Fixing LOD gaps, T-junctions

Question

I'm creating a heightmap renderer. One of the examples for solving gaps when doing LOD I found is this:

(from Game Programming Gems 2 - Greg Snook - Simplified Terrain using Interlocking Tiles)

Wouldn't this still produce a gap, if the three vertices encircled with red were not co-linear? Shouldn't the middle triangle be split into two, as I marked with the orange line?

Am I misunderstanding the problem, or is there a mistake in the example?

Answer 1

The answer is "it depends" - if the diagram is about t-junctions then the middle red circle is probably intended to be a T-junction, in other words all three red circles are indeed collinear by definition. You are also right that it isn't a fully general solution.

You can take a look at TerrainLOD.ppt for a presentation-overview of some typical terrain rendering problems and common solutions.