Masking Objects

A mask conceals, or covers, parts of an object behind the mask, yet it reveals parts of the object behind it as well. For example, a mask that you wear to a masquerade party might cover parts of your face, yet some parts might not be covered. A mask in Illustrator works in much the same way: You can use just about any shape or object in Illustrator to mask other elements in your design. After a mask is applied, you can edit the art behind the mask or the mask itself, independently of each other.

You can apply basically three types of masks in Illustrator: a clipping mask, a layer clipping mask, and an opacity mask. Each of these masks has its own specific uses and capabilities.

Clipping Masks

The clipping mask, the simplest kind of mask in Illustrator, can be made of any vector shape. A clipping mask works in this way: You have a shape that sits on top of your objects. When the mask is created, any objects that fall within the boundary of the top shape (the mask) are visible, but anything that falls outside that boundary is hidden. It's important to realize that the hidden art is not deletedit's still there, but it's hidden from view.

To create a clipping mask, draw a shape for your mask and bring it to the front of the stacking order. Position your mask over the art you want the mask to affect and select both the art objects and the mask shape above it (see Figure 7.109). Then choose Object, Clipping Mask, Make to create the mask (see Figure 7.110). When the mask is applied, both the objects and the mask above it are grouped together; however, you can use the Group Selection tool to select just the mask or just the objects to make edits. This is useful when you want to reposition the mask to reveal a different part of the objects below it.

Figure 7.109. The objects selected, with the object that will be the mask on top.

Figure 7.110. The masked art.

To release a mask (and have all the objects below the mask revealed again), select the group of the mask and objects, and choose Object, Clipping Mask, Release.

Layer Clipping Masks

Layer clipping masks are very similar in concept to clipping masks, except that they are applied in the Layers palette and affect all items on a specific layer. To create a layer clipping mask, simply create a shape that's on the same layer as the art you want to mask, and click on the layer listing in the Layers palette to highlight it. Then click the Make/Release Clipping Mask button at the bottom of the Layers palette (see Figure 7.111).

Figure 7.111. Creating a layer clipping mask.

Any object you add to that layer automatically becomes masked as well. Likewise, any object you pull out of that layer no longer is masked. A mask object is indicated by a listing in the Layers palette with an underline (see Figure 7.112). To release a layer clipping mask, highlight the layer in the Layers palette and click the Make/Release Clipping Mask button.

Figure 7.112. An object with an underline in the Layers palette indicates that it's a clipping mask.

Opacity Masks

As Emeril Lagasse says, let's kick it up a notch! Opacity masks are like clipping masks on steroids. First of all, you can use just about anything as an opacity maskeven gradients and photographs. If you remember how we discussed creating alpha channels and layer masks inside of Photoshop, you'll quickly see that opacity masks inside Illustrator are practically the same thing.

Did you Know?

Because opacity masks use the luminosity values of the mask itself, you can actually create a photograph or a vector object that truly fades to transparent. Simply use a black-to-white gradient as your opacity mask.

The same basic rule applies to opacity masks, in that you place the mask at the top of the stacking order and then select both the mask and objects under it before applying the mask.

When you have the objects selected, open the Transparency palette and expand the palette so that you can see all the options. You'll see a thumbnail of your selection on the left side of the palette (see Figure 7.113). From the Transparency palette flyout menu, choose Make Opacity Mask (see Figure 7.114). You'll now see that a second thumbnail appears in the Transparency palette beside the original one, which is the mask itself (see Figure 7.115).

Figure 7.113. The fully expanded Transparency palette with the thumbnail visible.

Figure 7.114. Creating an opacity mask.

Figure 7.115. After the opacity mask is created, both thumbnails are visible.

Did you Know?

The thumbnail icons in Illustrator's Transparency palette function exactly the same way layer mask icons work in Photoshop. For example, if you Shift+click on the mask thumbnail, the mask is disabled.

After an opacity mask is applied, you have several options in the Transparency palette:

To edit the art, click on the left thumbnail. You can tell that the thumbnail is highlighted if you see a black outline around it. Any edits you make on the artboard will affect the art objects themselves, and there's no way for you to access or edit the mask object itself.
To edit the mask object itself, click on the right thumbnail. There are two ways you can tell that you're editing the mask itself and not the art under it. The mask thumbnail is highlighted with a black outline, and the Layers palette changes to display only a single itemthe opacity mask itself (see Figure 7.116). While you're editing an opacity mask, you cannot make any edits to other art in your file.
Figure 7.116. The Layers palette, when an opacity mask thumbnail is highlighted.
Between the two thumbnails, there is a link icon (on by default). When the art and the mask are linked, they will be transformed together. So if you move the art, the mask will move with it as well (so that the appearance will be the same). However, you can click on the link icon to disable the link, at which time you can move the mask and the art independently.
The Clip option uses the shape of the mask to also clip the art under it (basically giving the same effect as a clipping mask in addition to the opacity levels).
The Invert Mask option enables you to reverse the luminosity values of the mask object. In simple terms, when you toggle this option, anything that was previously visible through the mask becomes hidden, and everything that was hidden becomes visible.

An object with an opacity mask applied to it is indicated in the Layers palette with a dashed underline (see Figure 7.117). To release an opacity mask, select the masked object and choose Release Opacity Mask from the Transparency palette flyout menu.