VeraLux HyperMetric Stretch - an open discussion - Forums

A few days ago I released my script "VeraLux HyperMetric Stretch" for Siril, now available in version v1.2.0 under GNU GPL-3.0-or-later.

A project that was born as a personal challenge is becoming a widespread topic. I am enormously grateful for the attention shown to my work and I am happy if, in any way, it can be useful to any of us (in addition to myself).

However, I am convinced that such a script must take into account the feedback derived from daily use, and should not be vitiated by the perception of those who wrote it (that is, exclusively by my judgement).

Therefore, I would like to open a discussion here, to ask you if you have had a chance to try it, what you think, or what you think are its strengths or criticalities. For me it would be essential to have your feedback, which is the reason why I wanted to make my work public.

Thanks to anyone who will dedicate some of his time to this discussion ;-)

Clear skies!
Riccardo

Hi Riccardo,

Thanks for your effort, and making it available! I’ve tried it on a couple targets and as others have noted, the colors are quite saturated, but I’ve also read/heard that it actually protects the natural color while stretching and that it looks over saturated because we’re used to de-saturated colors from other types of stretching. Can you explain more about that? Also, there’s a Saturation slider that defaults at 90%. What percentage yields ‘natural’, or faithful to the data, color?

Cheers,
Scott

Hi Scott, and thank you for your message! Just a quick clarification first: the official VeraLux script for Siril does not have any saturation slider. The one you are referring to is the unofficial PixInsight port (in case, I’d be grateful if you could confirm), and I cannot guarantee its accuracy. In the official version, color saturation is not modified at all. What sometimes looks “highly saturated” is simply the real chromatic balance preserved during the stretch, while other stretches tend to wash it out by compressing the channels. This is why some people feel they are seeing stronger colors: they were already there in the data, only less visible before.

If someone wants visually softer stars, the official script now includes a control called “Color Grip”: at 1.00 (default) is physically faithful color preservation (default); lower than 1.00: smoother, more traditional star appearance.

So:,natural and faithful color is always at 1.00. Everything below that is a personal artistic choice.

If you want, feel free to send me an example image and I can show you how the official version behaves on your data.

Riccardo Paterniti · Dec 10, 2025, 01:16 PM
Hi Scott, and thank you for your message! Just a quick clarification first: the official VeraLux script for Siril does not have any saturation slider. The one you are referring to is the unofficial PixInsight port (in case, I’d be grateful if you could confirm), and I cannot guarantee its accuracy. In the official version, color saturation is not modified at all.

You’re correct, I was using the (unofficial) PI version. Do you have any plans to release it officially for PI? And do you know if the unofficial version works any differently, other than the saturation slider?

I’ll take you up on your offer to show what it does with one of my images, but at work so won’t be until this evening.

Cheers,
Scott

PI version (my porting): nope
How it works the unofficial one: don’t know
Your files: it will be a pleasure, whenever you want ;-)

So far, in limited testing, it seems to work really well. I happen to like the way it makes colors pop and you can always desaturate if it’s not to taste. Thank you for the time and effort involved with this project.

Thank you Tony! Have you tried the Siril v1.2.0 with Color Grip function? If yes, I’d be interested in your feedback :-)

Tony Gondola · Dec 10, 2025, 03:00 PM
So far, in limited testing, it seems to work really well. I happen to like the way it makes colors pop and you can always desaturate if it’s not to taste. Thank you for the time and effort involved with this project.

Official or unofficial version? Maybe why the saturation slider was added in the unofficial version.

Cheers,
Scott

Scott Badger · Dec 10, 2025, 03:05 PM
Tony Gondola · Dec 10, 2025, 03:00 PM
So far, in limited testing, it seems to work really well. I happen to like the way it makes colors pop and you can always desaturate if it’s not to taste. Thank you for the time and effort involved with this project.
Official or unofficial version? Maybe why the saturation slider was added in the unofficial version.
Cheers,
Scott

The official version in Siril.

I saw a demo by Rich (Deep Space Astro) which prompted me to update my Siril version to check it out.

I tried it on a couple of images so far and generally liked the result.

I intend to try it on some images of dark nebulae and dust to see if it can pull out the really dim structures and contrast with the really dark.

I have not yet tried it on a starless image which is my normal process. Might also see what it does with the extracted stars.

So far I like what I am seeing. Thanks for developing this tool! At some point I’d like to dive into the theory behind it.

Nice work!

Riccardo Paterniti · Dec 10, 2025, 03:05 PM
Thank you Tony! Have you tried the Siril v1.2.0 with Color Grip function? If yes, I’d be interested in your feedback :-)

Yes and that function works very well. I’ve found that setting it to zero will give a color intensity that’s closer to what people are used to seeing post stretch so there’s a lot of flexibility there. The adjustment range is perfect.

Dean Ostergaard · Dec 10, 2025 at 04:17 PM
I saw a demo by Rich (Deep Space Astro) which prompted me to update my Siril version to check it out.
I tried it on a couple of images so far and generally liked the result.
I intend to try it on some images of dark nebulae and dust to see if it can pull out the really dim structures and contrast with the really dark.
I have not yet tried it on a starless image which is my normal process. Might also see what it does with the extracted stars.
So far I like what I am seeing. Thanks for developing this tool! At some point I’d like to dive into the theory behind it.
Nice work!

