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Using Layers for Transparency and Clipping in Illustrator CS4

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Do you think of layers strictly as a selection tool? You're missing out. David Karlins, author of Adobe Illustrator CS4 HOW-TOs: 100 Essential Techniques, points out a number of ways in which layers can be used to edit artwork in Illustrator.
Using a Clipping Mask in a Layer

Using a Clipping Mask in a Layer

An object within a layer can act as a clipping mask over other objects within that layer. To be clear: You do not use an entire layer (with a bunch of objects) as a clipping mask over other layers; instead, you use an object within a layer as a clipping mask over the rest of the layer.

One more rule: The clipping object must be a vector; placed bitmaps cannot be used as clipping objects.

With those provisos, here's how to create a clipping mask within a layer:

  1. Make sure that the vector object that will act as the clipping mask is at the top of the layer's stacking order. If it isn't, drag the object to the top of the set of objects within the layer.
  2. Click on the layer name in the Layers panel to select the layer.
  3. Click the Make/Release Clipping Mask icon at the bottom of the Layers panel (see Figure 3).

Figure 3 Preparing to use the top object in a layer as a clipping mask.

After you apply the clipping mask, only the section(s) of the underlying objects covered by the clipping mask object are visible. The object used as a clipping path is underlined in the Layers panel (see Figure 4).

Figure 4 When a clipping mask is applied within a layer, the clipping object is underlined in the Layers panel.

To undo a clipping mask applied in a layer, choose Edit > Undo from the Illustrator menu. Releasing a clipping mask by clicking the Make/Release Clipping Mask icon in the Layers panel undoes the clipping mask, but doesn't restore the original stroke, fill, or other attributes of the object that was used as the clipping mask.

As noted earlier, you can move objects from one layer to another by selecting them in the Layers panel and dragging them (in the Layers panel) into a different layer. The implication for masking is that any object dragged into the masked layer will have the clipping mask applied to it.

You can change the visible section of the masked object by clicking and dragging—not on the masking object, but on the masked object. For example, I can click and drag on the (hidden, invisible) masked sketch in my illustration to change the part of the illustration that will "show through" the clipping mask (see Figure 5).

Figure 5 Moving a masked object underneath a mask.

You can apply masking without creating a layer mask: Simply place one object on top of another, select both objects, and choose Object > Clipping Mask > Make. But here's an interesting fact: The clipping mask you generate when you use this technique (as opposed to a layer mask) works differently. If you create a mask in Illustrator CS4 without doing so as a layer mask, you cannot select the underlying object and move it.

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