UPDATE: This video is quite old now, it will still work, but for even better results and the ability to render out with an alpha, you can grab a much more advanced version from my website at:
[ Ссылка ]
Regarding the method shown in this video I did forgot to mention how to control the transparency of the shadow. To do this, on the colour ramp that's connected to the shader to rgb node, change the left colour from black to a gray.
