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Getting Around in GIMP - Luminosity Masks Revisited

Brorfelde landscape by Stig Nygaard (cb) After adding an aggressive curve along with a mid-tone luminosity mask.

I had previously written about adapting Tony Kuyper’s Luminosity Masks for GIMP. I won’t re-hash all of the details and theory here (just head back over to that post and brush up on them there), but rather I’d like to re-visit them using channels. Specifically to have another look at using the mid-tones mask to give a little pop to images.

Luminosity Masking in darktable:
PIXLS.US - Luminosity Masking in darktable

Let’s Build Some Luminosity Masks!

The way I had approached building the luminosity masks previously were to create them as a function of layer blending modes. In this re-visit, I’d like to build them from selection sets in the Channels tab of GIMP.

For the Impatient:
I’ve also written a Script-Fu that automates the creation of these channels mimicking the steps below.

Once installed, you’ll find it under:
Filters → Generic → Luminosity Masks (patdavid)

[Update]:
Yet another reason to love open-source - Saul Goode over at this post on GimpChat updated my script to run faster and cleaner. You can get a copy of his version at the same Registry link above. (Saul’s a bit of a Script-Fu guru, so it’s always worth seeing what he’s up to!)

Pat David Luminosity Mask GIMP Tutorial
Mountains as Far as the Eye Can See by Western Arctic National Parklands on Flickr (cb)

We’ll start off in a similar way as we did previously.

Duplicate your base image

Either through the menus, or by Right-Clicking on the layer in the Layer Dialog:

Layer → Duplicate Layer

Pat David GIMP Luminosity Mask Tutorial Duplicate Layer

Desaturate the Duplicated Layer

Now desaturate the duplicated layer. I use Luminosity to desaturate:

Colors → Desaturate…

Pat David GIMP Luminosity Mask Tutorial Desaturate Layer

This desaturated copy of your color image represents the “Lights” channel. What we want to do is to create a new channel based on this layer.

Create a New Channel “Lights”

The easiest way to do this is to go to your Channels Dialog.

If you don’t see it, you can open it by going to:

Windows → Dockable Dialogs → Channels

Pat David GIMP Luminosity Mask Tutorial Channels Dialog
The Channels dialog

On the top half of this window you’ll see the an entry for each channel in your image (Red, Green, Blue, and Alpha). On the bottom will be a list of any channels you have previously defined.

To create a new channel that will become your “Lights” channel, drag any one of the RGB channels down to the lower window (it doesn’t matter which - they all have the same data due to the desaturation operation).

Now rename this channel to something meaningful (like “L“ for instance!), by double-clicking on its name (in my case it’s called “Blue Channel Copy”) and entering a new one.

This now gives us our “Lights“ channel, L :

Pat David GIMP Luminosity Mask Tutorial L Channel

Now that we have the “Lights” channel created, we can use it to create its inverse, the “Darks” channel…

Create a New Channel “Darks”

To create the “Darks” channel, it helps to realize that it should be the inverse of the “Lights” channel. We can get this selection through a few simple operations.

We are going to basically select the entire image, then subtract the “Lights” channel from it. What is left should be our new “Darks” channel.

Select the Entire Image

First, have the entire image selected:

Select → All

Remember, you should be seeing the “marching ants” around your selection - in this case the entire image.

Subtract the “Lights” Channel

With the entire image selected, now we just have to subtract the “Lights” channel. In the Channels dialog, just Right-Click on the “Lights” channel, and choose “Subtract from Selection”:

Pat David GIMP Luminosity Mask Tutorial L Channel Subtract

You’ll now see a new selection on your image. This selection represents the inverse of the “Lights” channel…

Create a New “Darks” Channel from the Selection

Now we just need to save the current selection to a new channel (which we’ll call… Darks!). To save the current selection to a channel, we can just use:

Select → Save to Channel

This will create a new channel in the Channel dialog (probably named “Selection Mask copy”). To give it a better name, just Double-Click on the name to rename it. Let’s choose something exciting, like “D“!

More Darker!

At this point, you’ll have a “Lights” and a “Darks” channel. If you wanted to create some channels that target darker and darker regions of the image, you can subtract the “Lights” channel again (this time from the current selection, “Darks”, as opposed to the entire image).

Once you’ve subtracted the “Lights” channel again, don’t forget to save the selection to a new channel (and name it appropriately - I like to name subsequent masks things like, “DD”, in this case - if I subtracted again, I’d call the next one “DDD” and so on…).

