How can I make a shadow disappear in Cycles? I'm doing an acetate animation effect: Text on 'transparent plastic' lays down on a white surface, and as the text lays down its shadow, from a sun lamp, plays on the white surface.

All achieved. A subdivided plane, cloth sim, UV text, the usual diff/transparency/factor node set up.

The problem is I want there to be no residual shadow once the text has hit the white base plane. And the cloth sim won't let me hit the base plane, due to the cloth sim tolerances. It floats just above the base plane.

So I'm looking for a way to 'fade' this thin drop-shadow away in the compositor.

I can't change the world surface node colour (which gives the shadow colour) from black to white: that affects the ambient light of the entire scene. I can't raise or lower the sun lamp strength for the same reason.

I guess I'm looking for more info on the shadow pass... I'm not sure how to use it in this example. I'm looking for a parameter that allows me to fade out the lamp's ability to cast a shadow... or to fade out the ability of the base plane to receive a shadow. Any tips?

Blend file: https://dl.dropboxusercontent.com/u/13900881/flop2.blend
This affects the object casting the sadow not the lamp.

A slightly simpler solution exists: you can set the object you want not to cast shadows not to interact with shadow rays. You can then render your animation with and without the shadow. Finally you mix the two animations to control the fading.

The parameters are in Object > Ray Visibility > Shadow. See this qustion for more info about this:

How to make a cycles light emisson object invisible to the camera?