Yes there's the occasional gap in the render, these are places where the ray is refracted by the object, but not reflected. It is a bit like what happens if a prism or a lens changes the direction of a ray. This change of direction will cause it to miss the other opaque objects. There's no clear indication whether or not these rays should be opaque or not. We can introduce another parameter for the maximal ray deflection but this will introduce other artefacts like seams where the deflection just exceeds that limit. I don't know if there's a satisfactory solution to this problem.
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Roeland