Remember the last time you gave your walls a fresh coat of paint? You probably used a roll of blue or beige masking tape to cover the baseboards and windows so you wouldn't get paint on them. Masking tape's digital equivalent--called a layer mask--is a timesaving feature in Adobe Photoshop, Photoshop Elements (version 9+), and the affordable yet powerful Pixelmator. Here's a quick primer on how to use this great tool in your digital image editing program of choice.
Layer masks make for a far more flexible editing experience because you're hiding pixels instead of erasing them. For example, say you're treating yourself to some head-swapping (fantastic for breakups, pranks, etc). If you use the Eraser tool and accidentally remove an ear, there's no getting it back without some heavy undo action. However, if you use a layer mask to hide pixels instead, you can easily fix an accidental ear cover-up.
You can perform all manner of non-destructive techniques with layer masks. For example, you can make text appear behind an object in a photo, or isolate a picture's subject and alter the background. You can fix exposure problems by using a layer mask to restrict the change to certain areas, or control an area affected by a color tint or a filter's effect. You can also use masks to combine images, give a photo rounded or painterly edges, change the color of a single object in a photo, create a partial color effect, and more.
Note: You may not realize it, but many apps use automatic masking in their effects tools to change part of a photo instead of the whole thing. For example, iPhoto for iOS uses masking for its Brushes tool and three of its Artistic Effects. Likewise, Adobe Photoshop Lightroom and Camera Raw use masking for their Adjustments Brush, Gradient and Radial filters.
Add a layer mask
Whether you're working in Adobe Photoshop or Elements, adding a layer mask is identical: activate the layer and click the circle-within-a-square icon at the bottom of the Layers panel in Photoshop, or at the top of the Layers panel in Elements' Expert mode (Full Edit in earlier versions). In Pixelmator, click the gear icon at the bottom of the Layers panel and choose Add Layer Mask.
The resulting mask looks like a white rectangle in your Layers panel and serves as a miniature representation of your document. You make your mask by adding digital black paint, obscuring certain parts of the photograph. You can use multiple methods to do this, like the Brush tool (set to paint with black), the Gradient tool (set to a black and white gradient), or by creating a selection and filling it with black.
A helpful tip: When you're working inside a layer mask, black paint conceals, white reveals. If you make a mistake and conceal too much of the image with black paint, you can reveal it again by painting atop those areas with white. What a forgiving way to edit!
Create a soft-edge vignette collage
Layer masks can be used in many ways to create more interesting photographs. For example, you can use masks to blend two images together into a soft, oval vignette; it's a handy technique and great for photographers, designers, and hobbyists.
Start by opening two images: the main image, and the one you want for the vignette. In the main image document, press Command-A to select all and then press Command-C to copy the image. In the other document, press Command-V to paste the main image as a new layer. Activate the Elliptical Marquee tool, click atop the image and drag diagonally downward to draw an oval selection. (Tip: Press and hold the spacebar to reposition the selection as you draw it).
In Photoshop: Click the "Add layer mask" icon and then double-click the mask thumbnail in the Layers panel to summon the Masks panel. Drag the Feather slider rightward to soften the mask's edge.
In Elements: Click the Refine Edge button in the Options bar at the bottom of its workspace. In the resulting dialog, press L to preview the mask on all layers, make sure all sliders are at 0, and then drag the Feather slider rightward. From the Output To menu at the bottom, pick Layer Mask and click OK.
In Pixelmator: Choose Edit > Refine Selection. In the resulting dialog, drag the Feather slider rightward and click OK. In the Layers panel, click the gear icon and choose Add Layer Mask. Press Command-D to dismiss the selection.
Use adjustment layer masks
Adobe Photoshop and Elements These two programs include adjustment layers, which let you make color and lighting changes that are self-contained--your original image remains untouched. A layer mask automatically tags along with each adjustment layer, allowing you to hide the effects of that particular adjustment. (In Photoshop, you also get an automatic mask when using Smart Filters, enabling you to hide a filter's effects from portions of the photo.)
To create a partial color effect in either program, open an image and then press the D key to set the color chips at the bottom of your Tools panel to the default of black and white, then press the X key until black is on top.
Click the half black/half white circle at the bottom of the Layers panel in Photoshop (or the bottom of the Layers panel in Elements) and choose Gradient Map. Your image is instantly transformed into black and white. With the mask active (it'll have a light-colored outline), press the B key to grab the Brush tool, and then paint across the area of your image that you want to keep in color. Tap the left bracket key ([) to decrease brush size; tap the right bracket key (]) to increase it.
Pixelmator: While Pixelmator doesn't have adjustment layers, you can create this effect by choosing Layer > Duplicate Layer, and in the Effects Browser, double-clicking the "Black & White" thumbnail.
Add a layer mask to the black and white layer, then press the B key to activate the Brush tool. Click the color swatch in the Tool Options at the top of your screen and choose black. Paint atop the area that you wish to remain in color, while using your left/right bracket keys to control brush size. If you reveal more color than you want, set the Tool Options' color swatch to white and paint back across that spot.
Use clipping masks
All three programs support clipping masks, which are like digital stencils; they let you take one layer's contents (a photo) and push it through the contents of the layer underneath it (text, a shape you've drawn, or a brush stroke). As long as that lower layer has some transparent (empty) areas, the layer above it is "clipped" so that you only see through the solid parts of the lower layer.
Here's how to use a clipping mask to push a photo through text: Open a photo and, if necessary, double-click the Background layer in the Layers panel to unlock it. Next, press the T key to grab the Type tool. In Photoshop or Elements, click atop the document and enter a short word; in Pixelmator, click and drag to draw a text box and then enter a word.
Use a thick font like Impact, Arial Bold, or Helvetica Black, but don't worry about text color. In the Layers panel, activate the photo layer and drag it above the text layer. Option-click the dividing line between the two layers to create a clipping mask. (To release the clipping mask, Option-click the dividing line between the two layers again.)
As simple as that
As you can see, with just a few steps, you can conquer masking and use it to gain editing flexibility. Until the next time, may the creative force be with you all.