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Luminosity Masks in GIMP

Oct 7, 2011, 19:41
Post: #1
Luminosity Masks in GIMP
I saw on the GIMP Flickr group a discussion thread a while back about how to utilize luminosity masks as described by Tony Kuyper in GIMP.

I personally use them quite a bit, and figured I would finally get off my butt and post a tutorial covering the basics of creating and using luminosity masks (I use a split-tone tutorial as an example of how they work to help those who may be new to these types of masks).

So, the tutorial is here:

patdavid.net: Getting Around in GIMP - Luminosity Masks

And, the rest of my tutorials can be found here:

patdavid.net: Getting Around in GIMP

I would love any feedback if there are any others who use these types of masks in their normal workflow (or if you just want to provide feedback in general)!
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Oct 7, 2011, 20:09
Post: #2
RE: Luminosity Masks in GIMP
Thanks, I will definitely give you some feedback soon. Looks - at first glance - very complete!

[Image: 6216880253_3c00e6165e.jpg]

Love your Gimp!
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Oct 7, 2011, 20:53
Post: #3
RE: Luminosity Masks in GIMP
Is it possible to use this method to selectively control levels or curves? If I only wanted to adjust the highlights of an image could I do it using Layer Masks or Luminosity Masks? I posted this question on the tutorial, but I thought I'd post it here in case anyone else had some cool ideas. Thanks.

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Oct 8, 2011, 03:26
Post: #4
RE: Luminosity Masks in GIMP
(Oct 7, 2011 20:53)austin Wrote:  Is it possible to use this method to selectively control levels or curves? If I only wanted to adjust the highlights of an image could I do it using Layer Masks or Luminosity Masks? I posted this question on the tutorial, but I thought I'd post it here in case anyone else had some cool ideas. Thanks.

I answered you on the tut, but I'll post here as well for completeness.

Yes you can use this to selectively control level/curves on portions of your image. In fact, this is really one of the main points of usefulness of these masks. As mentioned in Tony's original tutorial, you can try making adjustments to the Mid tones mask with heavy s-curves and really make the contrast and colors pop.

Some things I have been trying lately that look interesting are to run an unsharp mask on a layer, and filter it with the darks luminosity mask (thereby only applying the unsharp to the dark tones).
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Oct 11, 2011, 18:37 (This post was last modified: Oct 11, 2011 18:40 by patdavid.)
Post: #5
RE: Luminosity Masks in GIMP
It's usually quicker for me to just quickly dup and desat a layer, and adjust as necessary there on each of my adjustment layers, though your way is essentially just another path to the same solution.

If the adjustments are made to narrow the tonal ranges selected on the mask as applied, you'll get instant feedback on the adjustments in your image stack, which is nice.

Also, using curves to isolate mid-tones is a neat idea, but I can't help but notice that the mid-tone curves will not be a function of the light and dark masks as I described (as opposed to generating the mid-tones mask from the difference in the light and dark masks). You'll have a heavier shift in tonal isolation towards mid-tones due to the smoothing of the curves tool when pulling up the middle (unless you freehand the result - and can expect banding issues from that).

The steps are not too bad compared to what you suggest in terms of complexity:

1) duplicate the image layer
2) desaturate - Light Mask
3) duplicate desaturated layer
4) invert color - Dark Mask
5) set layer mode to difference
6) create new from visible
7) invert colors on new visible layer - Mid Mask

That's it. Copy/paste as needed into layer masks to use!

I would have been interested in following his procedure exactly, but it appears that channel intersections work across the entire image domain, and I think PS will intersect based on selected region - that's why he can just subtract Light and Dark from the entire image to obtain his Mids. For the life of me I couldn't get GIMP channels to reproduce those steps...
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