Editing Shadow volume (section)

=== Depth pass ===

Heidmann proposed that if the front surfaces and back surfaces of the shadows were rendered in separate passes, the number of front faces and back faces in front of an object can be counted using the stencil buffer. If an object's surface is in shadow, there will be more front facing shadow surfaces between it and the eye than back facing shadow surfaces. If their numbers are equal, however, the surface of the object is not in shadow. The generation of the stencil mask works as follows:

# Disable writes to the [[Z-buffering|depth]] and color buffers.
# Use [[back-face culling]].
# Set the stencil operation to increment on depth pass (only count shadows in front of the object).
# Render the shadow volumes (because of culling, only their front faces are rendered).
# Use front-face culling.
# Set the stencil operation to decrement on depth pass.
# Render the shadow volumes (only their back faces are rendered).

According to [[Euler characteristic]], all lit surfaces will correspond to a 0 in the stencil buffer, where the numbers of front and back surfaces of all closed manifolds (shadow volumes) are equal.

This approach has problems when the eye itself is inside a shadow volume (for example, when the light source moves behind an object). From this point of view, the eye sees the back face of this shadow volume before anything else, and this adds a −1 bias to the entire stencil buffer, effectively inverting the shadows. This can be remedied by adding a "cap" surface to the front of the shadow volume facing the eye, such as at the front [[clipping plane]]. There is another situation where the eye may be in the shadow of a volume cast by an object behind the camera, which also has to be capped somehow to prevent a similar problem. In most common implementations, because properly capping for depth-pass can be difficult to accomplish, the depth-fail method (see below) may be licensed for these special situations. Alternatively one can give the stencil buffer a +1 bias for every shadow volume the camera is inside, though doing the detection can be slow.

There is another potential problem if the stencil buffer does not have enough bits to accommodate the number of shadows visible between the eye and the object surface, because it uses [[saturation arithmetic]]. (If they used [[arithmetic overflow]] instead, the problem would be insignificant.)

Depth pass testing is also known as '''z-pass''' testing, as the [[depth buffer]] is often referred to as the z-buffer.