I'm using Blender's internal renderer, trying to get some transparent shadow using a spot light with deep or irregular buffer switched on. Contrarily to this answer, the manual for 2.77 states this about the Irregular Buffer:
This method supports transparent shadows. To do so, you will first need to setup the shadow setting for the object which will receive the transparent shadow.
and this about the Deep Shadow Buffer:
Deep Shadow buffer supports transparency and better filtering, at the cost of more memory usage and processing time
(That last bit is even in the button's tooltip.)
Yet, when I map this png image onto a plane:
then switch my light to spot and the shadows to deep/irregular buffer, making sure that the plane underneath is set to receive transparent shadows, I do not get transparent shadows, but this:
So my question is, is there any extra step needed to get it to work, or is the feature broken, or am I missing something or what?