ParticleFieldKernelConstantSurfletAPI
Defines the constant 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 as 1.0 if p.length <= 1,
and 0.0 otherwise. The splat support for this kernel is a bounded circular
disk on the XY plane of radius 1.
Per-splat opacity is multiplicative with the step-function falloff. Rotation and scale will transform the disk kernel into a planar ellipse, and position moves the splat center from the origin.