what you want is a shadow catcher, which we don’t have (yet) in Cycles. To effectively cut out the rendered image, you need to assign a holdout shader to the object - but you won’t have the shadows. You can get them using the shadow pass on another renderlayer, like Richard said.
For the shadows, you can render the scene (or just the eyes using the render layers) with a complete white diffuse shader, and use the render as alpha.
Do you mean i have to add a diffuse shader to the node (i posted above)? With what am i supposed to connect that diffuse shader? Sorry, i’m a noob in those things.
two render layers, one for the solid surface and the other for the transparency. In the composite, I mixed the luminance from the second layer with the alpha from the first.
Blender’s UV/Image Editor. Use the file output node (in the compositing context of the node editor) to save the shadow pass as separate pngs, then mix them (also in the compositor) on top of your regular render frames.
As for the curve, it seems you just need to invert the endpoints’ values.
Tell me what you don’t understand, I will explain more in detail. Do you know how to use the compositor ? The video you linked uses Blender Internal, it’s not really reproducible with Cycles unfortunately.