Fill rules

When you fill a compound path in which the subpaths overlap, or a path in which a single path crosses itself a few times, you can tell Expression Design how to determine which portions of the path should be filled. For example, if you draw two circles, overlap them slightly, and then combine them into a compound path, the center area (where they overlap) is considered inside the object (and therefore filled) or outside the object (not filled). This control is called a fill rule.

You can choose between the two fill rules by selecting an object, expanding the Appearance category in the Properties panel (click  ) to view the Object category. You can select between two options in the the Fill Rule drop-down list: Winding Rule and Even-Odd Rule.

Applying the Even-Odd Rule (left) versus the Winding Rule (right)

Technically, these two fill rules are based on a winding number count. To determine whether a particular point is considered inside or outside a path, count the number of times that point is encircled by the path, with each clockwise turn canceling out a counter-clockwise turn. (The path direction is typically based on the order in which you drew its points, and reversing a path's direction affects the fill rule.) If you choose Winding Rule, Expression Design will fill every space that has a non-zero winding number count. If you choose the Even-Odd Rule, it only considers points with an odd number to be inside the path.