I've been looking through some of the lighting threads, and it seems like there is conflicting information. Should you use IBL if you are using IDL?

edit: I'm using P8 if it matters

IBL is OK to use, that is the first answer, but you did not ask if it was OK. You asked if you should. There is no reason to say you should use them together. You can use them together. I do sometimes use them together. IBL is a way to add environmental lighting to the scene so that it contributes non uniform lighting information. This looks more realistic. But you can also add environmental lighting with an environment sphere or environment box, and use no lights at all. You can also have an interior scene with an actual room all around, in which case secondary lighting will result from bounced light of the interior light sources. In any case, if you have a scene taking place inside a closed surface, whether that surface is a room, or a bunch of boxes, or an environment sphere, then IBL will not contribute to the lighting of that interior, unless you also make the surrounding geometry partially transparent. Think about it this way. When IDL is enabled, IBL is effectively a giant sphere of light surrounding your scene. If other things block that light, then it has no purpose and does not matter. If scene objects do not surround your subject,, then that IBL will contribute.

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) ?

enabling IDL causes FFRender to calculate the occlusion shadows (previously faked with IBL/AO) and diffuse bounces (not easy to fake with IBL/AO).  my vote is to skip IBL and use bill's envsphere with hdri.

So, when using IDL and an enclosed space (which from what I've read, you always should do for best light reflection and bounces) IBL is either unnessissary or over kill.

Thanks.

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.

Quote - So, when using IDL and an enclosed space (which from what I've read, you always should do for best light reflection and bounces) IBL is either unnessissary or over kill.

 

Thanks.

I don't agree that you must always use an enclosed space. My point was you can provide lighting information, recorded as an image taken from an actual real-world scene, and fill the blank spots of a non-enclosed space. This is what IBL is for - filling in the blanks of your scene.

I have done some great portraits with no enclosed space at all - just a floor and one wall, and some IBL to supply the rest.

As well, people believe that all scenes have light bouncing from everywhere and therefore require a fully enclosed scene. This is hardly true. 

Outdoors - what is the enclosing surface at the beach? Not the sky - it does not enclose in the sense used here. It does not bounce light. It supplies light - period. In fact, it is the second most dominant light source in a beach scene, second only to the sun itself. Nothing from the ground or the people bounces up and reflects back from the sky. The sky is a light source - purely - no bouncing. 

In such cases, an IBL can supply the sky lighting just fine.