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Re: [Closed REQ 5573]: thanks for Geomview (and a question)

  • To: software@geom
  • Subject: Re: [Closed REQ 5573]: thanks for Geomview (and a question)
  • From: "Stuart Levy" <slevy>
  • Date: Thu, 28 Sep 1995 17:30:19 -0500

> I want certain parts of my geometric model to appear semitransparent, but
> even though I use alpha values of about 0.25 in some of the face
> descriptions of the OFF files, Geomview appears to ignore them.

You need to do both: specify alpha values, *and* enable transparency mode
(with the "Transparent" button on the Materials panel, or the 
"+transparent" property in an appearance setting).

Unfortunately transparency doesn't really work; to behave properly, 
geomview would need to display polygons in order of depth, and it doesn't.
In some cases the result doesn't look too bad, in others it's unrecognizable.

For the best chance of a good result, try this.  Enable transparent
mode only on those parts of the model which are transparent.  (Since
transparency disables Z-buffering, that ensures that opaque objects
do get Z-buffered.)

Then, make sure the transparent parts are displayed last.  That is, if
you have several separately-loaded Geomview objects, put the
transparent pieces in separate object(s) and load them after all the
opaque parts.  Geomview displays multiple objects in the order that they
appear in the object browser.

Or, if you have a single composite object, say a collection of pieces
gathered together in a LIST, then make the transparent portions be the
last component(s) of the LIST.

Finally, apply the +transparent attribute to just those portions where
transparency is used.  For example:

    { < hdodec.off }
    { < csquare.quad }
    { appearance { +transparent } < cube.off }
    { appearance { +transparent } { QUAD 1 0 0  1 1 0  0 1 0  0 0 0 } }

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