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I believe I already know the answer to this question, but I want to make sure I'm understanding this correctly. Within a physically based rendering pipeline, you calculate your indirect lighting from an hdri image (which is transformed into an irradiance map in engine (from what I can tell so far)).

Therefore if one had a scene which was outside for example and you were using some form of daytime sky hdri, but then you transitioned into a cave where there's very little or no light, you would then need a separate irradiance map for this location as well, correct?

Or if you wanted to emulate complete darkness, use a solid black irradiance map (or simply a shader that uses black for what would have been the calculated value of the irradiance map) and have your direct lighting (point lights, area lights, etc) be your only source of light?

Do I seem to be grasping the concept or am I missing something?

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The answer to your first question is yes.

Your second question is a little vague. Yes, they would be the only source of light, but them being the only source does not solve the use case for the irradiance map. Irradiance map is needed for accounting ambient light. In a cave setting, ray would bounce around and create ambient light. Thus you might still need the irradiance light to stimulate the effect of ray bouncing around creating the ambient light. I would say anytime the ambient light has a significant role for the scene, irradiance map would be something good to have.

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