CoreGraphics draw an image on a white canvas

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:

1. Set canvas size to the size of your image + 2*margins
2. Fill the whole canvas with background color (white in your case)
3. 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;
}