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I never thought there is a difference where I start creating a rectangle in Illustrator.

However I've just realised that if I start from Left-Upper corner I create "outer corner style", and when I start from Left-Lower corner I create "inner corner style" rectangle.

There is no any visible difference between these two rectangles, but when I apply a pattern brush - it can be clearly seen.

My questions are -

Is there a way to know about type of a path created - "outer" vs "inner"?

Does it applied to other shapes and where it does matter aside of pattern brush?

Proper terminology and any link to the info would be highly appreciated.

5

As they say on Facebook, "It's complicated." This has to do, somewhat, with one of the more esoteric (and mathematical) aspects of Postscript called a "Winding Rule," which determines what is the "inside" and what is the "outside" of a closed path segment, but mostly with the fact that any path has a direction. You determine the direction by dragging out from top right or bottom left.

There's a discussion about this in the Illustrator Help file under "Compound Paths," which is the only place Illustrator really exposes reversing the direction of a path in the UI. (Ironically, InDesign makes this much simpler. It's in the main menu as Object > Paths > Reverse Path.)

If you create a compound path, such as by creating outlines for an uppercase letter "B," you can change whether a particular segment is filled or not by selecting it with the Direct Selection Tool and reversing the path via the Attributes panel. In your particular example of a simple rectangle, you can switch from one version to the other using Object > Transform > Mirror or use the ridiculously complex method (RCM): Object > Compound Path > Make, open the Attributes panel and turn on Nonzero Winding and Reverse Path.

For a geeks-only technical exposition on the two winding rules, see the Wikipedia entries for Nonzero Rule and Even-Odd Rule, or Google either of those terms.

  • Great question! This is one of those things that's so nicely hidden, and so awesomely geeky, it can completely baffle anyone. – Alan Gilbertson Jul 23 '14 at 23:10

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