Thanks Hans
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Sweet Home 3D is generally unable to compute the shadow where there are too many trees. I didn't find any algorithm who could help me improve this yet. 
Emmanuel, rather than compute the triangles for the simple, second setting, built in render, why not just ignore the triangles whenever a tree is identified in a scene, instead, the trees X & Y size can be used to scale a simple rough edge png shape (like the example (complete with alpha channel) png below) to provide a simple shadow.
db4tech
