Question-and-Answer Resource for the Building Energy Modeling Community
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Nathaniel's answer is very detailed and accurate. I would like to add some details on the _overture calculation_. The overture calculation is used to populate the ambient cache only. As such, the rendering size may be extremely small (50px square). We don't care about the output rendering from this pass (in fact, it can be immediately thrown away, or if on unix, simply piped to /dev/null), it's purely for some rays to get traced through the scene to generate some values for the ambient cache. Be sure to save an ambient cache, and to refer to it on the subsequent run!
