Research

The Alchemy Screen-space Ambient Obscurance Algorithm

"The Alchemy Screen-space Ambient Obscurance Algorithm"
Morgan McGuire (NVIDIA), Brian Osman (Vicarious Visions), Michael Bukowski (Vicarious Visions), Padraic Hennessy (Vicarious Visions), in High-Performance Graphics 2011, August 2011
Research Area: 3D Graphics
Author(s): Morgan McGuire (NVIDIA), Brian Osman (Vicarious Visions), Michael Bukowski (Vicarious Visions), Padraic Hennessy (Vicarious Visions)
Date: August 2011
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Abstract:

Ambient obscurance (AO) produces perceptually important illumination effects such as darkened corners, cracks, and wrinkles; proximity darkening; and contact shadows. We present the AO algorithm from the Alchemy engine used at Vicarious Visions in commercial games. It is based on a new derivation of screen-space obscurance for robustness, and the insight that a falloff function can cancel terms in a visibility integral to favor efficient operations. Alchemy creates contact shadows that conform to surfaces, captures obscurance from geometry of varying scale, and provides four intuitive appearance parameters: world-space radius and bias, and aesthetic intensity and contrast. The algorithm estimates obscurance at a pixel from sample points read from depth and normal buffers. It processes dynamic scenes at HD 720p resolution in about 4.5 ms on Xbox 360 and 3 ms on NVIDIA GeForce580.