Masks based on the brightness values of an image are very useful for tonal corrections. They can limit the corrections to the highlights or the shadows. Furthermore, they can be created from the brightness values belonging to the composite channel or an individual channel. To give one example of their use, you can limit a Curves adjustment to just the highlights by taking the following steps:
1. Ctrl-click (Windows), ^-click (Mac OS) on the composite channel to load the luminosity mask, or Ctrl+Alt+Tilde (Windows), ^+Opt+Tilde (Mac OS). The Tilde key is the one marked with a sign. To be precise, you are using the grave accent "v" key, but tilde is commonly used to refer to the key.
2. Choose Layer ^ New Adjustment Layer ^ Curves, or click the Create New Fill or Adjustment icon in the Layers palette and then choose Curves. Photoshop creates an adjustment layer and an accompanying mask thumbnail and includes a grayscale image of the composite channel in the mask.
3. In the Curves dialog box, adjust the curve and click the OK button to exit.
When you make any adjustments in the Curves dialog box, the changes made to the highlights show in accordance with the density of the mask, but the changes applied to the shadows are masked.
To affect the shadows, press Ctrl+Shift+I (Windows), 8€+Shift+I (Mac OS) after step 1 to invert the mask, or you can click the mask thumbnail at any time and invoke the preceding shortcut.
After becoming aware of this technique, you can quite easily find other uses for it. For example, you can use it to confine the effect of layer blends, Hue/Saturation adjustments, global sharpening, colorized layers, or sepia-like effects.
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