John M. Dlugosz wrote:
> 
> Ken wrote in message <36E34714.CA08E2BC@pacbell.net>...
> > DXf has the ability to store either face or line info or both. Sometimes
> >the program exporting the model will only export one or the other sometimes
> >both. Conversion programs also have their own way to handle the data in the
> >dxf file and it varies with who wrote it.
> 
> But does the face info include normals?  That is, is one "face" a plain
> triangle (three points) or a smooth triangle (three points and three
> normals, as in POV and OpenGL)?  For that matter, how do mesh-based editing
> programs define such things?  I see a mesh of triangles, but how does it
> know those approximate real surfaces?  Or are they just facets, period, and
> it uses the joining angle idea like the import setting you told me about, to
> smooth things implicitly (which would make it difficult to represent a sharp
> edge that is below the critical angle!)
> 
> 'learning,
> --John

  It does not store surface normal informations that I'm aware of.
It is just face information. As you say the conversion program will
evaluate each junction and if it falls within the defined angle it
becomes acted upon. There are times that a large specified smoothing
angle will take out every sharp edge on a model. That's where trial
and error come into play I'm afraid. I believe 3DWin has a default
smoothing angle of 70, unless you change it from the shipped setting
and it provides a good general purpose smoothing amount.
  You can immediatly see when smoothng is turned off on models that
have a low face count and it's pretty amazing what a large angle can
do to correct it. I think the use of the smooth triangle function needs
to be evaluated on a per model basis as it can add in both parsing time
and quite considerably in the file size produced.

-- 
Ken Tyler

mailto://tylereng@pacbell.net

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