Given:
ctx
) with frame {0,0,100,100}
r
), with frame {25,25,50,50}
It's easy to clip the context to that rect:
CGContextClipToRect(ctx, r);
to mask out the red area below (red == mask):
But I want to invert this clipping rect to convert it into a clipping mask. The desired outcome is to mask the red portion below (red == mask):
I want to do this programmatically at runtime.
I do not want to manually prepare a bitmap image to ship statically with my app.
Given ctx
and r
, how can this be done at runtime most easily/straightforwardly?
Read about fill rules in the “Filling a Path” section of the Quartz 2D Programming Guide.
In your case, the easiest thing to do is use the even-odd fill rule. Create a path consisting of your small rectangle, and a much larger rectangle:
CGContextBeginPath(ctx);
CGContextAddRect(ctx, CGRectMake(25,25,50,50));
CGContextAddRect(ctx, CGRectInfinite);
Then, intersect this path into the clipping path using the even-odd fill rule:
CGContextEOClip(ctx);