Application of SPCC and MGC plus SPCC, MGC and SPCC to an image any difference?

[fwm891]

I've taken an image (the Helping Hands nebula) shot recently and after some back-n-forth with a couple of other members about when processes are applied I thought I'd try myself.

There are four images below:

A straight Integration of 50x 300s subs processed in PI with no other processing applied.

Image 2, Same integration with MGC only applied

Image 3, Same integration with SPCC then MGC applied

Image 4, Same integration with SPCC, MGC and a further SPCC applied

I have my own opinions on this so I won't bias anyones thinking but comments are welcome.

Francis

@Ouroboros @Vroobel

[Anne S]

I always apply gradient removal processes as soon as possible stacking. Then I apply SPCC followed by BlurXterminator. That's for mono imaging. If colour I unlink the colour channels before SPCC. I don't think there's any benefit in applying SPCC twice.
 

I haven't used MSG much as I've not got any data to play with, no clear skies since October!

[Vroobel]

I like it!

We can exclude the first image as it shows a vignetting, which is corrected with the MGC in the second image but (!) it looks like stars there have a greenish cast so I wouldn't accept it as well. 

It's hard to choose between the third and fourth images, they aren't obtained according to my taste but in general, the third image (SPPC then MGC) seems cooler or brighter while the fourth one (SPCC MGC SPCC) looks warmer, which I like. Unfortunately, it also looks darker which may be a disadvantage.

 

 

[Ouroboros]

@fwm891 Nice thing to try. 

Erm … well … they’re all pretty similar. 2,3 and 4 are obviously better, as one would hope.  To my eye number 4 is marginally better than number 3 in having an ever so slightly better  balanced background. It is marginal though. Both 3 and 4 are better than 2.

Have you reset the stf for each image and are they linked or unlinked channels?

I wonder whether the fact that you have captured a lot of (very nice) dust in the background of this image makes it more difficult to differentiate what affect the MGC and SPCC processes are having, 

[fwm891]

Thanks for the comments so far I'll add my thoughts on the SPCC/MGC mixing here.

@Anne S  I agree I don't think there is any benefit in applying SPCC twice to an image.

@Vroobel Your final comment here about the SPCC MGC SPCC image is to me probably the most significant. The image appears darker. SPCC applies a background neutralisation to the image as a final stage. Surely that negates what MGC has just done to the image when it's applied the MARS data to the image background. The MARS data is surely not neutral across the image?

@Ouroboros You too seemed to perceive number 4 as having a slightly better balanced background! That was my first thought but then I thought it can't be right because as above I kept thinking the SPCC has just destroyed the MARS constructed background?

So I'm going to stick my neck on the block and say that (for me) number 3 is the better image ( at this processing stage)

Its star colouring has been corrected by the use of SPCC (Gaia DR3), its background by the use of MGC (MARS). It obviously still needs other treatments from PI's storehouse.

Time to try the same on a range of images with different backgrounds...

Further comments welcome

Edited January 7 by fwm891
Text added

[Stuart1971]

Number 4 looks the best to my eye, and on my Screen, and that’s the issue really as everyone’s viewing on a different screen so it’s very hard to see what others see, and almost impossible to form an objective opinion without all seeing the exact same image on the same screen.

i use an iPad pro, and the screen is excellent, but the image above looks totally different on my PC screen which is also a top end 4k monitor…and very different again on my phone, again an Apple device…🤷🏼‍♂️

But to me number 4 takes the win..👍🏻

[Vroobel]

9 minutes ago, Stuart1971 said:

Number 4 looks the best to my eye, and on my Screen, and that’s the issue really as everyone’s viewing on a different screen so it’s very hard to see what others see, and almost impossible to form an objective opinion without all seeing the exact same image on the same screen.

i use an iPad pro, and the screen is excellent, but the image above looks totally different on my PC screen which is also a top end 4k monitor…and very different again on my phone, again an Apple device…🤷🏼‍♂️

But to me number 4 takes the win..👍🏻

You are right, it depends on the screen. I work with a DELL UltraSharp U2724D in the sRGB colour space which is (according to the Wikipedia) a standard numerical encoding of colors, based on the RGB (red, green, blue) color space, for use on monitors, printers, and the World Wide Web. I believe that it meets the requirements, as no custom settings are applied, although I know that I cannot be sure without a calibration kit.

Anyway, the difference between the 3rd and 4th images is so subtle that I would close eye to it. 😉

 

[fwm891]

3 hours ago, Stuart1971 said:

Number 4 looks the best to my eye, and on my Screen, and that’s the issue really as everyone’s viewing on a different screen so it’s very hard to see what others see, and almost impossible to form an objective opinion without all seeing the exact same image on the same screen.

i use an iPad pro, and the screen is excellent, but the image above looks totally different on my PC screen which is also a top end 4k monitor…and very different again on my phone, again an Apple device…🤷🏼‍♂️

But to me number 4 takes the win..👍🏻

@Stuart1971, These images were all processed on a 4K iMac running Sequoia 15.2 if that gives you any guide.

[fwm891]

Four more images to contemplate...

[Stuart1971]

I think the 4th one again for me personally..👍🏻

[Stuart1971]

1 hour ago, fwm891 said:

Four more images to contemplate...

Have you tried MGC first and then SPCC, as that should give you the same result as your number 4 images, but with one lees step..? 

[fwm891]

@Stuart1971 No. Will try later. But I suspect that (to me) SPCC will overwrite the MGC background by applying it's background neutralisation....

[Ouroboros]

On 07/01/2025 at 17:07, Stuart1971 said:

Have you tried MGC first and then SPCC, as that should give you the same result as your number 4 images, but with one lees step..? 

I have done that and indeed found it necessary because MGC alone doesn’t remove the colour cast in an image.  

[fwm891]

I’ve tried a few combinations on different image types now and I’m coming to the view that MGC followed by SPCC yields the cleanest results in most cases.

[Stuart1971]

14 minutes ago, fwm891 said:

I’ve tried a few combinations on different image types now and I’m coming to the view that MGC followed by SPCC yields the cleanest results in most cases.

This is my conclusion too, and I think this is the way it’s supposed to be done, or at least in my mind anyway..👍🏻

[Vroobel]

I agree. If I'm not wrong, PixInsight's YT tutorials mention the same way.

[Stuart1971]

Yea I think in my mind, it’s logical to remove the background gradient, before SPCC tries to correct the colours, as it does not want any skewed colours form gradient and LP to interfere with its calculation, it needs to see a clean image of the object and stars and calculate from that for correct colours. 🤷🏼‍♂️

[Stuart1971]

Just found this on the PI forum, from one of the PI Admins, may be of interest.

“GradientCorrection can be applied anytime in the linear stage. We usually apply any gradient correction technique before the color calibration. The reason is simple: when you have gradients, you don't have a reliable background sky reference. That reference can be easily set up once the gradients are gone. But, on the contrary, in some cases, a wrong color calibration can make it difficult to assess if the gradient model is affecting the color of the extended objects. This is especially true when the unlinked RGB channels in STF give you the right contrast but the wrong color balance. I will show this in an upcoming video. 

In case you apply the color calibration before GC, just select the most plausible area for background reference. Then, after GC, use BackgroundNeutralization to perform a precise background calibration. We show it in one of the currently available videos.

As a final note, it is important to understand that GC performs a subtractive correction that does not affect star photometry. The gradients will not affect the calculated RGB ratios as well. So you can apply always SPCC before GC. It all depends on the image”.