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Questions about star mask & star colors

Posted: Mon Dec 03, 2012 4:28 am
by mcbbcn
Hi Ivo,

In the same way, you have an approach to get rid of all of the stars in a picture (Heal), would there be a way to do the opposite? Create an image that is stars only, then to increase saturation, fix shape & fix halos, and then to lay it over the final image? It got me thinking that increasing the saturation to get the color of the stars has a significant impact to the rest of the image but if there was a way to isolate the stars, it wouldn't affect the whole image. Now, the tricky part would be to figure out a way to blend them (layers?) correctly.

It may be a dumb idea, but I thought it was worth sharing it.

Cheers,

M.

Re: Questions about star mask & star colors

Posted: Mon Dec 03, 2012 7:05 am
by admin
mcbbcn wrote:Hi Ivo,

In the same way, you have an approach to get rid of all of the stars in a picture (Heal), would there be a way to do the opposite? Create an image that is stars only, then to increase saturation, fix shape & fix halos, and then to lay it over the final image? It got me thinking that increasing the saturation to get the color of the stars has a significant impact to the rest of the image but if there was a way to isolate the stars, it wouldn't affect the whole image. Now, the tricky part would be to figure out a way to blend them (layers?) correctly.

It may be a dumb idea, but I thought it was worth sharing it.

Cheers,

M.
Not a dumb idea at all! What you could do is heal the image so you have an image without stars, then 'subtract' the new image from the original image in the Layers module. This way you'll be left with the star component.
Be sure to save the new (healed) image. Then modify the star component to your tatse. When you're done, layer it back on top of the healed image using 'addition'.

Re: Questions about star mask & star colors

Posted: Mon Dec 03, 2012 5:55 pm
by mcbbcn
Awesome! I'll give it a try! Cheers, M.

Re: Questions about star mask & star colors

Posted: Mon Dec 03, 2012 10:48 pm
by admin
One thing; make sure you set 'New darker than old' in the Heal module. This way the Heal module will only generate background pixels to heal the stars with that are actually darker than the star itself. It'll get you better results.

Re: Questions about star mask & star colors

Posted: Tue Dec 04, 2012 5:28 am
by mcbbcn
OK, so I guess what you mean by 'heal' is remove the stars. So, I'm trying this option is some kind of aggressiveness level about how many pixels to remove, do you have a specific recommended value? Thanks for all your help! M.

PS: By the way, when I press 'help' on this option, I don't get a help message...

Re: Questions about star mask & star colors

Posted: Tue Dec 04, 2012 6:19 am
by mcbbcn
I just did my first star layer extraction, and addition to my final image. It was quick and rough, but I could the results. My star cores got a bit bigger, and I tried to the 'Magic' but it was not shrinking them enough...Anyway, my original stars were not great to star with, but I was able to complete the process just fine...I spent a bit more time getting those stars at the right time during the processing, I should be able to layer them just fine...

This is getting quite additive. I never had so much fun processing! :D

Cheers,

M.

Re: Questions about star mask & star colors

Posted: Tue Dec 04, 2012 10:23 pm
by admin
This is getting quite additive. I never had so much fun processing! :D
That's awesome to hear Miquel - little comments like these really make my day! :D

Re: Questions about star mask & star colors

Posted: Thu Dec 06, 2012 1:06 am
by mcbbcn
I'm glad to hear...I've been having so much joy that I had to share it...

By the way, I've been meaning to ask you a question about selecting stars, and using the heal and subtract to isolate the stars into a layer. You mentioned something about the setting "New must be darker than", and I've been experimenting with it, but I can't tell very well what it does, and the question mark (help) button does not seem to work. However my challenge is this...I don't seem to be able to extract the stars with all of its strength and luminosity & I think the "New must be darker than" must have something to do it...not sure...but how should I "Heal" to fully isolate the stars, so I can get bright strong stars in a separate layer after I do the subtraction?

Thanks for your help,

M.

Re: Questions about star mask & star colors

Posted: Thu Dec 06, 2012 3:32 am
by admin
mcbbcn wrote:I've been meaning to ask you a question about selecting stars, and using the heal and subtract to isolate the stars into a layer. You mentioned something about the setting "New must be darker than", and I've been experimenting with it, but I can't tell very well what it does, and the question mark (help) button does not seem to work.
"New must be darker than old" instructs the healing algorithm to conjure up pixels that are darker than the original (but still look plausible). This way, if you subtract the healed version from the original, you will always end up with a positive difference.

The similarly named "New must be darker than" parameter instructs the healing algorithm to conjure up pixels that are darker than a specific luminance value (100% = pure white, 0% = pure black). This way, the healing algorithm can be restrained to not generate new stars (which would be brighter than a specific value). This parameter is extremely useful when healing out dust donuts.
However my challenge is this...I don't seem to be able to extract the stars with all of its strength and luminosity & I think the "New must be darker than" must have something to do it...not sure...but how should I "Heal" to fully isolate the stars, so I can get bright strong stars in a separate layer after I do the subtraction?
What you end up with by subtracting the 'healed' luminance from the original 'luminance', is the brightness difference that the star makes compared to the background. So what you get is the difference the star makes compared to the background, but not the star plus the background. Why is this useful? You have now completely isolated the star component from the background and you can manipulate either (or both) it at will. If you still had the background component in there as well, you would be manipulating the background component along with the star, which would look weird once you would add back your modified star on top of your background.

Take this crop from your image for example;
Miquel_Heart_PreLife_Post2_crop.jpg
Miquel_Heart_PreLife_Post2_crop.jpg (58.82 KiB) Viewed 6028 times
I removed the stars, did some large scale Wavlet Sharpening and added the stars back in;
Miquel_Heart_PreLife_Post2_crop_bg_modified.jpg
Miquel_Heart_PreLife_Post2_crop_bg_modified.jpg (61.95 KiB) Viewed 6028 times
Neat huh? Note I left the Cap Mode at 'Clip' - this blows out the stars a little bit in areas that have had their background brightened. Set it to 'Normalize' if you don't want that.

Re: Questions about star mask & star colors

Posted: Thu Dec 06, 2012 6:05 am
by mcbbcn
Very cool!!! Thanks for another great tip!