details, doesn't seem to clear them completely, whilst the "Amp glow" preset (on the lowest setting) seems to work better, but erases the fainter stuff along with the gradients. I sometimes struggle to find a compromise adjusting the settings, hence me trying the other tools.

That said, I'll look into weaning myself off SCNR and Background Extraction and see if I can further refine my ST workflow.

Thanks!
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Re: Colour Module

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Melty wrote: Fri May 13, 2022 2:04 pm Hi Ivo,

Ah that makes sense now. I had no idea that was the purpose of oversaturating the image...

The main reason I've been using SCNR and Background Extraction lately on linear data is that, at least in my hands, Wipe seems to struggle with complex gradients in my multi session images.

The "Basic" preset, whilst retaining faint details, doesn't seem to clear them completely, whilst the "Amp glow" preset (on the lowest setting) seems to work better, but erases the fainter stuff along with the gradients. I sometimes struggle to find a compromise adjusting the settings, hence me trying the other tools.

That said, I'll look into weaning myself off SCNR and Background Extraction and see if I can further refine my ST workflow.

Thanks!
Providing ST the most untouched data as possible is extremely important.

Complex gradients are usually the result of an acquisition issue, and tend to not be celestial in nature. Multi-session datasets may indeed exacerbate this, imprinting such issues in multiple directions in different parts of the image, thereby multiplying amplitudes and/or undulation frequencies. Indeed, in such cases where gradients cannot be objectively distinguished, by any sort of algorithm, you may run into limitations.

I'm always on the lookout for "problem" datasets to make StarTools better, so if you have any such datasets that you feel Wipe is choking on, then I would greatly appreciate having a look!
Ivo Jager
StarTools creator and astronomy enthusiast
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