Understanding Luminosity Masks in Photoshop

Luminosity masks are powerful tools to successfully blend your images

There are many ways to select an area of an image. In this article, we will take a detailed look at what luminosity masks are, how to create them and how to use them effectively in Photoshop.

Luminosity masks are one of the most powerful tools when it comes to selection. If you are new to Photoshop or unfamiliar with layers and layer masks, we have a comprehensive beginner’s guide to Photoshop. This article requires an understanding of layers and layer masks, both of which are covered in the beginner’s guide.

What is a Luminosity Mask?

As the name applies, a Luminosity Mask is a selection made with respect to luminosity in an image. We can understand it better with an example:

In the above image, the foreground on the left falls into the shadows. Let us say I want to add a Curves adjustment layer to pull out more details from the shadows. In the layer mask linked to the curves, I want to mask out all the highlights, because including them will eventually blow them out. To select the darks and the shadows, I can just use the brush tool to paint over the mask with a desired opacity.

But using a brush tool has a few drawbacks. First, if we are not extremely careful while painting on the areas to show or hide, we might end up with halos around extreme contrast selections. Second, it is practically impossible to make a complex selection like the illustrative image below:

When we look at the selection above, we can understand that such complex selections are not possible with simple selection tools. The above image is the layer mask view of the image. With layer masks, we know that whites show, blacks hide, and the opacity changes with respect to the amount of grey in the selection.

You can see that the darks are the brightest with varying degrees of brightness between the darks and the shadows. You can also see that the bright clouds are almost completely masked out.

By adding a curves adjustment layer with a luminosity dark mask applied, we can easily bring out the details in the shadows without affecting the highlights. Take a look at the image below – we have seamlessly brightened the shadows. Doing so with the help of luminosity masks would take only a few minutes:

How to Create Luminosity Masks

We will start with 50% grey and create masks based on luminosity, all the way to selecting only the brightest of the brights. Generally, six different masks or channels would do the job. Similarly, we will create six different masks for darks starting from middle grey. Finally, we will create six masks for mid-tones.

Creating Bright Channels

Now let us take a step-by-step approach of creating bright masks. To make it easier to understand, I have taken a gradient image as an example, so that we can clearly understand which areas are selected and which areas are masked out.

Step 1:
Go to the Channels tab (marked red) which is placed right next to the Layers tab. You can see the default channels. The first one is the RGB Channel, which is the composite of Red, Green & Blue channels below. Make sure that the RGB Channel is selected and you will see all the four channels selected.

Step 2:
With RGB selected, left click on the RGB channel while pressing Alt+Control+Shift in PC or Option+Command+Shift in Mac. Please keep in mind that if you check any of the Red, Green or Blue channels, only that channel will be selected. Whereas when you select the RGB channel, you can observe that all four default channels are selected. When you make the selection, you can see the marching ants appear around the selected area. In the example, you can see the selection of brights starting approximately from 50% grey.

Step 3:
Now with the selection made, click on the Save selection as channel Icon (marked red in the illustrative image below). Now you can see a new channel called Alpha1 (marked green). Rename it as Brights1 for Convenience.

Step 4:
Now, select the Brights1 channel. Make sure the selection of Brights1 is still highlighted by the marching ants. Left-click on Brights1 while pressing the Alt+Control+Shift/Option+Command+Shift as we did in Step 2. You will see that the selection changes. In this example, more mid-tones are ignored in the selection. Now, we save this selection as a Channel. Since we have renamed the previous channel as Brights1, this new channel will be named Alpha 1 by default. If we don’t rename, every consecutive channel will be named as Alpha 1, Alpha 2, Alpha 3 and so on. Rename the new Alpha 1 as Brights 2.

Step 5:
Let’s repeat the same. Now by selecting Brights2, hit Alt+Control+Shift+Click, save this selection as a channel and rename it as Brights3. A selection with an area smaller than the previous channel can be seen.

Step 6:
Repeat the above procedure until we get to Brights6. If we look at the selected areas starting from Brights6 to Brights1, we can see that we have made progressive selection from extreme brights to almost middle grey progressively.

Now that we are done with the brights, let us get to creating the dark channels. Dark channels are basically the opposite of brights. There are two ways in which we can do it. We can just inverse the selections of the respective brights. Invert the selection Brights1 as Darks1, Invert the selection Brights2 as Darks2 and so on. Another way of doing it is by reversing Brights1 and then progressively refining it all the way to the darkest of the darks darks as we did with the bright channels. Below is a step-by-step procedure to select the darks.

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