Oblique Frustum Culling is Cool

Every now and then I come across a graphics algorithm so out there that it just blows my mind - it feels like cheating the laws of physics or something.  Oblique Frustum Culling is one of those algorithms.

Here's the situation: to make a pretty water reflection, you need to reflect the camera position around the water plane - take a picture, and then use that projected texture later to draw the water reflections.  There's one hitch: you have to clip out everything below the water.  If you don't, you get this.

The inset picture is the "from-the-water" view...note that the pillars of the bridge go way below the water (so one art asset can be used at many elevations).  But since they are not obscured by the water (when viewed from below the water), they show up in the reflection texture as the dark areas behind the bridges.  (This is because the reflection texture's projection doesn't know what "forward" and "backward" are along the projected light ray.)

You can't fix this using depth buffering or by drawing the water - the water would occlude everything you do want.  So you need a user-specified clip plane.  Clip everything below the water and go home happy, like this.

As I have blogged before, clip planes and shaders don't always work well together.  (And by don't always, I mean never.)  If I remember correctly, one vendor requires you write the clip vertex while the other requires you don't.  To make it more fun, the latest GLSL specs will deprecate the clip vertex entirely in terms of gl_ClipDistance (which makes more sense actually).  Mix that with drivers for OS X and for Win/Lin, and users who aren't on the latest GLSL compilers, and you have enough anarchy to want to kill yourself.

That's where oblique frustum culling comes in.  Basically it's an algorithm that takes the near clip plane of the view frustum and distorts/bends/twists/performs voodoo on it so that it is your user clip plane.  Quite literally everything below water is now "too close" to the camera.

What I really love about this algorithm is: no extra state - we don't have to have conditionals in our shaders for the (rare) time we need our user clip plane - we just bend the frustum and move on with life.  How cool is that?!!