Shadow maps – Casting shadows on transparent plane

Hi,

I need to cast shadows on a transparent plane in order to get only the shadow shown and not the material applied to the plane (I want it fully transparent). Is this possible in three.js? I am using THREE.SpotLight as light source.

As a workaround solution I think I could apply a fully transparent texture map as a material and see if works as expected but was wondering if I could manage to get this effect just tweaking standard material properties.

Thanks