There is a somewhat subtle aspect to this example. Recall that the TCS is executed once for each output vertex in the patch. Therefore, assuming that we are rendering cubic Bezier surfaces, this TCS will be executed 16 times for each patch. Each time it is executed, the value of depth will be slightly different because it is evaluated based on the z coordinate of the vertex. You might be wondering, which of the 16 possible different tessellation levels will be the one that is used? It doesn't make sense for the tessellation level to be interpolated across the parameter space. What's going on?
The gl_TessLevelInner and gl_TessLevelOuter output arrays are per-patch output variables. This means that only a single value will be ...
