Particle volume rendering becomes very slow for nonuniform particles #593
Description
We use OSPRay version 2.10. The purpose is visualizing SPH based fluid simulation results, so we import the particles as a particle volume and specify an isosurface as the fluid surface. When we use fixed size of particles, the rendering is good. When we modulate the particle radii with a variable, the raytracer becomes very slow, even though the surface is set to be opaque. For example, in a dataset, the average particle size is 2e-3, while the smallest size is 1e-8. In such a situation, it even hangs. Filtering out the small particles is helpful, but it is hard to set a perfect threshold.