  New web technologies have enabled the deployment of powerful GPU-based
computational pipelines that run entirely in the web browser, opening a new
frontier for accessible scientific visualization applications. However, these
new capabilities do not address the memory constraints of lightweight end-user
devices encountered when attempting to visualize the massive data sets produced
by today's simulations and data acquisition systems. In this paper, we propose
a novel implicit isosurface rendering algorithm for interactive visualization
of massive volumes within a small memory footprint. We achieve this by
progressively traversing a wavefront of rays through the volume and
decompressing blocks of the data on-demand to perform implicit ray-isosurface
intersections. The progressively rendered surface is displayed after each pass
to improve interactivity. Furthermore, to accelerate rendering and increase GPU
utilization, we introduce speculative ray-block intersection into our
algorithm, where additional blocks are traversed and intersected speculatively
along rays as other rays terminate to exploit additional parallelism in the
workload. Our entire pipeline is run in parallel on the GPU to leverage the
parallel computing power that is available even on lightweight end-user
devices. We compare our algorithm to the state of the art in low-overhead
isosurface extraction and demonstrate that it achieves 1.7x-5.7x reductions in
memory overhead and up to 8.4x reductions in data decompressed.
