A Directional Occlusion Shading Model for Interactive Direct Volume Rendering

Volumetric rendering is widely used to examine 3D scalar fields from scanners and direct numerical simulation datasets. One key aspect of volumetric rendering is the ability to provide shading cues to aid in understanding structure contained in the datasets. While shading models that reproduce natural lighting conditions have been shown to better convey depth information and spatial relationships, they traditionally require considerable (pre-)computation. In this paper, we propose a novel shading model for interactive direct volume rendering that provides perceptual cues similar to that of ambient occlusion, for both solid and transparent surface-like features. An image space occlusion factor is derived from the radiative transport equation based on a specialized phase function. Our method does not rely on any precomputation and thus allows for interactive explorations of volumetric data sets via on-the-fly editing of the shading model parameters or (multi-dimensional) transfer functions. Unlike ambient occlusion methods, modifications to the volume, such as clipping planes or changes to the transfer function, are incorporated into the resulting occlusion-based shading. Scientific Computing and Imaging Institute, University of Utah Technical Report No UUSCI-2008-009 A Directional Occlusion Shading Model for Interactive Direct Volume Rendering Mathias Schott1, Vincent Pegoraro1, Charles Hansen1, Kévin Boulanger2, Josh Stratton1, Kadi Bouatouch2 1Scientific Computing and Imaging Institute, School of Computing, University of Utah, USA 2IRISA, Université de Rennes I, France Figure 1: From left to right: a) Visible male data set with occlusion of solid and transparent materials (3.4 FPS, 996 slices) b) CT scan of an engine block where a clipping plane was used to show the exhaust port (13.3 FPS, 679 slices) c) Bonsai data set of which complex features are exposed by our ambient occlusion approximation (4.1 FPS, 1492 slices) Abstract Volumetric rendering is widely used to examine 3D scalar fields from scanners and direct numerical simulation datasets. One key aspect of volumetric rendering is the ability to provide shading cues to aid in understanding structure contained in the datasets. While shading models that reproduce natural lighting conditions have been shown to better convey depth information and spatial relationships, they traditionally require considerable (pre-)computation. In this paper, we propose a novel shading model for interactive direct volume rendering that provides perceptual cues similar to that of ambient occlusion, for both solid and transparent surface-like features. An image space occlusion factor is derived from the radiative transport equation based on a specialized phase function. Our method does not rely on any precomputation and thus allows for interactive explorations of volumetric data sets via on-the-fly editing of the shading model parameters or (multi-dimensional) transfer functions. Unlike ambient occlusion methods, modifications to the volume, such as clipping planes or changes to the transfer function, are incorporated into the resulting occlusion-based shading.Volumetric rendering is widely used to examine 3D scalar fields from scanners and direct numerical simulation datasets. One key aspect of volumetric rendering is the ability to provide shading cues to aid in understanding structure contained in the datasets. While shading models that reproduce natural lighting conditions have been shown to better convey depth information and spatial relationships, they traditionally require considerable (pre-)computation. In this paper, we propose a novel shading model for interactive direct volume rendering that provides perceptual cues similar to that of ambient occlusion, for both solid and transparent surface-like features. An image space occlusion factor is derived from the radiative transport equation based on a specialized phase function. Our method does not rely on any precomputation and thus allows for interactive explorations of volumetric data sets via on-the-fly editing of the shading model parameters or (multi-dimensional) transfer functions. Unlike ambient occlusion methods, modifications to the volume, such as clipping planes or changes to the transfer function, are incorporated into the resulting occlusion-based shading.