UIEdgeInsets ignored on CGContextDrawImage within a UIGraphicsBeginImageContextWithOptions

Question

I am struggling getting Image Insets to work when drawing to an off screen buffer.

Using the resizableImageWithCapInsets: on an UIImage directly setImage: into a button works fine for me:

UIImage * base = [UIImage imageNamed:@"button.png"];
UIImage * img = [base resizableImageWithCapInsets:UIEdgeInsetsMake(20,20,20,20)];
[self setImage:img forState:UIControlStateNormal];

As shown below (left is raw scaling, right is scaled with insets):

Image stretched without ado Image stretched with decent insets

So the right one is fine - the lines top/botton/left/right are equally spaced. So far so good.

Now if I try the very same thing with an image that is drawn and then captured to an off screen buffer with:

UIImage * base = [UIImage imageNamed:@"button.png"];
base = [base resizableImageWithCapInsets:UIEdgeInsetsMake(20,20,20,20)];

UIGraphicsBeginImageContextWithOptions(self.bounds.size, NO, 0);
ctx = UIGraphicsGetCurrentContext();

CGContextDrawImage(ctx, CGRectMake(0,0,self.bounds.size.width,
         self.bounds.size.height), [base CGImage]);

img = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext(); 
[self setImage:img forState:UIControlStateNormal];

I get below (again left is raw scaling, right is with insets):

raw scaling, side buffer scaling with inset, side buffer (WRONG!) .

So here it seems that the insets are ignored.

Anyone any suggestions as to what is going wrong here ? Fully functional example at http://people.apache.org/~dirkx/insetSample.zip and just the key code at http://pastebin.com/rm8h6YFV.

Any and all suggestions appreciated.

Dw

Robin Summerhill · Accepted Answer · 2012-09-24 10:51:12Z

13

I would guess that stretchable UIImages are handled at a higher level than Quartz2D and so using CGContextDrawImage is not going to work.

You could try using -[UIImage drawInRect] instead. This will draw to the current UIGraphics context, which is your bitmap context that you create in your code snippet.

The relevant line is:

[base drawInRect:CGRectMake(0,0,self.bounds.size.width,
                                    self.bounds.size.height)];

edited Sep 24, 2012 at 10:51

answered Sep 24, 2012 at 10:44

Robin Summerhill

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3 Comments

Robin Summerhill Over a year ago

I just tried editing the project you supplied and making this change fixes your problem.

Dirk-Willem van Gulik Over a year ago

Thank you - that is indeed spot on. Works lovely with that change (for future reference - pastebin.com/FjZkfb9z is the updated file/code.

Dirk-Willem van Gulik Over a year ago

Actually - when doing this with the real image (which is sort of an illumated button as seen on vision mixers (flickr.com/photos/peterprice/121342192) - one needs to make sure things are rotated - as the two drawing layers have the Y origin at top resp. bottom.

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