CODEWITHSUNDEEP
objective-ciosipaddrawingcgcontext
Lindsey
Lindsey

Reputation: 235

Erasing after drawing with CGContext

I'm trying to do a simple drawing app for the iPad where you can draw on a picture, and I'm using CGContext stuff to do it but the way I originally planned on handling erasing was to just draw over stuff with white...except I just realized today that it doesn't work when you're drawing onto another image because then when you "erase" you'll also "erase" the background image as well.

Is there any way to support actual erasing?

Thanks!

Upvotes: 10

Views: 6087

Answers (3)

Ofir Malachi
Ofir Malachi

Reputation: 1286

Clear all CGContextRef drawings:

CGContextRef context = UIGraphicsGetCurrentContext();
CGContextClearRect(context, self.bounds);
[self setNeedsDisplay];

Upvotes: 0

Eden
Eden

Reputation: 1120

I also needed erasing functionality. Based on @Jeremy's answer, here is what worked for me:

CGContextRef cgref = UIGraphicsGetCurrentContext();

if(erase == TRUE) // Erase to show background
{
    CGContextSetBlendMode(cgref, kCGBlendModeClear);
}
else // Draw with color
{
    CGContextSetBlendMode(cgref, kCGBlendModeNormal);
}

Upvotes: 26

Jeremy W. Sherman
Jeremy W. Sherman

Reputation: 36143

Display the user's drawing in a layer above the image. Then erasing is as simple as drawing a transparent patch on the drawing layer in order to let the image pixels below it show through.

Upvotes: 4

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