Extinction-Optimized Volume Illumination

Abstract

We present a novel method to optimize the attenuation of light for the single scattering model in direct volume rendering. A common problem of single scattering is the high dynamic range between lit and shadowed regions due to the exponential attenuation of light along a ray. Moreover, light is often attenuated too strong between a sample point and the camera, hampering the visibility of important features. Our algorithm employs an importance function to selectively illuminate important structures and make them visible from the camera. With the importance function, more light can be transmitted to the features of interest, while contextual structures cast shadows which provide visual cues for perception of depth. At the same time, more scattered light is transmitted from the sample point to the camera to improve the primary visibility of important features. We formulate a minimization problem that automatically determines the extinction along a view or shadow ray to obtain a good balance between sufficient transmittance and attenuation. In contrast to previous approaches, we do not require a computationally expensive solution of a global optimization, but instead provide a closed-form solution for each sampled extinction value along a view or shadow ray and thus achieve interactive performance.

Decoupled Opacity Optimization for Points, Lines and Surfaces

“…First, the squared sum of all importance values g all is computed, which is used in the second pass to compute the squared sum of importance values g b behind the current fragment. (Pseudo code adapted from [AZD16].) self-occlusion handling: off self-occlusion handling: on Figure 5: In the CTBL data set, each geometry is represented as one segment.…”

“…Ament et al [AZD16] extended opacity optimization to volume data, naming it extinction optimization. They not only optimized the extinction along view rays, but also along shadow rays.…”

“…Analytic Solution. Without a smoothness term, the opacity values of the individual fragments are independent of each other [AZD16]. Hence, the energy can be rearranged to…”

“…At line crossings, however, the occlusion terms will reduce the opacities of the fragments in the linked list, which creates unwanted discontinuities. Smoothing the importance [AZD16] across the geometry does not remove this artifact, since the opacity in pixels with a single fragment is not affected. Instead, we smooth the opacities across the geometry, for which we use the precomputed object-space discretization.…”

“…Thus, different geometry types were not easy to combine in practice. Recently, Ament et al [AZD16] extended opacity optimization to volume data by minimizing the energy in ray space (per pixel) and reformulating the objective function. They had the insight that when removing the smoothness term, the optimal solution can be calculated analytically.…”