ParticleFieldKernelGaussianSurfletAPI
Defines the Gaussian surflet kernel for a ParticleField.
An un-transformed kernel (i.e. identity position, scale, rotation, opacity)
will define opacity at point ‘p’ on the XY plane by
g(u=0;o=1;x = p.length()), with opacity off the XY-plane defined as 0.
Note that since the standard deviation is 1, the 3-sigma point is 3.0
and 99.7% of the splat support is within a circular disk on the XY plane
of radius 3.
Per-splat opacity is multiplicative with the Gaussian falloff; rotation and scale will transform the Gaussian disk kernel into a planar ellipse, and position moves the per-splat peak falloff from the origin.