This chapter is from the book
This chapter is from the book
This chapter is from the book
Creating Compound Paths
A compound path is a single path that consists of more than one path. That sounds like an oxymoron, no? Think of the letter O in the alphabet. It appears to be a large circle with a smaller circle cut out from its center. How is such a shape created with Illustrator? The answer is by drawing two circles and combining them to become a single compound path. You do this by choosing Object > Compound Path > Create. The result is a shape with a hole cut out of the middle (Figure 4.33). Compound paths are treated as one entity, and therefore, both paths that make up this compound path take on the attributes of the bottommost path. If your compound path consists of multiple shapes, Illustrator does its best to figure out which paths become hollow and which appear solid.
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Figure 4.33 An example of a compound path. The hole in the center is actually cut out from the path, and objects that appear beneath the compound shape are visible through the hole.
Illustrator uses one of two methods to decide which paths of a compound shape are hollow and which are solid. The default method is the Non-Zero Winding Fill Rule method; Illustrator can also use another method, the Even-Odd Fill Rule method. You'll find both of these buttons in the Attributes panel, and you can choose them when a compound path is selected on the artboard (Figure 4.34). By default, Illustrator uses Non-Zero Winding Fill Rule and makes the bottommost path clockwise and all the other selected paths counterclockwise.
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Figure 4.34 You can use the Attributes panel to choose one of the two supported compound path methods for determining hollow and solid areas.
When you create a compound path and click Non-Zero Winding Fill Rule, you can manually reverse the path direction to control whether a shape is hollow or solid. Use the Direct Selection tool to select the path you need, and click the appropriate button in the Attributes panel (Figure 4.35).
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Figure 4.35 Using the Attributes panel to manually reverse the direction of a path, you can specify whether a part of a compound path using Non-Zero Winding Fill Rule is hollow or solid.
Featured Matchup: Non-Zero Winding Fill Rule vs. Even-Odd Fill Rule
More math! When you compare them to each other, Even-Odd Fill Rule seems more intuitive, and it is easier to predict which areas will be filled and which areas will be hollow. Although you have more flexibility with Non-Zero Winding Fill Rule—you can manually control the result—this rule is more difficult to understand, and the result is harder to predict.
With Even-Odd Fill Rule, every area inside an even number of enclosed areas becomes hollow, and every region inside an odd number of enclosed areas becomes solid (Figure 4.36). An enclosed area refers to an area enclosed by another path (or the loop of a path in a self-intersecting shape). The outermost enclosed area is always numbered 1, and therefore a regular path is filled (it is enclosed by a single area, which is an odd number).
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Figure 4.36 When you're using Even-Odd Fill Rule, Illustrator labels areas using odd and even numbers to determine hollow and solid areas.
In contrast, the Non-Zero Winding Fill Rule takes into account the direction of a path: An area enclosed by a clockwise loop counts as +1, and an area enclosed by a counterclockwise loop counts as –1. When the sum of these counts is zero, that area becomes hollow. When it is anything else, that area becomes solid (Figure 4.37). Because you can manipulate the path direction to get different results from the same shapes, Non-Zero Winding Fill Rule is more flexible, but it's an exercise of trial and error since you can't see the direction of a path on the artboard.
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Figure 4.37 When you're using Non-Zero Winding Fill Rule, Illustrator takes into account the direction of the path when it determines the hollow and solid areas of a compound path. The arrows indicate path direction.
Although the results in most cases are the same whether you use the Non-Zero Winding Fill Rule or Even-Odd Fill Rule setting, sometimes the result is different (Figure 4.38).
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Figure 4.38 Some compound shapes appear different, depending on the fill rule specified, especially with self-intersecting paths.
—Special thanks to Teri Pettit of the Adobe Illustrator team for helping explain these rules.