transparency in geomview

[slevy]

I looked at what would be involved in adding transparency to geomview,
and noticed that it's in fact already available, though not in the most
convenient way.  If you create a file, say "transparent", containing

	(merge-ap World { +transparent })

and possibly "opaque" containing

	(merge-ap World { -transparent })

then loading "transparent" will enable transparency in MinneView style,
and "opaque" will restore opacity.

Any version of geomview should behave this way.

Caveats:
  1) The above commands only *allow* transparent objects to be drawn;
for objects to be actually transparent, you must explicitly specify
their colors, setting the fourth component to be something less than 1.
(The same arrangement was required for MinneView.)
For example a half-transparent square could be written as
  CQUAD
  -1 -1 0	1 1 1 .5
  -1  1 0	1 1 1 .5
   1  1 0	1 1 1 .5
   1 -1 0	1 1 1 .5

It should be possible to specify a default opacity per object, but
that can't be done at the moment.  [Actually, due to a bug, that's not quite
true.  The "red" component of specular color actually becomes the default
object opacity.  There's no way to set this from geomview's control panels,
but you could make all objects half-transparent by default by setting
	(merge-ap World { material { specular .5 .5 .5 } })
at the expense of also dimming all specular highlights.]

  2) Setting the object's color in geomview will override any transparency!
	[That is, unless you use the "specular" bug mentioned in the
	previous set of brackets.]

  3) Transparency should be effective on Indigos, and on GTX and VGX Irises;
	it does not and will not work on Personal Irises.

The next geomview release will permit setting opacity directly.

   Stuart