Interactive Volume Rendering with Dynamic Ambient Occlusion and Color Bleeding

Abstract

We propose a method for rendering volumetric data sets at interactive frame rates while supporting dynamic ambient occlusion as well as an approximation to color bleeding. In contrast to ambient occlusion approaches for polygonal data, techniques for volumetric data sets have to face additional challenges, since by changing rendering parameters, such as the transfer function or the thresholding, the structure of the data set and thus the light interactions may vary drastically. Therefore, during a preprocessing step which is independent of the rendering parameters we capture light interactions for all combinations of structures extractable from a volumetric data set. In order to compute the light interactions between the different structures, we combine this preprocessed information during rendering based on the rendering parameters defined interactively by the user. Thus our method supports interactive exploration of a volumetric data set but still gives the user control over the most important rendering parameters. For instance, if the user alters the transfer function to extract different structures from a volumetric data set the light interactions between the extracted structures are captured in the rendering while still allowing interactive frame rates. Compared to known local illumination models for volume rendering our method does not introduce any substantial rendering overhead and can be integrated easily into existing volume rendering applications. In this paper we will explain our approach, discuss the implications for interactive volume rendering and present the achieved results.

Volumetric ambient occlusion for volumetric models

“…This limitation is overcome by Ropinski et al [11] and Hernell et al [3], using a local volumetric shadowing effect. Ropinski et al compute for each voxel a local histogram from the voxel's neighbourhood, by accumulating intensities weighted by inverse squared distances.…”

Image Plane Sweep Volume Illumination

“…The first occurrences of this concept had the goal to improve the spatial comprehension of isosurfaces [1,24,38]. Ropinski et al [32] exploit an expensive pre-processing which involves computation and clustering of local histograms, which could have been accelerated later on [22]. Diaz et al [11] have adopted screen-space ambient occlusion techniques within their vicinity occlusion map approach.…”

“…The approaches tackling this area can be roughly divided into two groups. First, approaches exploiting volume preprocessing, which can be done either on the CPU [32] or, for increased performance, on the GPU [14]. Techniques belonging to this group precompute the illumination information and store it in an additional volume.…”

Viewpoint information channel for illustrative volume rendering

“…Wyman et al [28] presented a method that supports the simulation of direct lighting, shadows and interreflections by storing pre-computed global illumination in an additional volume to allow viewpoint, lighting and isovalue changes. Ropinski et al [18] computed a local histogram for each voxel from the voxel neighbourhood, by accumulating intensities weighted by inverse squared distances. These local histograms can be combined interactively with the user-defined transfer function to give an effect similar to local ambient lighting.…”