====== SkyDome ======

A scene node that models a uniform (isotropic) overcast sky hemisphere as 72 directional light sources arranged in 6 rings × 12 azimuths.

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The ring geometry and per-ring solid-angle weights follow the Turtle sky discretization used in the model (Buck-Sorlin &  Henke, 2009/2013).  

For each ring($r$) the power density of each of the 12 constituent directional lights is:

$$ powerDensity_i = totalPower * RING_{FRACTION}[r] / sin(elevation[r]) $$

So that the sum over all 72 lights equals the totalPower (total diffuse horizontal irradiance).