how to ignore transparency of image when clicking/raycasting on it in 2D/uGUI?

Earlier this year, i found a post about a new method (since unity 5.6 if I correctly remember) to allow clicks to ignore parts of an image when raycasting on it, according to its alpha value. I can't find the method nor the post anymore? Is someone aware of the function i m thinking about and could you provide an example?

Thanks a lot