Creating luminosity masks by intersection

[Bill Martz]

I have read the Tony Kuyper article, "Different Masks for Different Tones," at http://goodlight.us/writing/luminosi...tymasks-5.html and don't understand how he creates his additional luminosity masks "by progressively intersecting selections of the same mask. Intersection is when Photoshop creates a new selection by finding the pixels that two selections have in common."

It seems to me that if you intersect a selection with itself, you get the original selection. I am a GIMP user, and I have tried it, and I get the same selection. How is it different with Photoshop?

[Manfred M]

Sorry - I have very limited experience in GIMP. Does GIMP have the same channels functionality as Photoshop as one keeps building up the Alpha Channel to build up the required mask? I haven't used that method in years, but the link you show seems to use that traditional method.

There are better ways to build Luminosity Masks in Photoshop. The Select Colour Range functionality makes it very easy to create luminosity masks for highlights, mid-tones and shadows.

[Bill Martz]

Thanks for replying, Manfred. There's no date on the Tony Kuyper article, so I guess it is very old and Photoshop now can do what he was describing through new features. I was really curious about the use of intersection. Both Photoshop and GIMP describe it the same way, as the creation of a selection of the pixels in common. I don't see how intersecting a selection with itself does anything other than give you the original selection.

[Manfred M]

Bill - I don't remember the specifics of this approach; too many years (probably 2009 or 2010) when I studied this formally. I vaguely remember doing some work using the alpha channel and repeating this a number of times to build up the intensitiy of the mask. There were all kinds of strange keystrokes involved and unless GIMP and Photoshop are set up exactly the same way, I can see how the results from the two different piece of software could give completely different results. I know some people love using the luminosity mask, but I rarely use them.

Photoshop has a number of different intersection operations, all of which have specific mathematical relationships applied to the two layers being integrated. Some remove pixels, others ignore pixels above or below a certain threshold, others add them while others do other strange and wonderful things (i.e. blending modes).

Sorry I can't be of more help. I suggest reaching out to the GIMP community. There are some GIMP users on this site.

[DanK]

There are better ways to build Luminosity Masks in Photoshop. The Select Colour Range functionality makes it very easy to create luminosity masks for highlights, mid-tones and shadows.

Thanks for pointing this out. I use select_color_range frequently to select by color, but I have never played with its ability to select highlights, midtones, and shadows. It's very flexible because of the range control. Some seople use luminosity masks to select more than three areas, but I never have, and this is far, far simpler.

[Bill Martz]

That helps a lot, Manfred. I think that explains why, even though the term intersect is used, it is not just the common pixels. There's more to the meaning, and that is why repeated intersections of the same selection produces new selections and not just the same one.

[wannoo]

I think you skipped an important step.

Once you ctrl-click the RGB channel you've selected the highlights based on that channel.
Now you have to make an alpha channel (click the icon to the right of the selection icon in PS).
You'll see the alpha channel with it's marching ants.
Intersecting means that you now want to select only the lights from that alpha channel (and not the RGB channel again 'cause that will make no difference since it will reselect the lights of that channel again).

So click on that alpha channel. Now when you ctrl-click the thumb on that alpha channel you'll see the marching ants selection shrink.
What have you done: first step you created a lights selection (RGB channel) - then you created a lights selection (alpha channel) of those lights = a lights-lights selection.
Repeating those steps (intersecting) will refine your highlights selection to even narrower lights (bright highlights).

So make sure you select the alpha channel first.

[Bill Martz]

Thanks, wannoo. Not being a PS user, I didn't realize the significance of that step. Is there some relationship so you know, or can control, how much difference there is between lights and lights-lights?

[wannoo]

There is a relationship since all actions in PS (and other software) depend on calculations/algoritms that will determine which pixelvalues will be affected and by how much.
The difference between lights1 and lights2 (lights-lights) also depends on the tonality of the photo itself. Typically repeating the steps 5 times (lights6) will give an almost completely black mask except the brightest highlights (sun etc...).

You can't control the step from lights1 to lights 2. However you can tweak any alpha channel to margins too small to notice with your eyes.
To do so you'll be diving in the wonderfull but daunting world of adding and substracting alpha channels (ex. darks2 and lights2) and also the aplly image, calculations and blend-if commands.
Some are straightforward while others will test your cognitive ability.

I've been exploring them for some years now and still "discover" new moves/techniques on a regular basis.