LAYERSI think Carlos put it nicely when he said "PI's masking system is fully capable of doing that, but perhaps not as "user friendly" or fast."
I agree 100% with that. Also, if I have to choose between a tool that accomplishes X that takes me certain effort and the funometer reads 10, and a different tool that also accomplishes X but with less effort and for which the funometer also reads 10 or maybe more, and it also doesn't degrade my learning experience, why should I use the tool that's harder to use?
Hopefully my message can be read between the lines :-)
IMHO PS's layering system sucks BTW. And it does because AFAIK, you're stuck with 8-10 "blending modes" and their "opacity" (plus a bunch of effects that we don't use in astro images like glows, bevels, shadows, etc). I doubt PI's layering approach would be so limited, whenever it happens...
MESSING UP WITH MASKSI've done all sort of crazy things to my images in my "olden" days. Yet, they ended up looking like crap, most of them at least

Now I don't nearly mess up with them at all, and surprisingly, they look a lot better.
But... but!!! I'm going to tell you why in my last image for example I PAINTED over a mask and why I don't have any regrets at all... (doesn't mean you'll agree with me, which BTW is fine with me of course

)
Here's a tiny version of a crop of the image (
full version here):

Now.. notice the area (top-right) above those two large arches... You can see it's an area with a darker background...
Originally, after I was done with most of my processing, that area was flat dark, in other words, now you can see that despite being darker than the rest, there's some "smoke" in there... (if you stretch the image wildly or see it against a black screen background you'll notice better).
So how did I get all that faint "smoke" to actually be at least somewhat visible? (barely, but visible)...
Quick wrap-up: broke the image into small and large scale structures, applied a histogram adjustment to the large scales image, and added the images back together with a mask... (it's a bit more complex than that, but that's a +/- fair wrap-up). Sounds ok... Now.. Here's how I built the mask:
1.- Extracted luminance
2.- Extracted large scale stuff using ATWT to "blur" it.
3.- Did a histogram adjustment.
4.- Binarized the image to a point where ONLY that area AND a few small black "islands" in other parts of the image were left black - everything else was white.
5.- Blurred the mask with ACDNR
6.- I PAINTED white those smaller black islands.
Juan appeals to the documentary value of an image... Well, here's the thing the way I see it...
After the very reproducible "data based" processes steps 1-5 this is what I ended up with:

You can see the area above the arches nicely unprotected, and you can also see the other "black islands"...
Now, these 1-5 steps may be 100% reproducible and "data based", but just because something is "data based" doesn't make it right. You can do all sort of crazy things with luminance masks!! I personally think I just went overboard with this mask, but hey, it's not painted, so I feel safe

That is not my point though...
Now, let's go back a bit... I am about to apply a histogram stretch with a mask!!! Ok? That's not in my book of supercool things to do. In fact, I assure you that except for protecting the galaxies, no selective stretch has been done on the original image at all during the processing until now....
This means I have already made the decision that I want to brighten some areas in the image while not doing so in others (not only that, I'm going to be stretching only very large scale structures, so I'm even more selective). But you can ignore that if that already bothers you, because that's not my point either.
Now, by examining in the original image the areas covered by these "black islands" I immediately notice that if I leave those areas unprotected, they are going to suffer. I'll explain... Before this processing, they were darker than their surrounding areas, and show a nice contrast, which I believe is of "documentary value". If I apply this process without protecting those areas as well, I believe I will be degrading the image both, from an artistic and a documentary perspective, because the faint signal in those areas is stronger than the faint signal above the arches, and if left unprotected, the stretch will basically make those areas to blend with their surroundings, as if there was not a decrease in IFN illumination in those areas - which there is. I know... because I tried it.
So I have a decision to make... Either
1) I don't apply this process at all, in which case the faint "smoke" above the arches will remain unseen. I really want those faint details to be a bit visible, and I think that by doing so, the image will have MORE documentary value. It is after all the goal I have with this processing session.
2) I apply it with this mask that has been created "only from the data" but knowing it will degrade areas of the image (the areas under the "black islands") that IMHO have more value, again, from both an artistic and a documentary perspective, if left untouched.
3) I pull the superduper clone stamp and KILL those "black islands" so those areas are not affected from what I'm about to do, and have a crowd of "purists" lynch me in a big way.
Since the goal of this particular processing session was to extract very faint details from where originally we couldn't see any, and if we leave the mask as it is, IMHO the stretch done on those "black island" areas will NOT produce this result, and instead, this processing step will in fact "flatten" those areas (a case might be made about leaving unprotected the "hole" under the left arch, maybe), it made sense FOR ME - both from an artistic and documentary perspective - to make sure these areas are not affected by this processing session.
So I PAINTED the mask by reducing those "black islands" to pure white and leave unprotected only the area above the arches. And again, IMHO I believe I didn't degrade the documentary value of the image any less than anything done prior to that with a plethora of "data based" masks.
And BTW, I can 100% reproduce this "effect" anytime, though I already knew that 100% replication doesn't guarantee anything.
And of course, I feel right about it because in my very humble opinion, I made the right decision.
And yes, I tried several things prior to pulling the clone stamp... Who knows? It might have been possible to do it without the clone stamp. And I had FUN trying... And I failed. And so I went back to that 3 points decision, and made the decision that I felt was the best. In the end, the improvement is so subtle, I would probably find pointless an argument about my decision.
But if still, anyone would claim that "assuming your image was of documentary value prior to this step, I believe that after doing this it has crossed the threshold of acceptability" my answer would be:
Want to do science with this? Here's my raw data.
Want to give me some feedback? Please do so.
Want to enjoy my work and my interpretation? Look at it.
Want to tell me this image no longer is acceptable AP? Remind me not to discuss this topic with you again

And they lived happily ever after, the end 8)
EDIT: BTW I forgot to mention... In the end, I toned down the effect of this modification A LOT, so in my final published image, the "stuff" above those arches is much less noticeable than in the screen shot above. Funny things the mind does sometimes :-)
EDIT 2: And the screenshots above are focused only on that area. To see my final published image
click here...
EDIT 3: And the screenshot of the mask (which covers the full image) does NOT match the first screen shot - which is a crop. Which sucks because then you cannot see well the correlation between the "black islands" and the area they cover in the real image, but hopefully you'll figure it out by using the link I gave in EDIT 2. SORRY!!!