  Direct Volume Rendering (DVR) using Volumetric Path Tracing (VPT) is a
scientific visualization technique that simulates light transport with objects'
matter using physically-based lighting models. Monte Carlo (MC) path tracing is
often used with surface models, yet its application for volumetric models is
difficult due to the complexity of integrating MC light-paths in volumetric
media with none or smooth material boundaries. Moreover, auxiliary
geometry-buffers (G-buffers) produced for volumes are typically very noisy,
failing to guide image denoisers relying on that information to preserve image
details. This makes existing real-time denoisers, which take noise-free
G-buffers as their input, less effective when denoising VPT images. We propose
the necessary modifications to an image-based denoiser previously used when
rendering surface models, and demonstrate effective denoising of VPT images. In
particular, our denoising exploits temporal coherence between frames, without
relying on noise-free G-buffers, which has been a common assumption of existing
denoisers for surface-models. Our technique preserves high-frequency details
through a weighted recursive least squares that handles heterogeneous noise for
volumetric models. We show for various real data sets that our method improves
the visual fidelity and temporal stability of VPT during classic DVR operations
such as camera movements, modifications of the light sources, and editions to
the volume transfer function.
