Sidebar Sidebar
Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.

iOS Erasing with CGContext

S

spaceman816

macrumors member
Original poster
Jul 29, 2009
30
0
I've looked all over for this but nobody seems to have posted a working answer:
How can I make an eraser for a drawing made using CGContext?
I can't just draw the background color over the drawing because I need the background to remain transparent.
People have suggested using transparent paint, but that won't work.
I need to replace what's drawn with transparent paint.

Thanks.
 
You need to set the CGBlendMode correctly. I have code that does this (draws a colour then overdraws in transparent) but I'm not at home right now so I can't confirm which blend mode I am using (or that this is what I am doing).
 
CGContextClearRect() might work, but not sure if it would work along with a clipping (for erasing non-rectangles). I would assume it would but haven't tested.
 
kainjow said:
CGContextClearRect() might work, but not sure if it would work along with a clipping (for erasing non-rectangles). I would assume it would but haven't tested.
Click to expand...

I meant to mention that I tried this out, but it's a really ugly square eraser that doesn't fit my needs. I need a round cap.
 
Trying creating a path, adding it to the context, and using CGContextClip() to clip, then do the erase.
 
spaceman816 said:
I meant to mention that I tried this out, but it's a really ugly square eraser that doesn't fit my needs. I need a round cap.
Click to expand...

And, of course, you've investigated the solution I proposed? Thought not.

This is what I do:

Code: 
	CGContextSetStrokeColorWithColor(c, [[UIColor clearColor] CGColor]);
	CGContextSetFillColorWithColor(c, [[UIColor clearColor] CGColor]);
	CGContextSetBlendMode(c, kCGBlendModeClear);
	CGContextAddEllipseInRect(c,CGRectMake(size.width-(size.width/2)-(size.width/10), (size.width/10), (size.width/2), (size.height/2)));
	CGContextClosePath(c);
	CGContextDrawPath(c, kCGPathFillStroke);
 
robbieduncan said:
And, of course, you've investigated the solution I proposed? Thought not.

This is what I do:

Code: 
	CGContextSetStrokeColorWithColor(c, [[UIColor clearColor] CGColor]);
	CGContextSetFillColorWithColor(c, [[UIColor clearColor] CGColor]);
	CGContextSetBlendMode(c, kCGBlendModeClear);
	CGContextAddEllipseInRect(c,CGRectMake(size.width-(size.width/2)-(size.width/10), (size.width/10), (size.width/2), (size.height/2)));
	CGContextClosePath(c);
	CGContextDrawPath(c, kCGPathFillStroke);
Click to expand...

I hadn't gotten a chance to look into it- but this works perfectly.
Thanks so much.
 
the eraser actually jumps a little too much (if you drag your finger quickly, a circle gets erased here and there with space left in the middle). is there any way to smooth the path?
- sorry if i'm asking dumb questions...im new to objective c and haven't been able to find anything that fixes this.
 
spaceman816 said:
the eraser actually jumps a little too much (if you drag your finger quickly, a circle gets erased here and there with space left in the middle). is there any way to smooth the path?
- sorry if i'm asking dumb questions...im new to objective c and haven't been able to find anything that fixes this.
Click to expand...

You will need to track the path and interpolate between the points you are actually picking up to provide a smooth path. There is no built in way to do this: it's 100% up to you.
 
You must log in or register to reply here.
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.
Top Bottom
Back