Quote - What does IDL contribute, then, if IBL is in use and contributing to the scene (ie as you say, there are no other scene objects blocking the light) ?

Miss Nancy has given the answer, but I'd suggest that you should try it.

It is astonishing how much the floor contributes light differently with IDL versus without it. 

The real point is not to ask what IDL contributes, but rather what does IBL contribute. There are few cases where IDL needs IBL, whereas almost every case where IBL needs IDL.

IBL is just a source of lighting information.

IDL is the accurate use of that lighting information, in context with the objects in the scene.

Whether or not you use IBL as a source of lighting info, IDL is how you use it right.

You can also use infinites, and spots, and point lights, more accurately with IDL than without it.

IBL is just one of the tools in your lighting source tool kit. 

IDL is how light actually gets processed.

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