Adobe Illustrator CS5 One-on-One

Working with Opacity Masks

If you’ve ever used Photoshop, you may know that you can erase

temporary holes into a layer using a layer mask. Illustrator offers

something similar, but because it’s not restricted to a layer, it goes

by a different name: opacity mask. In many ways, an opacity mask

is like a path in a compound shape set to Subtract or Intersect, carv-

ing a hole or containing other paths. The differences between those

modes and an opacity mask are that the latter works with paths

that include all varieties of fi lls and strokes (whereas all paths in a

compound shape must subscribe to a single set of attributes); it can

convey fades and levels of translucency; and you can apply it to a

layer, group, text block, blend, gradient mesh—in short, to anything.

In return for such fl exibility, the opacity mask demands that you

come to terms with its unusual ways. Which is why I’ve organized

this exercise differently from the others. I’ll start things off with a

few steps that—while they don’t get us any nearer to our ultimate

goal—demonstrate how the opacity mask works. Then you’ll use

this knowledge to clip away the jacket ele-

ments along Sam’s head, clip away Sam’s ex-

cess pant legs, and create a fading piano. As a

bonus, I’ll show you a great use for blending

between groups.

1.

Open your last-saved fi le. Con-

tinue working with the fi le that

you saved at the end of the pre-

ceding exercise. Or open my version of

the illustration in progress, The knockout

group.ai, found in the Lesson 08 folder.

2.

Turn on the Circle path. First, a quick tu-

torial on how opacity paths work. If nec-

essary, in the Layers panel, twirl closed

the Jacket sublayer. Then click in the fi rst

column in front of the Circle path (just

above Jacket) to turn it on. You’ll see a

white circle with a 40-point black stroke,

as in Figure 8-12. What it lacks in beauty,

in makes up for in educational value.

3.

Select the Circle and Jacket items. Again

in the Layers panel, click the meatball for

the Jacket sublayer to select it. And then

Shift-click the meatball for the Circle path.

Figure 8-12 .

251

Working with Opacity Masks

The circle’s

white fi ll

maintains

opacity

The black stroke

and beyond

result in

transparency

The circle expressed as

an opacity mask

4. Combine the selected objects into an opacity

mask. Return to the Transparency panel and

click the in the panel’s top-right corner. Then

choose the command Make Opacity Mask.

Illustrator applies the top path in the selection,

Circle, as an opacity mask to the selected object

below. These are the repercussions:

• The Circle path disappears from the Lay-

ers panel.

•

Illustrator gives the Jacket sublayer a dashed

underline, showing that it harbors an opac-

ity mask. (Remember this: When reviewing

archived art or fi les from other designers,

a dashed underline in the Layers panel

means “opacity mask in effect.”)

•

In the Transparency panel, the white circle

appears to the right of the sublayer thumb-

nail, as shown at the top of Figure 8-13.

•

Look closely, and you’ll see that the Knock-

out Group check box in the Transparency

panel appears as . This time, it’s accurate:

Some items in the selection are knockout

groups; others aren’t. Leave it as is.

•

In the document window, everything out-

side the white fi ll of the circle goes trans-

parent, also shown in Figure 8-13.

An opacity mask defi nes transparency using

brightness. White is opaque, black is transpar-

ent, and gray is an intermediate level of translu-

cency. Which is why the circle’s fi ll leaves Sam’s

clothing opaque and anything black—includ-

ing the stroke and the empty area outside the

path—makes the clothing transparent.

5.

Experiment with the check boxes. When work-

ing with an opacity mask, the Transparency

panel makes available two mysterious check

boxes, Clip and Invert Mask. Perform the fol-

lowing and you’ll understand how they work:

•

So you can see what you’re doing, press

Ctrl+H (or -H) to hide the selection edges.

Figure 8-13 .

252

Lesson 8: Working with Transparency