@CONFERENCE\{IMM2004-03349,
    author       = "B. Jakobsen and N. J. Christensen and B. D. Larsen and K. S. Petersen",
    title        = "Boundary Correct Real-Time Soft Shadows",
    year         = "2004",
    booktitle    = "Computer Graphics International",
    volume       = "",
    series       = "",
    editor       = "",
    publisher    = "",
    organization = "",
    address      = "",
    url          = "http://www2.compute.dtu.dk/pubdb/pubs/3349-full.html",
    abstract     = "This paper describes a method to determine correct shadow
boundaries from an area light source using umbra and penumbra
volumes. The light source is approximated by a circular disk as
this gives a fast way to extrude the volumes. The method also
gives a crude estimate of the visibility of the area light source
as seen from a point in the shadow region. Instead of rendering
the volumes to the stencil buffer, we use an extended shadow map -
a so-called {D-}buffer, which among other things stores distances
from the center of the light source to the umbra and penumbra
volumes. The method is suited for implementation on most
programmable hardware. Though some crude approximations are used
in the visibility function, the method can be used to produce soft
shadows with correct boundaries in real time.",
    isbn_issn    = "0-7695-2171-1"
}