trying to make fake shadow with RTT texture applied to a mesh

[MichaelJPCT]

i am trying to make fake shadow. my approach is like this:
stick a transparent mesh on an object3D which should be a shadow receiver, but in fact the transparent mesh is the receiver;
render the shadow caster object into a texture and project the texture onto the transparent mesh;
the RTT should have alpha information - the part covered by shadow caster is opaque and black, other part is fully transparent.

now i wonder how to render into the alpha channel of RTT texture - i want to use openGL hardware framebuffer for the RTT task (faster) so the texture should be directly used in GPU (not downloaded for further process).
is setAsShadowMap() meant for this?

[EgonOlsen]

I'm not sure how this is supposed to work!? Do you have a drawing that illustrates it? Concerning the alpha channel...it might be a better idea to just render into the texture and then apply the projected result with a shader instead of relying on the alpha. setAsShadowMap() makes the texture act as a depth map. This is needed if you want to implement shadow mapping without using the ShadowHelper, but in your case, you don't seem to rely on depth at all, so I guess that you don't need it.

[MichaelJPCT]

my approach is just using projective texture, except the receiver is a seperate small simple  object3d instead of a large complex object3d.
i found i could clear the alpha channel of rtt by a full window quad mesh with shader: gl_fragcolor=vec4(0,0,0,0)
then i can draw opaque object in the rtt.
using shader to render the receiver without alpha information can work too.