This is a very old example of mine actually (creation timestamp says Sep 2010), which I've recently fixed & improved by request.
It demonstrates a simple algorithm to cast shadow from instance in platformer game. So when player jumps, shadow is displayed on ground below (realism!).
For the sake of simplicity and performance, shadows are checked against bounding boxes by default. Pixel-perfect checking is added as a separate condition, requiring affected objects to be children of obj_complex.
If you're interested in how this works behind-the-scenes, here's an illustration:
Outlined instances are ones used for finding top-most position below the player (which shadow will be cast onto). Red line is a number of point checks to indicate top-most point on objects with precise masks.
As another random visual tweak, there's a piece of code added to make shadow shrink as caster gets further from ground. A minimal touch, but these do stack.