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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!

like image 365
Lindsey Avatar asked May 09 '11 15:05

Lindsey


2 Answers

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);
}
like image 172
Eden Avatar answered Nov 11 '22 19:11

Eden


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.

like image 44
Jeremy W. Sherman Avatar answered Nov 11 '22 20:11

Jeremy W. Sherman