Thank you Dean! Sometimes, it works better on images with stars (it’s subject dependent), but to be honest, most of the times, on starless images it does its best. Obviously in my personal opinion :-)

Tony Gondola · Dec 10, 2025 at 04:19 PM
Riccardo Paterniti · Dec 10, 2025, 03:05 PM
Thank you Tony! Have you tried the Siril v1.2.0 with Color Grip function? If yes, I’d be interested in your feedback :-)
Yes and that function works very well. I’ve found that setting it to zero will give a color intensity that’s closer to what people are used to seeing post stretch so there’s a lot of flexibility there. The adjustment range is perfect.

Thank you Tony!

While it addresses the unofficial PI version you may have a look at the fairly positive review of view into space on youtube: https://www.youtube.com/watch?v=QTK6Qv6_5mE

I would appreciate an official version for PI but fully understand the hassle to support two versions. Maybe it’s time for PI to finally start supporting Python scripts …

Clear skies

Wolfgang

kuechlew · Dec 11, 2025 at 06:14 AM
While it addresses the unofficial PI version you may have a look at the fairly positive review of view into space on youtube: https://www.youtube.com/watch?v=QTK6Qv6_5mE
I would appreciate an official version for PI but fully understand the hassle to support two versions. Maybe it’s time for PI to finally start supporting Python scripts …
Clear skies
Wolfgang

I’m in contact with Sascha - who always makes excellent content! - after seeing his video. He’s been very kind to me ;-)

kuechlew · Dec 11, 2025 at 06:14 AM
Maybe it’s time for PI to finally start supporting Python scripts …

Long overdue, if you ask me.

Yes, but Siril did it :).

Thanks a million, Riccardo.

I discovered your script yesterday from cloudynights. I use Siril 1.4.0 latest release.

Gave it a try on two of my stacks (pre-starnet++) just to see how it works, with default settings. I like the ease of use.

Usually, I use the Seti Astro’s statistical stretch for the pre-stretch of my starless layers (Ha, Oiii, Sii or their continuum subtracted derivatives). Further, use the curves and GHS before combining them.

Will try your script in a similar manner.

Thanks for your time and efforts.

ZigZagZebraz · Dec 11, 2025 at 12:05 PM
Thanks a million, Riccardo.
I discovered your script yesterday from cloudynights. I use Siril 1.4.0 latest release.
Gave it a try on two of my stacks (pre-starnet++) just to see how it works, with default settings. I like the ease of use.
Usually, I use the Seti Astro’s statistical stretch for the pre-stretch of my starless layers (Ha, Oiii, Sii or their continuum subtracted derivatives). Further, use the curves and GHS before combining them.
Will try your script in a similar manner.
Thanks for your time and efforts.

Thank you! Consider VeraLux a (pretty) good starting point, then curves is the best option (without touching white or black points), especially in Scientific mode, that needs further tone mapping. Let me know ;-)

Thanks again, Riccardo.

On Ha, Sii and Oiii layers, applied VeraLux. Selected my sensor (IMX571 - did not seem to differ much with the default settings, if I change only this setting), Scientific (Preserve) selected and Auto-Calc Log D makes it easier to process. Used curves for each layer and recombined. Was able to get better details.

Riccardo Paterniti · Dec 11, 2025, 12:14 PM
Thank you! Consider VeraLux a (pretty) good starting point, then curves is the best option (without touching white or black points), especially in Scientific mode, that needs further tone mapping. Let me know ;-)

ZigZagZebraz · Dec 11, 2025 at 02:51 PM
Thanks again, Riccardo.
On Ha, Sii and Oiii layers, applied VeraLux. Selected my sensor (IMX571 - did not seem to differ much with the default settings, if I change only this setting), Scientific (Preserve) selected and Auto-Calc Log D makes it easier to process. Used curves for each layer and recombined. Was able to get better details.
Riccardo Paterniti · Dec 11, 2025, 12:14 PM
Thank you! Consider VeraLux a (pretty) good starting point, then curves is the best option (without touching white or black points), especially in Scientific mode, that needs further tone mapping. Let me know ;-)

When you work with mono narrowband the sensor profile matters much less, simply because each frame contains only one channel and there is no RGB cross-talk to correct. In OSC, the sensor matrix defines how the R/G/B channels are weighted and preserved. In mono, you are feeding VL a pure signal, so the sensor setting mostly becomes a neutral scaling preset. That’s why IMX571 vs Rec.709 looked almost identical in your test

Thanks for the explanation. 😀

where can i download it?

Robert Houghton · Dec 11, 2025, 07:09 PM
where can i download it?

If you’re asking for the Siril plugin:

Open Siril, on the right top, Hamburger (three lines) icon, Preferences, Scripts, below there there will be a table of all the plugins available. You can type in the little search bar in the menu “VeraLux” and it will show up below. Select it with the tick mark on the right. It will be installed.

Some could not find it, in which case, have to delete the scripts directory on the top of the menu.