Detect touches only on non-transparent pixels of UIImageView, efficiently

Question

How would you detect touches only on non-transparent pixels of a UIImageView, efficiently?

Consider an image like the one below, displayed with UI

Answer 1

Here's my quick implementation: (based on Retrieving a pixel alpha value for a UIImage)

- (BOOL)pointInside:(CGPoint)point withEvent:(UIEvent *)event {
    //Using code from https://stackoverflow.com/questions/1042830/retrieving-a-pixel-alpha-value-for-a-uiimage

    unsigned char pixel[1] = {0};
    CGContextRef context = CGBitmapContextCreate(pixel,
                                                 1, 1, 8, 1, NULL,
                                                 kCGImageAlphaOnly);
    UIGraphicsPushContext(context);
    [image drawAtPoint:CGPointMake(-point.x, -point.y)];
    UIGraphicsPopContext();
    CGContextRelease(context);
    CGFloat alpha = pixel[0]/255.0f;
    BOOL transparent = alpha < 0.01f;

    return !transparent;
}

This assumes that the image is in the same coordinate space as the point. If scaling goes on, you may have to convert the point before checking the pixel data.

Appears to work pretty quickly to me. I was measuring approx. 0.1-0.4 ms for this method call. It doesn't do the interior space, and is probably not optimal.