Originally bombs and other ordnance didn't use shadow models at all. They were introduced for ordnance by DT. For a small bombs shadows are definitely not necessary, as they hardly will be ever noticed by player.
Shadow model usually is built from LOD+1 or LOD+2 model. All cavities on the shadow model should be covered with polygons, all internal parts removed.
Shadow model doesn't require any texture, therefore, alpha channel can not be used on shadow models. The material name for it should start with word "shadow". For example, you may have 2 materials: "shadow1" for 1-sided shadow parts and "shadow2" for 2-sided parts. Obviously, the 2-sided shadow should be used only on 0-thickness parts. On ground object shadow models all polygons, which are parallel to the ground and facing towards it, should be removed (as they won't be ever visible).
Building models do not require shadow models. They are generated automatically from building LODs by export tools.
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