Transfer-function-independent acceleration structure for volume rendering in virtual reality

Abstract

Visualizing volumetric medical datasets in a virtual reality environment enhances the sense of scale and has a wide range of applications in diagnostics, simulation, training, and surgical planning. To avoid motion sickness, rendering at the native refresh rate of the head-mounted display is important, and frame drops have to be avoided. Despite these strict requirements and the high computational complexity of direct volume rendering, it is feasible to provide a comfortable experience using volume ray casting on modern hardware. Many implementations use precomputed gradients or illumination to achieve the targeted frame rate, and most rely on acceleration structures, such as distance maps or octrees, to speed up the ray marching shader. With many of these techniques, the opacity of voxels is baked into the precomputed data, requiring a recomputation when the opacity changes. This makes it difficult to implement features that lead to a sudden change in voxel opacity, such as real-time transfer function editing, transparency masking, or toggling the visibility of segmented tissues. In this work, we present an empty space skipping technique using an octree that does not have to be recomputed when the transfer function is changed and performs well even when more complex transfer functions are used. We encode the content of the volume as bitfields in the octree and are able to skip empty areas, even with transfer functions that cannot efficiently be represented as a simple range of voxel values. We show that our approach allows arbitrarily editing of the transfer function in real-time while maintaining the target frame rate of 90 Hz.