  We present a voxel-based rendering pipeline for large 3D line sets that
employs GPU ray-casting to achieve scalable rendering including transparency
and global illumination effects that cannot be achieved with GPU rasterization.
Even for opaque lines we demonstrate superior rendering performance compared to
GPU rasterization of lines, and when transparency is used we can interactively
render large amounts of lines that are infeasible to be rendered via
rasterization. To achieve this, we propose a direction-preserving encoding of
lines into a regular voxel grid, along with the quantization of directions
using face-to-face connectivity in this grid. On the regular grid structure,
parallel GPU ray-casting is used to determine visible fragments in correct
visibility order. To enable interactive rendering of global illumination
effects like low-frequency shadows and ambient occlusions, illumination
simulation is performed during ray-casting on a level-of-detail (LoD) line
representation that considers the number of lines and their lengths per voxel.
In this way we can render effects which are very difficult to render via GPU
rasterization. A detailed performance and quality evaluation compares our
approach to rasterization-based rendering of lines.
