In this series of experiments I look at how to generate swarms of "things" that
circulate around the performers, passing behind them in one half of their orbit
and in front of them in the other half.  The  orbiting  things are usually just
glowing globes, but can be tailored to be shape.

Currently there are just two series of these experiments, one if which only has
a single member, but the other has comprises several sub-series using different
shapes for the swarm or its elements.

The A series is based on the e#21916.3 from GlslSandbox.com,  by  an annonymous
author. This shader generated a series of glowing balls that orbited in an ever
changing pattern. I have generalised it so that both the shape of the swarm and
of its constituent elements is configurable. The .scn files for scenes that are
based on this shader use it twice, firstly to generate those parts of the swarm
that are in the background and those that are in the  foreground.

Whether the the shader is being used for the foreground or background is passed
to it by a parameter, depth, specified in the .scn file.  This avoids having to
use two versions of the shader that differ by only one character.  It  would be
possible to pass more parameters this way,  reducing  the  number of .fsh files
even more,  but I have not done so - mainly because the ability to pass data to
a shader in this way is a new feature (Feb 2015) and I had already written most
of these shaders, so I only modified them to pass the depth parameter.

The A series is split into a number of sub-

   A1 - The swarm, of glowing balls, forms a narrow
        cylinder circulating around the performer.

   A2 - As A1, but the cylinder is full screen width.

   A3 - As A1, but the swarm elements bob up and down
        while they circulate.

   A4 - As A1, but the swarm narrows towards the top
        to form a dome over and around the performer.

   A5 - As A3, but the swarm elements are red hearts.
        Two forms of bobbing motion are used.

   A6 - As A3, but the swarm elements are flowers.

   A7 - As A3, but the swarm elements are stars.

   A8 - As A3, but the swarm elements are butterflies.

The B series is based on a different shader and produces  a  swarm  of  glowing
globes of different colours and sizes circulating around the performer.  As yet
there is only one scene in this series and it is without any easy configuration
mechanism.