Use a CoreGraphic Stroke as Alpha Mask in iPhone App

Answer 1

The method you want to call is CGContextClipToMask. Just draw your "steam" image and then the stroke on another CGImage. Then, clip the steam to the stroke. Something like this:

- (void)somewhereElse{
    UIImage *steam = [[UIImage imageNamed:@"steamImage.png"] retain];
    steamRef = steam.CGImage;
    //...
}

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextDrawImage(context, self.bounds, steamRef);         //draw the main image
    CGContextClipToMask(context, self.bounds, maskRef);         //respect alpha mask
    //where maskRef is a GCImage of your stroked path
}