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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!

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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 8, 2011, 08:18
Post: #5
RE: Luminosity Masks in GIMP
The technique is interesting, but his method to get the mask is complicated and your is even more so... Why don't you work directly on the channels?

Here is a quick write-up:

1) duplicate the image layer
2) desaturate it (use the desat method you want here, possibly one o fthe options of Colors/Desaturate)
3) Go to the Channels dialog
4) Grab one of the colors channels and copy it below, call it "Luminosity"
5) To make a luminosity mask, duplicate the channel, give it a name, and select it... the secret here is that the Color tools now apply to this channel, not to the image.
6) Open the Curves tool, the "curve" can be seen as a plot of the selection(Y axis) vs lightness (X-axis).
7) To select midtones, just make a curve that is low at both ends of the graph, and high in the middle. The slope of the step is the feathering of the selection, but you don't need much (with this method you can do much finer luminosity-based selection)
8) Click OK and leave the Curves tool, and use Channel-to-selection

With this method, you can always have pixels that are more that 50% selected, and avoid the problem of the invisible selection he alludes to on page 5 Smile

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Oct 11, 2011, 18:37 (This post was last modified: Oct 11, 2011 18:40 by patdavid.)
Post: #6
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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Oct 11, 2011, 20:01
Post: #7
RE: Luminosity Masks in GIMP
(Oct 11, 2011 18:37)patdavid Wrote:  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...
When I played with curves on the channels I got once burned because I still had a selection... so the selection is effective on channels when you edit them... but if you want the intersection of two channels inside the current selection, you are just intersecting three channels, one of them being the selection, so intersecting both channels one after another with the selection should do it. And f tyoiu are using channels yu don't even need that... you'll have your various luminosity channels, and if you want to play with one within the existing selection, you just intersect it with the selection to create the operational selection.

[Image: 2Tvo0.png]

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