Re: Erasing drawn content
Re: Erasing drawn content
- Subject: Re: Erasing drawn content
- From: Quincey Morris <email@hidden>
- Date: Wed, 9 Jun 2010 11:55:54 -0700
On Jun 9, 2010, at 11:06, Matej Bukovinski wrote:
> The clipping path approach doesn't really help. It turns out that you can only intersect the current clipping path (initially the view's bounds) with a new path and the intersected region is the region where drawing is visible. This means that when you're drawing a larger rectangle over a smaller one, you would need to set the clipping path to all areas NOT covered by the smaller rectangle. This is basically the same problem as calculating which parts of the larger rectangle to draw and just drawing there.
I think you can do it, but it just takes an extra step. For each overlay (front to back):
1. Intersect the rect for the current overlay with the "drawRect" (i.e. the rect passed to drawRect:) producing the effective "tintRect".
1a. If tintRect is empty, nothing to draw, so skip 2-4.
2. Fill the tint rect with the appropriate color.
3. Construct a new bezier compound-path object with one path from drawRect (+[NSBezierPath bezierPathWithRect:]), plus a path from the tintRect (-[NSBezierPath appendBezierPathWithRect:]). That essentially "inverts" the tintRect within drawRect. Set the even-odd winding rule on the path, so that you don't have to be concerned about subpath directions (although I believe creating a compound path in the order I described will give correct results with non-zero winding rule, too).
4. Intersect this compound path with the current clipping path (-[NSBezierPath addClip]).
Repeat for each overlay rect.
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