Extinction-Based Shading and Illumination in GPU Volume Ray-Casting

Abstract

Direct volume rendering has become a popular method for visualizing volumetric datasets. Even though computers are continually getting faster, it remains a challenge to incorporate sophisticated illumination models into direct volume rendering while maintaining interactive frame rates. In this paper, we present a novel approach for advanced illumination in direct volume rendering based on GPU ray-casting. Our approach features directional soft shadows taking scattering into account, ambient occlusion and color bleeding effects while achieving very competitive frame rates. In particular, multiple dynamic lights and interactive transfer function changes are fully supported. Commonly, direct volume rendering is based on a very simplified discrete version of the original volume rendering integral, including the development of the original exponential extinction into a-blending. In contrast to a-blending forming a product when sampling along a ray, the original exponential extinction coefficient is an integral and its discretization a Riemann sum. The fact that it is a sum can cleverly be exploited to implement volume lighting effects, i.e. soft directional shadows, ambient occlusion and color bleeding. We will show how this can be achieved and how it can be implemented on the GPU.

Correlated Photon Mapping for Interactive Global Illumination of Time-Varying Volumetric Data

“…However, these methods do not incorporate physically based global illumination effects. Schlegel et al [35] used summed area tables (SATs) to achieve interactive dynamic complex lighting. Ament et al [3] also utilize SATs with different sampling patterns to support coherent calculation of up to six orthodirectional light sources.…”

Fast global illumination for interactive volume visualization

“…Object space occlusion, for example [Kronander et al 2012], offers better quality while the cost of visibility estimation is still high. Summed area table (SAT) based techniques [Díaz et al 2010;Schlegel et al 2011] provide an efficient way to integrate occlusion/distinction in certain volume regions and can render global shadows. But the simplified approximation cannot produce good multiple scattering effects such that the overall shading quality is limited.…”

“…Many recent volume rendering techniques are also based on such framework (e.g. [Schlegel et al 2011;Kronander et al 2012]). Therefore, we also follow this strategy to derive a novel PDE-based volume illumination method.…”

“…Most real-time techniques try to reduce the cost of voxel radiance estimation by using simplified approximations (e.g. [Schlegel et al 2011]). Although such methods can produce smooth shadows, the shading quality and realism are limited without accurate and comprehensive light simulations.…”

Enhancing Salient Features in Volumetric Data Using Illumination and Transfer Functions

“…One example is the summed area table-based technique presented by Schlegel et al [63]. They precompute a summed area table which is co-registered with the volume data and use it to improve the performance of estimating the attenuation of light during rendering.…”