The Problem was, that our falsecolorshader uses a different type of Vertex (with other Information than Aardvark.Base.Rendering.Effects.Vertex). Therefore information was missing - in this case the SourceVertexIndex. Using the same Vertex type solved the problem.
Alternatively, add the SourceVertexIndex to your custom Vertex type - mixing and matching vertex types is OK as long as the inputs and outputs fit together
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