Hit Testing with CALayer using the alpha properties of the CALayer contents

Question

I\'m writing a game for Mac using Cocoa. I\'m currently implementing hit testing and have founds that CALayer offers hit testing, but does not seem to implement the alpha pr

Answer 1

Something I stumbled across while scratching my head over a similar problem is that CALayer uses containsPoint: when you send it hitTest:

Its default behaviour is to test against bounds, but we can override and get it to check the alpha channel, and just use CALayer's built in hit-testing to handle the rest:

- (BOOL) containsPoint:(CGPoint)p 
{
    return CGRectContainsPoint(self.bounds, p) && !ImagePointIsTransparent(self.contents, p)) return YES;
}

There's a discussion of testing for a single pixel's alpha at Retrieving a pixel alpha value for a UIImage

This worked for my purposes:

static BOOL ImagePointIsTransparent(CGImageRef image, CGPoint p)
{
    uint8_t alpha;

    CGContextRef context = CGBitmapContextCreate(&alpha, 1, 1, 8, 1, NULL, kCGImageAlphaOnly);
    CGContextDrawImage(context, CGRectMake(-p.x, -p.y, CGImageGetWidth(image), CGImageGetHeight(image)), image);
    CGContextRelease(context);

    return alpha == 0;
}

(If you're using renderInContext: to draw to the CALayer rather than setting its contents property, then it's going to be more complicated. This might be useful in that case: http://www.cimgf.com/2009/02/03/record-your-core-animation-animation/)