Applying Transparency to a Layer
As just noted, there are two ways to apply masking to an objectmake one object a mask, or apply masking through the Layers panel. In my opinion, editing a masked object is easier in Illustrator CS4 if you apply the mask through the Layers panel.
Similarly, you can apply transparency from the Layers panel, or by simply selecting one or more objects and applying transparency to the selection. The advantage to applying transparency through the Layers panel is that you can define transparency for all objects within the layer, and then you can apply and adjust that transparency at will.
The first step in applying transparency to objects through the Layers panel is to move all the objects to which transparency will be applied into the same layer (see Figure 6). That layer has to be above (on top of) any layer that has artwork that will be (partially) covered by the transparency.
Figure 6 Arranging objects to which transparency will be applied into a single layer, and moving that layer on top of another layer.
Next, target the layer to which transparency will be applied. Do this by clicking on the circle to the right of the layer's name, as shown in Figure 7.
Figure 7 Targeting a layer.
With the layer targeted, view the Transparency panel (choose Window > Transparency). The small preview window in the Transparency panel will preview all the objects in the targeted layer.
Adjust the Transparency slider to change the amount of opacity, as shown in Figure 8. (Opacity is the opposite of transparency; something with 100% opacity is completely opaqueyou cannot see through it.) You can also experiment with different modes of transparency by using the Normal pop-up in the Transparency panel.
Figure 8 Applying 25% opacity (using Lighten mode) to an entire targeted layer.
At any time, you can target the layer to which transparency has been applied, and adjust the transparency settings.