Erasing after drawing with CGContext

Question

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!

score 26 · Answer 1 · answered Jan 28 '12 at 08:35

26

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);
}

answered Jan 28 '12 at 08:35

Eden

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you are my hero, thank you so much for giving such a simple solution! – Bek Nov 22 '12 at 11:08
2

First off I'm going by Ray Wenderlich's tutorial for creating a simple drawing app. Everything works for it, but when I change the blend mode to kCGBlendModeClear all it does is erase the whole canvas once touches ended gets called. Any thoughts? – Ron Dear May 01 '14 at 22:14
Excellent, though too verbose. You can do the same with a one-liner: `CGContextSetBlendMode(UIGraphicsGetCurrentContext(), erase ? kCGBlendModeClear : kCGBlendModeNormal);` – SaltyNuts Oct 30 '14 at 17:16

score 4 · Accepted Answer · answered May 12 '11 at 23:00

4

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.

answered May 12 '11 at 23:00

Jeremy W. Sherman

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How do you draw a transparent patch? I have used `CGContextSetRGBStrokeColor` with an alpha of 0.0, but that doesn't seem to work. – Roberto Jun 10 '11 at 00:03
Play with the compositing mode (Copy instead of SourceIn, for example) and consider filling as well as stroking. – Jeremy W. Sherman Jun 10 '11 at 01:04
1

This is a nothing-burger. Should be a comment, not an answer. – Albert Renshaw Jul 28 '18 at 05:57

score 0 · Answer 3 · answered Jul 19 '20 at 21:15

0

Clear all CGContextRef drawings:

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

answered Jul 19 '20 at 21:15

Ofir Malachi

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Erasing after drawing with CGContext

3 Answers3