we present a novel contact-hardening shadow (CHS) approach
based on the AMD CHS sample. Our method is optimized for Shader Model 5.0
and is capable of rendering high-quality large shadow penumbras at a relatively
low cost. Section 1.4 describes our approach toward lit transparent particles
rendering.
Compute shaders (CSs) are a relatively new feature in graphics APIs, introduced
first in the DirectX 11 API. We have been able to gain substantial benefits
for postprocessing using CSs. We expound upon our experience with CSs in
Section 1.5.

Contact-hardening shadows (CHSs), similar to percentage-closer soft shadows
(PCSSs) [Fernando 05], are a shadow-mapping method to simulate the dynamic
shadows from area lights. The achieved effect is a sharper shadow as the shadow
caster and the receiver are closer to each other and a blurrier (softer) shadow as
the caster and the receiver are farther from each other (see Figure 1.10). The
implementation in Thief is based on the method from the AMD SDK [Gruen 10].
This method is easy to integrate because it uses the shadow map generated
by a single light source and can just replace ordinary shadow filtering. One of the
main drawbacks in this technique is the extensive texture fetching, and in Thief
we implemented an optimized method for Shader Model 5.0 that drastically limits
the access to the shadow map. The CHS process is divided into three steps, which
are the blocker search, the penumbra estimation, and the filtering.

[Gruen 10] Holger Gruen. “Contact Hardening Shadows 11.” AMD Radeon
SDK, http://developer.amd.com/tools-and-sdks/gr...cs-development/
amd-radeon-sdk/archive/, 2010.

This sample, contributed by AMD, presents a Direct3D 11 / Shader Model 5.0 accelerated technique that uses non-stationary / dynamic weight matrices for unique per-sample filter weights (these don't necessarily represent a separable filter) for a contact hardening shadow effect. The effect is comparable to PCSS (Percentage-Closer Soft Shadows [Fer05]) but does not suffer from banding or noise artifacts.

Introduction
Most games make use of uniform shadow filters that don't produce physically plausible shadows. This means, for example, that the blurriness of shadows does not depend on the distance between shadow caster and shadow receiver.

In order to achieve physically plausible shadows, PCSS (Percentage Closer Soft Shadows [Fer05]) were introduced as a real-time method to achieve contact hardening shadows. Typical implementations of PCSS suffer from noise and banding artifacts that result from the use of a Poisson disk of samples.

This SDK sample uses a large box (12Ч12) of shadow map samples in combination with a non-stationary filter weights matrix to achieve a transition from sharp to soft shadows. The sample accepts the fact that one needs to limit the size of the light source in order to achieve high quality results. Since the sun can usually be treated as a relatively small light source, the technique works well for a directional light.

The Shader Model 5.0 instructions GatherRed()and GatherCmpRed()are used to accelerate the computation of average-blocker-depth and to accelerate the non-stationary and non-separable dynamic filter operation.

Blocker Search
The search for blockers for a NЧN shadow map filter footprint is carried out using only (N/2)Ч(N/2) GatherRed() operations. The shader in the sample performs 36 of these instructions. A Shader Model 4.0 implementation would need to perform 144 point samples in order to obtain the same information.

Filtering with a Dynamic Filter Matrix
Based on the average blocker depth and the size of the light source, a factor between 0.0 (sharp) to 1.0 (totally blurry) is computed. This factor is used to compute a dynamic weight matrix that results from feeding four matrices into a cubic Bezier function. The math from [GS09] is used to compute the filter, reducing the necessary ALU through the use of GatherCmpRed().

References
[GS09] Gruen Holger, Story Jon, "Taking Advantage of Direct3D 10.1 to Accelerate Performance and Enhance Quality," AMD sponsored session at Eurographics 09, available online at http://developer.amd.com/gpu_assets/Taking...fDirect3D10.pps

[Fer05] Fernando Randima, ”Percentage-closer soft shadows”, July 2005, SIGGRAPH '05: SIGGRAPH 2005 Sketches