Alpha Map Shadows
 

How do you get an alpha map to cast shadows? I experimented with an alpha tree. I put the alpha map on the transparency but the whole plane casts the shadow instead of the tree. Even though only the tree shows in the render.

This my friend is what we call a gobo, or a cocoloris. It stems from theatre and to do it in maya i just map my alpha channel to the color attribute of my spot lights. You can centre, adjust and repeat the image in the 2d texture placement node as expected but remember to set the default color appropriately.

You dont actually need any shadow calculations involved at all.