[exDM69]

You should understand that conceptually stencil-then-cover is the point-in-poly algorithm using line segment intersections. It's just that the GPU triangle rasterizer solves the line equations (or half plane equations) and accumulates the per-pixel result in the stencil buffer.

The Doom 3 shadow volume algorithm is just the same, extended into 3d and using a little bit of depth buffer and front/back face culling to make some edge cases work (self-shadowing, shadow volume caps).

If you want to go read the original paper (please share a link!), you can circumvent the paywall with sci-hub.io if you don't mind a little piracy.