Erasing drawn content
Erasing drawn content
- Subject: Erasing drawn content
- From: Matej Bukovinski <email@hidden>
- Date: Tue, 8 Jun 2010 14:32:04 +0200
Hi,
In a cocoa application that I'm developing I have a custom NSView subclass that I use as an overlay view for annotating a WebView. The overlay view draws semi-trasparent rectangles in its drawRect: method for DOM elements that were selected by the user.
Since the fill color for the drawn rectangles is semi-transparent, the pixel values get added up when a smaller rectangle is drawn over a larger one (here's an example http://cl.ly/4fecb8fac0abff6ef6ac , as you can see the smaller rectangle has a darker background because of the larger rectangle, that was drawn first). This is especially problematic if the rectangles are of different colors, since color mixing occurs.
I would like to prevent this sort of behavior by somehow "erasing" the content that was perviously drawn (i.e., making the background transparent again) before drawing with a new element. Does anyone have an idea how to achieve this?
Thank you for your time.
Best regards,
Matej
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