Attached Link: http://www.renderosity.com/messages.ez?Form.ShowMessage=1700160

See attached link (Falcon's leaf technique). You should note that using the term "alpha map" is a misnomer, since it's usually used to describe a mask or channel in a 2D image, as used in Photoshop et al. In a 3D app like Carrara, it's called a transparency map, or opacity map. See also the original description of Carrara transparency by Antoine Clappier:

http://www.renderosity.com/messages.ez?Form.ShowMessage=547974