Use a CoreGraphic Stroke as Alpha Mask in iPhone App

Question

I'm basically looking to create something akin to a very simple version of iSteam/iFog alebit for a different purpose. In effect there will be two images, one of the subject matter and the other an image of condensation or some such. The user can then wipe their finger over the screen and it will "cut" that from the top layer to reveal the lower layer. So far I've been able to draw a basic line on the screen using CoreGraphics and strokes but I can't find a way to then use this as an alpha mask for the steam layer.

If anyone could give me advice on what to use or even better some sample code I'd be very grateful as right now I'm pulling my hair out. Here is what I have so far:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {


mouseSwiped = YES;
UITouch *touch = [touches anyObject];   
CGPoint currentPoint = [touch locationInView:self.view];
UIGraphicsBeginImageContext(self.view.frame.size);
CGRect rect = CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height);
[drawImage.image drawInRect:rect];
CGContextRef context = UIGraphicsGetCurrentContext();

CGContextSetLineCap(context, kCGLineCapRound);
CGContextSetLineWidth(context, 36.0);
CGContextSetRGBStrokeColor(context, 1.0, 1.0, 1.0, 1.0);
CGContextBeginPath(context);

CGContextMoveToPoint(context, lastPoint.x, lastPoint.y);
CGContextAddLineToPoint(context, currentPoint.x, currentPoint.y);
CGContextStrokePath(context);
drawImage.image = UIGraphicsGetImageFromCurrentImageContext();


}

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
}