Photoshop Luminosity Masks Part 2

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Photoshop Luminosity Masks Part 2

This week, in the last of a two part series, I’m going to cover setting up a Midtone Luminosity Mask and look at creating multiple shadow and highlight masks to target different levels of luminance for greater tonal control of your images.

I’ve deliberately chosen a dark image which could do with some tonal adjustment to open it up – if you want to work on the same image you can find it here on visualhunt.com.

curves adjustment layer in indesign

Let’s start by adding a ‘Curves’ Adjustment layer and lightening the midtones of the image – I’ve masked this so we can see the unadjusted image on the left.

As we can see, if we push the midtone adjustment too far we start to lose highlight detail and the clouds and lighter parts of the water start to bleach out.

adjusting curves in indesign

To protect our highlights and focus the adjustment on the midtone range we need to create a Midtone Luminosity mask. I’m going to delete this adjustment layer, select my Background layer, then swap over to the Channels Panel (Windows>Channels).

Firstly, we need to create our basic highlight and shadow masks following the method outlined in Part 1 of this series. Mouse over the top channel, our composite RGB. Hold down the Cmd (Mac) or Ctrl(PC) key and you’ll see the hand cursor now has a marquee indicating we can load a selection from the current channel. Click on the RGB channel and you’ll see a marquee, or marching ants, loaded over the highlight areas in our image. The quick-key for loading a selection from our composite channel is Cmd+Opt+2 for Mac / Ctrl+Alt+2 for PC.

creating a channel from a selection in photoshop

Click on the icon to save our selection to a new channel and call this ‘Highlights’. Now Duplicate this Channel and Invert it to create or ‘Shadows’ channel. If you need a recap on this process you can refer back to Part 1. You should now have 2 Alpha channels as below.

creating an alpha channel from selection in photoshop

To create our Midtone channel we need the intersection of the highlight and shadows. Load the selection from our Highlights channel and then mouse over the Shadows channel and hold Cmd+Alt+Shift (Mac) or Ctrl+Alt+Shift (PC). The hand cursor should show a marquee with an x in the middle indicating that an intersection will be created from our 2 selections. When you click on the channel you’ll see a warning dialogue. Once you click OK you won’t see a marquee but a selection has been created.

creating a mid tone channel in photoshop

Click on the icon to create a channel from our selection and call this ‘Midtones’. Depending on the image you’re working with your channel should look something like the image below – shadows and highlights will be masked and the grey area of the mask is where all the midtone data sits in the image.

shortcut for midtone channel

If you know you only need a midtone selection you can short-hand this process by skipping the creation of separate highlight and shadow channels – load the selection from the composite RGB channel, inverse it, then load the intersection by hiding down the modifier keys and clicking on the composite channel again.

Anyway, now we have our midtone alpha channel so let’s load the selection and swap back to our layer panel and create a ‘Curves’ Adjustment layer using our midtone selection. I’ll apply a similar correction to the midtones as before.

For the sake of comparison I’ve masked off the left-side to show the image unadjusted. Notice how the highlights remain intact even though I’m pushing the midtones right up into the highlight region.

retaining hilights in mid tone channel

We’ve now covered creating masks for Highlights, Midtones and Shadows which gives us global control over our 3 main tonal ranges. Now let’s look at drilling down to get more specific masks within each range.

The process is the same regardless of whether you want to work on shadows, midtones or highlights. You load the selection from your mask, then intersect it with itself to create a new mask giving you a tighter selection. With highlights this will create a mask that focuses on lighter regions, with shadows it will be darker.

Since this image needs work in the shadows I’ll use this to demonstrate the method but first I’ll delete our midtone adjustment layer. Now load the selection from the ‘Shadows’ Alpha channel, then use your modifier keys, Cmd+Alt+Shift (Mac) or Ctrl+Alt+Shift (PC), to load the intersection and click on the channel again.

Now create a new Alpha from this reduced selection and give it a name. Some people like to go for dark, darker, darkest but since you won’t know how many levels you might want to work with I think numbers make more sense. In the example below I’ve created 3 levels of Shadow mask – notice how the masks gets darker with each step as the selection reduces down to lower luminance levels.

reusing a selection in photoshop channels

Using the ‘Shadows 3’ selection I’ve applied a Curve Adjustment layer with the focus on lightening the shadow regions. In the side-by-side with the original image below notice how the detail in the sky is kept as is whilst the pebbles in the foreground are now much more open – details on the rock-face in the mid-ground are also now more evident.

convert alpha channe into mask

Once you’ve been through the process of setting up the masks a couple of times it will become second nature. The great thing about luminosity masks is that they don’t require any special skill to create as the mask is derived from the image – no need to spend hours mastering brush tools, opacity setting etc.

Load a selection from your composite channel and that’s your highlights.

Inverse the selection from your composite channel and that’s your shadows.

Intersect the highlights and shadows and that’s your midtones.

Intersect any selection with itself to create a tighter mask for smaller luminance ranges.

So there you have it. A powerful masking technique which only takes a few minutes to setup but gives absolute control over the tonal ranges in any image.