By Matthew Fife, Filip Strugar, published on March 27, 2013 

Original link is here: https://software.intel.com/en-us/blogs/2013/03/27/adaptive-volumetric-shadow-maps

Internet Wayback Machine link in case that breaks:
https://web.archive.org/web/20131227104350/https://software.intel.com/en-us/blogs/2013/03/27/adaptive-volumetric-shadow-maps

Adaptive Volumetric Shadow Maps use a new technique that allows a developer to generate real-time adaptively sampled shadow representations for transmittance through volumetric media.  This new data structure and technique allows for generation of dynamic, self-shadowed volumetric media in real-time rendering engines using today’s Microsoft DirectX* 11 hardware.

Each texel of this new kind of shadow map stores a compact approximation to the transmittance curve along the corresponding light ray.  The main innovation of the AVSM technique is a new streaming compression algorithm that is capable of building a constant-storage, variable-error representation of a visibility curve that represents the light ray’s travel through the media that can be used in later shadow lookups.

Another exciting part of this sample is the use of a new pixel shader ordering feature found in new Intel® Iris™ and Intel® Iris™ Pro graphics hardware. Two implementations are provided in this sample – a standard DirectX implementation, and an implementation that utilizes this new graphics hardware feature.

This sample was authored by Matt Fife (Intel), Filip Strugar (Intel) with contributions from Leigh Davies (Intel), based on an original article and samplet from Marco Salvi (Intel).

Adaptive Volumetric Shadow Maps, EGSR 2010 paper by Marco Salvi.