This series od experiments are devoted to the developement of fragment
shaders that can be configured to generate a wide range of tunnel like
backgrounds with the configuration being performed via parameters that
are defined in a scene file.

Currently only one such fagment shader has been  developed,  though  a
rather different one is under consideration.

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The "TheEmu - Tunnel A.fsh" fragment shader was originaly inspired  by
Inigo Quilez's Deform - Square Tunnel downloaded  from  ShaderToy.com,
but other than using the same Minkowski distance metric has nothing in
common with it, though it can be configured to produce the same tunnel
shape.

The shader works by tiling its output window  with  repetitions  of  a
distorted "tunnel wall" image. By default these will comprise a series
of nested rings giving the impression of a tunnel disapearing into the
screen.  Any  image may be used, though the effectiveness of the final
result will depend on selecting a suitable image.  Simple  images such 
as a rock wall should always be effective,  but  more complex pictures
such as a landscape can be more problematic - though it has been found
that most "straight on" views,  such as the 2D backgrounds provided by
Totem, are reasonable choices.

By default the shader generates a static image,  but  apparent  motion
along the tunnel's axis may be generated under control  of  the  speed
parameter.  Refer  to the comments in the shader source for details of
this parameter.

The shader can either wrap one or more repetitions of the tunnel  wall
image through a full 360 degrees or through 180 degrees and reflect it
verticaly, horizontaly or diagonaly.  This  is  controlled by the mode
and repeatAngle parameters. Refer to the comments in the shader source
for details of these parameters.

The shape, position and orientation of the tunnel and  the  length  of
each section can be configured by using the shape, center,  alpha  and
zScale parameters.  Refer  to  the  comments  in the shader source for
details of these parameters.

By default the copy of the image used to tile the tunnel wall has  the
same orientation.  If  the image has not been specialy desined for use
as a tile this will result in discontinuities where the tiles meet. It
is possible to reduce this by "flipping" alternate tiles so that there
are matching colours at the joins.  This tile flipping is controled by
the tileScheme parameter.  Refer  to the comments in the shader source
for details of this parameter.

By default the tunnel walls are tiled without  distorting  the  tunnel
wall image with one side of the image parallel to the tunnel axis.  If
normal images are used the tunnel looks like an untwisted tube (though
specialy designed images can change this appearance).  However,  it is
possible to distort the image such that when used to tile  the  tunnel
it appears to be twisted.  This  is controlled by the twist parameter.
Refer to comments in the source for details of this parameter.

By default the colours used for the tunnel walls are just those  taken
from the the supplied tunnel wall  image,  including  preserveing  any
transparent or partialy transparent area.  However, it is often better
to fade the colour towards black or  white  with  distance  along  the
tunnel increases as this increases the illusion of depth and also will
hide "pixilation" effects in the very small copies of the tunnel  wall
image that are used for the far end of the tunnel. It is also possible
to make the whole tunnel translucent.  These  effects are controled by
using the fade, fadeColour and translucency parameters.  Refer  to the
comments in the shader source for details of these parameters.

It is possible to replace the end of the tunnel with a vertical  wall,
or introduce gaps between tunnel sections.  The  end wall and gaps are
transparent by default but any colour may be used instead.  These  are
controled by the endSize, endColour, gapSize and gapColour parameters.
Refer to comments in the source for details of these parameters.

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In order to adequately cover the different configuration possibilities
a large number of experimental scenes are used. These are divided into
two main groups, A0 and A1. 

The A0 group is used to look at the effects of individual   parameters
using a static tunnel.  The  A1  group  looks at slightly more complex
combinations of parameters using a moving tunnel.

In most of the scenes the image used for the tunnel wall  is  selected
at random from the Images subdirectory.  In some cases a specific test
image is used,  one that clearly shows the effects of tile orientation
and twisting.

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