Capture to texture with transparent background

Hey,

I have jit.gl.nurbs object capturing to texture which I'm using later with jit.gl.videoplane. I'd like the entire background to be transparent, i.e. everything that comes out of jit.gl.nurbs should be 100% opaque. Right now I'm using alphaglue shader, but it generates alpha from luminosity, so it's no ideal. Is there any simpler way to do it?

Thanks,

zb