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Schlick's approximation 173
Schlick's approximation
In 3D computer graphics, Schlick's approximation is a formula for approximating the bidirectional reflectance
distribution function (BRDF) of metallic surfaces. It was proposed by Christophe Schlick to approximate the
contributions of Fresnel terms in the specular reflection of light from conducting surfaces.
According to Schlick's model, the specular reflection coefficient R is given by
where is half the angle between the incoming and outgoing light directions, and is the reflectance at normal
incidence (i.e. the value of the Fresnel term when ).
References
• Schlick, C. (1994). "An Inexpensive BRDF Model for Physically-based Rendering". Computer Graphics Forum
13 (3): 233. doi:10.1111/1467-8659.1330233.
Screen Space Ambient Occlusion
Screen Space Ambient Occlusion (SSAO) is a rendering
technique for efficiently approximating the well-known computer
graphics ambient occlusion effect in real time. It was developed by
Vladimir Kajalin while working at Crytek and was used for the
first time in a video game in the 2007 PC game Crysis made by
Crytek.
Implementation
The algorithm is implemented as a pixel shader, analyzing the
SSAO component of a typical game scene.
scene depth buffer which is stored in a texture. For every pixel on the screen, the pixel shader samples the depth
values around the current pixel and tries to compute the amount of occlusion from each of the sampled points. In its
simplest implementation, the occlusion factor depends only on the depth difference between sampled point and
current point.
Without additional smart solutions, such a brute force method would require about 200 texture reads per pixel for
good visual quality. This is not acceptable for real-time rendering on modern graphics hardware. In order to get high
quality results with far fewer reads, sampling is performed using a randomly rotated kernel. The kernel orientation is
repeated every N screen pixels in order to have only high-frequency noise in the final picture. In the end this high
frequency noise is greatly removed by a NxN post-process blurring step taking into account depth discontinuities
(using methods such as comparing adjacent normals and depths). Such a solution allows a reduction in the number of
depth samples per pixel to about 16 or less while maintaining a high quality result, and allows the use of SSAO in
soft real-time applications like computer games.
Compared to other ambient occlusion solutions, SSAO has the following advantages:
• Independent from scene complexity.
• No data pre-processing needed, no loading time and no memory allocations in system memory.
• Works with dynamic scenes.
• Works in the same consistent way for every pixel on the screen.
• No CPU usage – it can be executed completely on the GPU.