I’ll usually make 3 levels of “Darks” channels, D, DD, and DDD:

Pat David GIMP Luminosity Mask Tutorial Darks Channels
Three levels of Dark masks created.

Here’s what the final three different channels of darks looks like:

Pat David GIMP Luminosity Mask Tutorial All Darks Channels
The D, DD, and DDD channels

Lighter Lights

At this point we have one “Lights” channel, and three “Darks” channels. Now we can go ahead and create two more “Lights” channels, to target lighter and lighter tones.

The process is identical to creating the darker channels, just in reverse.

Lights Channel to Selection

To get started, activate the “Lights” channel as a selection:

Pat David GIMP Luminosity Mask Tutorial L Channel Activate

With the “Lights” channel as a selection, now all we have to do is Subtract the “Darks” channel from it. Then save that selection as a new channel (which will become our “LL“ channel, and so on…

Pat David GIMP Luminosity Mask Tutorial Subtract D Channel
Subtracting the D channel from the L selection

To get an even lighter channel, you can subtract D one more time from the selection so far as well.

Here are what the three channels look like, starting with L up to LLL:

Pat David GIMP Luminosity Mask Tutorial All Lights Channels
The L, LL, and LLL channels

Mid Tones Channels

By this point, we’ve got 6 new channels now, three each for light and dark tones:

Pat David GIMP Luminosity Mask Tutorial L+D Channels

Now we can generate our mid-tone channels from these.

The concept of generating the mid-tones is relatively simple - we’re just going to intersect dark and light channels to produce whats left - midtones.

Intersecting Channels for Midtones

To get started, first select the “L“ channel, and set it to the current selection (just like above). Right-Click → Channel to Selection.

Then, Right-Click on the “D“ channel, and choose “Intersect with Selection”.

You likely won’t see any selection active on your image, but it’s there, I promise. Now as before, just save the selection to a channel:

Select → Save to Channel

Give it a neat name. Sayyy, “M“? :)

You can repeat for each of the other levels, creating an MM and MMM if you’d like.

Now remember, the mid-tones channels are intended to isolate mid values as a mask, so they can look a little strange at first glance. Here’s what the basic mid-tones mask looks like:

Pat David GIMP Luminosity Mask Tutorial Mid Channel
Basic Mid-tones channel

Remember, black tones in this mask represent full transparency to the layer below, while white represents full opacity, from the associated layer.

Using the Masks

The basic idea behind creating these channels is that you can now mask particular tonal ranges in your images, and the mask will be self-feathering (due to how we created them). So we can now isolate specific tones in the image for manipulation.

Previously, I had shown how this could be used to do some simple split-toning of an image. In that case I worked on a B&W image, and tinted it. Here I’ll do the same with our image we’ve been working on so far…

Split Toning

Using the image I’ve been working through so far, we have the base layer to start with:

Pat David GIMP Luminosity Mask Tutorial Split Tone Base

Create Duplicates

We are going to want two duplicates of this base layer. One to tone the lighter values, and another to tone the darker ones. We’ll start by considering the dark tones first. Duplicate the base layer:

Layer → Duplicate Layer

Then rename the copy something descriptive. In my example, I’ll call this layer “Dark” (original, I know):

Pat David GIMP Luminosity Mask Tutorial Split Tone Darks

Add a Mask

Now we can add a layer mask to this layer. You can either Right-Click the layer, and choose “Add Layer Mask”, or you can go through the menus:

Layer → Mask → Add Layer Mask

You’ll then be presented with options about how to initialize the mask. You’ll want to Initialize Layer Mask to: “Channel”, then choose one of your luminosity masks from the drop-down. In my case, I’ll use the DD mask we previously made:

Pat David GIMP Luminosity Mask Tutorial Add Layer Mask Split Tone

Adjust the Layer

Pat David GIMP Luminosity Mask Tutorial Split Tone Activate DD Mask

Now you’ll have a Dark layer with a DD mask that will restrict any modification you do to this layer to only apply to the darker tones.

Make sure you select the layer, and not it’s mask, by clicking on it (you’ll see a white outline around the active layer). Otherwise any operations you do may accidentally get applied to the mask, and not the layer.

At this point, we now want to modify the colors of this layer in some way. There are literally endless ways to approach this, bounded only by your creativity and imagination. For this example, we are going to tone the image with a cool teal/blue color (just like before), which combined with the DD layer mask, will restrict it to modifying only the darker tones.

