Shadow Volumes and Silhouette

OK… I have my z-fail algorithm and i can cast shadow volumes. but i cast them for every single triangle.

lets say i have a polygon list with the polygons(triangles) that cast shadows. how i can compute the silhouete for the shadows volume?
thanks in advance!

You do the dot product between the normalised light vector and the normal of each tri in your mesh. Faces that have a dot >= 0 are facing towards the light, faces with a dot < 0 are facing away from the light, the edges that separate tris that are towards and tris that are away is your silhouette.