Programming iOS 4

Clipping

Another use of a path is to mask out areas, protecting them from future drawing. This is called clipping. By default, a graphics context’s clipping region is the entire graphics context: you can draw anywhere within the context.

The clipping area is a feature of the context as a whole, and any new clipping area is applied by intersecting it with the existing clipping area; so if you apply your own clipping region, the way to remove it from the graphics context is to wrap things with calls to CGContextSaveGState and CGContextRestoreGState.

To illustrate, I’ll rewrite the code that generated our original arrow (Figure 15-5) to use clipping instead of a blend mode to “punch out” the triangular notch in the tail of the arrow. This is a little tricky, because what we want to clip to is not the region inside the triangle but the region outside it. To express this, we’ll use a compound path consisting of more than one closed area — the triangle, and the drawing area as a whole (which we can obtain with CGContextGetClipBoundingBox).

Both when filling a compound path and when using it to express a clipping region, the system follows one of two rules:

Winding rule

The fill or clipping area is denoted by an alternation in the direction (clockwise or counterclockwise) of the path demarcating each region.

Even-odd rule (EO)

The fill or clipping area is denoted by a simple count of the paths demarcating each region.

Our situation is extremely simple, so it’s easier to use the even-odd ...