Detail of final image

Luminosity Masking

Intricate selections made automatically

A short while ago someone asked me about the luminosity masks I briefly mentioned in “Retouching Trudie Ann Pt 4.”  Luminosity masks are more difficult to explain than they are to use. This type of mask sounds like it must be part of very advanced image editing, partly because the technique is still relatively obscure, and largely because most people use software that can’t create luminosity masks. It is, however, very easy to get started using them in your photo editing workflow if you have the right software.

Luminosity masks are created directly from an image’s channels. Chances are pretty good that if you don’t have Photoshop, you can’t directly alter the channels in an image. GIMP, the free (and rather geeky) alternative to Photoshop (Win & Mac) does have channels and does allow luminosity masking. Google has plenty of tutorials for GIMP users. I’ve been told that GIMP is more user friendly than ever. Most image editors, however, don’t allow direct access to the channels.

Channels panel with one alpha channel
The Channels panel in Photoshop provides direct access to the channels themselves. Channels are greyscale versions of the image, and masks are based on grayscale. The “New Selection” channel is an alpha channel. Alpha channels are used to create and store masks. A Layer mask will also show up as an alpha channel whenever you have targeted one in the Layers panel.

To make a set of luminosity masks, you need to be able to load the luminosity of a channel and save it as an alpha channel, then add, subtract, or intersect with other channels. If you can also record Actions, as Photoshop can, you can record actions which will then repeatedly create a mask or series of masks with a simple click of a button. If you own Photoshop, you can buy a panel from a developer who has already created the actions for you and provided you with the buttons to click. I own Tony Kuyper’s TKActions panel and recently upgraded to v4, so that’s the panel I’ll show. But if you Google “luminosity masks,” you’ll find not only other panels, but also free sets of actions, complete instructions for making your own, and a lot of articles and video tutorials on using luminosity masks— YouTube is your friend for the videos.

Image of TKActions V4 panel
Panels such as TKActions by Tony Kuyper remove the chore of making all the luminosity masks and you simply push some buttons. TKActions also provides button access to many common functions (left) that help, such as show/hide the marching ants, add adjustment layers, etc. The masks themselves can be created in a variety of ways (right)— using the traditional range of Darks, Midtones, and Lights, the Zone System, individual color channels, and more.

Why are luminosity masks becoming increasingly popular with photographers? Because we often select a range of values, not just color, and the more precisely we can target them without working hard the better. Think of a stand of dark trees against a bright sky, with plenty of areas where the sky shows through the leaves and branches. Trying to select only the sky is extremely difficult, but a luminosity mask that is restricted to just the lightest lights doesn’t select too much, and is feathered according to the image’s luminance, not by an arbitrary pixel amount around the entire edge of the selection. It doesn’t take any time to make the selection, plus you’ll often get the most natural results compared to  other methods.

Detail from original photo
Detail showing the original image opened from Camera Raw before any further adjustments were made. Let’s assume the photo is going to sell the home, or is part of a special memory, and you know it wasn’t really this drab in person. You want a quick fix that looks natural, and luminosity masks will help you with that.

It’s the ability to more precisely target a range of values that makes all the actions sets and panels for luminosity masking so popular lately. The basic luminosity mask generated from a channel when Cmd/Ctrl-clicking on the thumbnail  selects a huge range of values—it divides the image according to values that are either 50% bright or 50% dark. Sometimes that’s useful, especially when tinting an image. But consider the sky example above—if there are any values in the image that are lighter than 50% gray, they’ll all be at least partially selected with that basic mask, even if the sky itself is no darker than 15% gray. Obviously, you’d rather generate a mask that selects only the lightest lights in the image. You need more luminosity masks to target narrower ranges of values, and making them without actions and/or a panel is a lot of work. Now that the actions are readily available in one form or another, photographers are jumping at exploiting their potential.Detail after adding Vibrance

Detail of Zone 3 luminosity mask
Detail showing the result of adding a Vibrance adjustment layer masked with a Zone 3 luminosity mask. White reveals— Black conceals. The lighter colors in this image are untouched, or nearly so, while the darker areas of color get a boost.

Another example of when you might want to use a luminosity mask is when you want to blend two different exposures together in a single image. Today, most people think of using HDR software, which generates the blend from either a series of bracketed images or, in some applications, a single image processed more than once. But using HDR software can be more difficult to get to look natural, usually is best only if you have bracketed your exposures in the camera, and you may find it difficult to adjust the results to your taste. Many photographers prefer, at least some of the time, to blend two  exposures manually. Luminosity masks help make that task both quick and precisely tailored to your vision of what makes the “right” exposure.

Luminosity masking, however, won’t replace all other methods for limiting an effect. Even with a targeted mask, a luminosity section can sometimes be too large. If you want to select just a red shirt or a blue car, you’re not going to choose to select all the values in the image that also show up in the shirt or car. Other selection methods are better for such simple selections. If the selection is going to be more complicated than that, using a luminosity mask for the initial selection, then deselecting part of it, or masking the results more conventionally, often works. And when you’re going to heavily manipulate an image to create photo art using textures and special effects, fine edits with luminosity masks can be overkill. However, since they’re quick to make, they’re also easy to experiment with.Detail of Photo Filter adjustment

Detail of midtones luminosity mask
Here a Photo Filter was applied through a luminosity mask to warm up the midtones and get the building closer to its native color. Later the mask was masked even further by using yet another luminosity mask to create an active selection which was then painted into.

At first you probably won’t think of using a luminosity mask instead of your usual techniques, but once you get used to how easy they are to work with, you’re pretty much guaranteed to start using them more often. Even if your image is going to be heavily manipulated, reaching for a luminosity mask to control an effect will become second nature. In the next article on luminosity masks, I’ll talk about the practical aspects of choosing the right mask for the job. And I’ll demonstrate using the TKActions panel to streamline the process, freeing you to quickly evaluate the choices you have for the effects you want to apply.

Showing Original, Layers panel, and Final image
A handful of adjustments with luminosity masks keeps the edits looking natural and takes very little time, unlike creating masks from intricate selections or tedious hand painting.
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