So I’ll use the Colorize option to tone the entire layer a new color:

Colors → Colorize

To get a Teal-ish color, I’ll pull the Hue slider over to about 200:

Pat David GIMP Luminosity Mask Tutorial Split Tone Colorize

Now, pay attention to what’s happening on your image canvas at this point. Drag the Hue slider around and see how it changes the colors in your image. Especially note that the color shifts will be restricted to the darker tones thanks to the DD mask being used!

To illustrate, mouseover the different hue values in the caption of the image below to change the Hue, and see how it effects the image with the DD mask active:

Mouseover to change Hue to: 0 - 90 - 180 - 270

So after I choose a new Hue of 200 for my layer, I should be seeing this:

Pat David GIMP Luminosity Mask Tutorial Split Tone Dark Tinted

Repeat for Light Tones

Now just repeat the above steps, but this time for the light tones. So duplicate the base layer again, and add a layer mask, but this time try using the LL channel as a mask.

For the lighter tones, I chose a Hue of around 25 instead (more orange-ish than blue):

Pat David GIMP Luminosity Mask Tutorial Split Tone Light Tinted

In the end, here are the results that I achieved:

Pat David GIMP Luminosity Mask Tutorial Split Tone Result
After a quick split-tone (mouseover to compare to original)

The real power here comes from experimentation. I encourage you to try using a different mask to restrict the changes to different areas (try the LLL for instance). You can also adjust the opacity of the layers now to modify how strongly the color tones will effect those areas as well. Play!

Mid-Tones Masks

The mid-tone masks were very interesting to me. In Tony’s original article, he mentioned how much he loved using them to provide a nice boost to contrast and saturation in the image. Well, he’s right. It certainly does do that! (He also feels that it’s similar to shooting the image on Velvia).

Pat David GIMP Luminosity Mask Tutorial Mid Tones Mask

Let’s have a look.

I’ve deleted the layers from my split-toning exercise above, and am back to just the base image layer again.

To try out the mid-tones mask, we only need to duplicate the base layer, and apply a layer mask to it.

This time I’ll choose the basic mid-tones mask M.

What’s interesting about using this mask is that you can use pretty aggressive curve modifications to it, and still keep the image from blowing up. We are only targeting the mid-tones.

To illustrate, I’m going to apply a fairly aggressive compression to the curves by using Adjust Color Curves:

Colors → Curves

When I say aggressive, here is what I’m referring to:

Pat David GIMP Luminosity Mask Tutorial Aggresive Curve Mid Tone Mask

Here is the effect it has on the image when using the M mid-tones mask:

Aggressive curve with Mid-Tone layer mask (mouseover to compare to original)

As you can see, there is an increase in contrast across the image, as well a nice little boost to saturation. You don’t need to worry about blowing out highlights or losing shadow detail, because the mask will not allow you to modify those values.

More Samples of the Mid-Tone Mask in Use

Pat David GIMP Luminosity Mask Tutorial

Pat David GIMP Luminosity Mask Tutorial
The lede image again, with another aggressive curve applied to a mid-tone masked layer (mouseover to compare to original)

Pat David GIMP Luminosity Mask Tutorial
Red Tailed Black Cockatoo at f/4 by Debi Dalio on Flickr (used with permission) (mouseover to compare to original)

Pat David GIMP Luminosity Mask Tutorial
Landscape Ballon by Lennart Tange on Flickr (cb) (mouseover to compare to original)

Pat David GIMP Luminosity Mask Tutorial
Landscapes by Tom Hannigan on Flickr (cb) (mouseover to compare to original)

Pat David GIMP Luminosity Mask Tutorial
Stitched photos of the Idaho landscape near Craters of the Moon National Monument by Frank Kovalchek on Flickr (cb) (mouseover to compare to original)

Mixing Films

This is something that I’ve found myself doing quite often. It’s a very powerful method for combining color toning that you may like from different film emulations. Consider what we just walked through.

These masks allow you to target modifications of layers to specific tones of an image. So if you like the saturation of, say, Fuji Velvia in the shadows, but like the upper tones to look similar to Polaroid Polachrome, then these luminosity masks are just what you’re looking for!

Just a little food for experimentation thought… :)

Stay tuned later in the week where I’ll investigate this idea in a little more depth.

In Conclusion

This is just another tool in our mental toolbox of image manipulation, but it’s a very powerful tool indeed. When considering your images, you can now look at them as a function of luminosity - with a neat and powerful way to isolate and target specific tones for modification.

As always, I encourage you to experiment and play. I’m willing to bet this method finds it’s way into at least a few peoples workflows in some fashion.