Cesium: Day-night lighting for models

[lilleyse]

Models are lit based on the sun direction but there's no recognition of a day-night cycle. Two of the problems are:

• The sun basically shines through the ground during night, lighting the bottom-facing part of the model
• The ambient lighting doesn't adjust to time of day.

Should there be a day-night lighting option on Models/3D Tiles?

Requested on the forum: https://groups.google.com/forum/#!topic/cesium-dev/8Y2lHMgJZVM.

[laurensdijkstra]

Considering glTF 2.0 with physically based rendering shaders it would be very logical to implement the lighting in a more physically based way as well IMO.

[emackey]

There's some chance I'll be looking into this in the next couple of weeks.