  We propose a volumetric representation based on primitives to model
scattering and emissive media. Accurate scene representations enabling
efficient rendering are essential for many computer graphics applications.
General and unified representations that can handle surface and volume-based
representations simultaneously, allowing for physically accurate modeling,
remain a research challenge. Inspired by recent methods for scene
reconstruction that leverage mixtures of 3D Gaussians to model radiance fields,
we formalize and generalize the modeling of scattering and emissive media using
mixtures of simple kernel-based volumetric primitives. We introduce closed-form
solutions for transmittance and free-flight distance sampling for 3D Gaussian
kernels, and propose several optimizations to use our method efficiently within
any off-the-shelf volumetric path tracer by leveraging ray tracing for
efficiently querying the medium. We demonstrate our method as an alternative to
other forms of volume modeling (e.g. voxel grid-based representations) for
forward and inverse rendering of scattering media. Furthermore, we adapt our
method to the problem of radiance field optimization and rendering, and
demonstrate comparable performance to the state of the art, while providing
additional flexibility in terms of performance and usability.
