CoreGraphics draw an image on a white canvas - objective-c

Question

I have a source image which has a variable width and height, which I must show on a fullscreen iPad UIImageView but with addition of borders around the image itself. So my task is to create a new image with a white border around it, but not overlapping on the image itself. I'm currently doing it with overlapping via this code:
- (UIImage*)imageWithBorderFromImage:(UIImage*)source
{
CGSize size = [source size];
UIGraphicsBeginImageContext(size);
CGRect rect = CGRectMake(0, 0, size.width, size.height);
[source drawInRect:rect blendMode:kCGBlendModeNormal alpha:1.0];
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBStrokeColor(context, 1.0, 1.0, 1.0, 1.0);
CGContextSetLineWidth(context, 40.0);
CGContextStrokeRect(context, rect);
UIImage *testImg = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return testImg;
}
Can anyone tell me how do I first draw a white canvas that is 40 pixels bigger in each direction than the source image and then draw that image on it?

Answer 1

I adjusted your code to make it work. Basically what it does:
Set canvas size to the size of your image + 2*margins
Fill the whole canvas with background color (white in your case)
Draw your image in the appropriate rectangle (with initial size and required margins)
Resulting code is:
- (UIImage*)imageWithBorderFromImage:(UIImage*)source
{
const CGFloat margin = 40.0f;
CGSize size = CGSizeMake([source size].width + 2*margin, [source size].height + 2*margin);
UIGraphicsBeginImageContext(size);
[[UIColor whiteColor] setFill];
[[UIBezierPath bezierPathWithRect:CGRectMake(0, 0, size.width, size.height)] fill];
CGRect rect = CGRectMake(margin, margin, size.width-2*margin, size.height-2*margin);
[source drawInRect:rect blendMode:kCGBlendModeNormal alpha:1.0];
UIImage *testImg = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return testImg;
}