  We present an efficient raycasting algorithm for rendering Volumetric Depth
Images (VDIs), and we show how it can be used in a remote visualization setting
with VDIs generated and streamed from a remote server. VDIs are compact
view-dependent volume representations that enable interactive visualization of
large volumes at high frame rates by decoupling viewpoint changes from
expensive rendering calculations. However, current rendering approaches for
VDIs struggle with achieving interactive frame rates at high image resolutions.
Here, we exploit the properties of perspective projection to simplify
intersections of rays with the view-dependent frustums in a VDI and leverage
spatial smoothness in the volume data to minimize memory accesses. Benchmarks
show that responsive frame rates can be achieved close to the viewpoint of
generation for HD display resolutions, providing high-fidelity approximate
renderings of Gigabyte-sized volumes. We also propose a method to subsample the
VDI for preview rendering, maintaining high frame rates even for large
viewpoint deviations. We provide our implementation as an extension of an
established open-source visualization library.
