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

On github, you can find a project by Ole Begemann which extends UIButton so that it only detects touches where the button's image is not transparent.

Since UIButton is a subclass of UIView, adapting it to UIImageView should be straightforward.

Hope this helps.