Creating murky liquid
It is possible to change an object's surface visibility based on its distance from the camera. You can also get the distance to the next surface underneath it or behind it, even where it varies pixel for pixel. Some kinds of netted curtain fabric exhibit this effect; where the fabric is close to the window it is see through, and where it's further off the surface, it isn't. In a game environment, a fringe of transparency created by a DepthBiasedAlpha node is often used at the converging edge of water and land so that some of the shore is visible under shallow water, while the water opacity increases further out. It is also possible to create holographic or scanning effects using this node. In this recipe and the next we'll do both, creating a murky pool and a passageway scanner.
Coverage of DepthBiasedAlpha and other depth based Material nodes can be found at http://udn.epicgames.com/Three/MaterialsCompendium.html. Look up SceneDepth and PixelDepth too.
