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Another Photoshop Masking Technique: Luminosity Masking

Last week, we looked at a Photoshop equivalent of the traditional darkroom technique known as contrast masking. This week, we'll look at a similar Photoshop masking technique known as luminosity masking.

First off, unlike last week's tip, this one is Photoshop only. No Photoshop Elements I'm afraid. The reason why, is that luminosity masking creates an actual layer mask.

A basic layer mask comes with every adjustment layer you create. Click on the "new adjustment layer" icon (the one that looks like a half-black, half-white circle) at the bottom of the layers palette or use the Layer >> New Adjustment Layer menu equivalent, select a layer type, and you'll get a new adjustment layer. The layers palette entry for it will have two icons, one for the adjustment type selected (levels, curves, or what have you), and one to its right for the associated layer mask. To begin with, that layer mask will be a blank white square which means that the levels or curves will affect the entire image underneath. But if you click with your mouse on the layer mask icon and paint with black or shades of gray you can selectively mask that effect from part of the image. The effect is applied proportionally to the brightness of each point in the layer mask. Paint with pure black and the effect will be blocked completely at that point, or paint with pure white and the effect to show in full. Anywhere in between black and white will allow the effect to show accordingly. It's entirely up to you.

For example, if we create a levels adjustment layer to brighten up an image, we can use the associated layer mask to limit things to just the shadow areas of the image. All we need is an effective way to select just the shadow portions of our image. One option would be to paint with shades of gray on the layer mask as already mentioned. But this can be tedious and difficult to achieve results that blend well throughout the image. Ideally, we want the full effect of the levels adjustment to show in the darkest shadows and to show progressively less in areas where the lighting is already better.

Photoshop could instead create the mask for us based on a selection. If we use the lasso and other selection tools to select the shadow areas before clicking on the new adjustment layer icon, the resulting layer mask will automatically match our selection. We could then modify the layer mask as before but the same difficulty in realistically blending the effect would remain.

To solve this problem, we'll make use of a Photoshop's ability to convert a channel into a selection. At the bottom left of the channels palette is a small dotted-circle icon. Clicking on it will load the currently selected channel as a selection. If all three channels are selected, the luminosity of the composite image is used to form the selection. If you're a fan of keyboard shortcuts, you can do the same thing via Ctl-Alt-~ (tilde) on Windows, or Command-Option-~ on Mac OS. Each point in the image will be selected relative to how bright it is. A pure white point in the image will be completely selected while a pure black point won't be selected at all, and so on. As a selection, you can't really tell this of course since Photoshop shows selections by enclosing selected areas in the "marching ants" dotted line based on a fifty percent threshold. Any point that is at least fifty percent selected will appear to be part of the selection.

As soon as we create an adjustment layer with this selection active though its true nature will be revealed in the new layer mask which will appear as a black and white version of our image. But this is exactly the opposite of what we really want since now the highlights are selected more than the shadows are. Remember, white means selected and black means masked. Before we create the adjustment layer then we need to invert the selection via Select >> Inverse. Now when we create the adjustment layer, the layer mask will show as a negative of the image. A point that was completely black in the original will be pure white in the inverted mask with every other point being selected not on how bright it was, but on how dark.

If we create a curves or levels adjustment using an inverted luminosity mask we can increase the brightness of the shadows without affecting the highlights. Or at least not affecting them very much. Remember, even a point that was fairly bright to begin with was likely not pure white so will remain partially selected in the inverted mask. But if we click on the layer mask icon we can use Image >> Adjustments >> Levels or Curves to modify the mask as needed to more tightly limit the effect to just the shadows.

To give it a try, first select the luminosity channel for the image and then invert the selection. Now create a new curves adjustment layer but don't change the curve from its default. Click on "OK." Now Alt-click (Option-click on Mac OS) on the layer mask in the layers palette next to the new adjustment layer to see what it looks like. You should have a black and white negative version of your image. Now use Image >> Adjustments >> Curves on the layer mask. Pull the curve down sharply to turn increase the contrast of the mask. Click on "OK" to complete the conrast adjsutment of the layer mask. Now click on the adjustment layer icon to the left of your layer mask in the layers palette to see the image itself again. Then return to the curves adjustment for the adjustment layer by double clicking on it in the layers palette. You should now be back to the straight diagonal line default curve you initially created. Pull upwards on it to brighten the image. Since the effect will be constrained by the layer mask though, we can safely make major changes to the shadows without changing the image highlights at all.

The shadows are all blocked up in this original capture from Frenchman Coulee in eastern Washington
The shadows are all blocked up in this original capture from Frenchman Coulee in eastern Washington
  The 'marching ants' selection line after selecting the luminosity channel
The 'marching ants' selection line after selecting the luminosity channel
Icon at the bottom of the channels palette load a channel as the selection
Icon at the bottom of the channels palette load a channel as the selection
  The initial layer mask
The initial layer mask
Curves adjustment to increase contrast of the mask (not the image itself)
Curves adjustment to increase contrast of the mask (not the image)
  The modified layer mask
The modified layer mask
Curves adjustment to brighten the shadows of the image itself
Curves adjustment to brighten the shadows of the image itself
  The final image showing much better shadow detail
The final image showing much better shadow detail

Since a digital camera records a significant amount of detail that remains hidden in the shadows, you can make big improvements by using a luminosity mask to resurrect that lost detail.


Date posted: November 19, 2006

 

Copyright © 2006 Bob Johnson, Earthbound Light - all rights reserved.
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Previous tip: Contrast Masking in Photoshop Return to archives menu Next tip: Doing Even More with Luminosity Masks

Related articles:
Contrast Masking in Photoshop
Doing Even More with Luminosity Masks
 

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