Re: Attn. Pro/Engineer users!! Attn. Pro/Engineer users!!

From: Anthony Martin (California State  University, Long Beach), Peter Gien (Pogo  International, Inc.)
To: RP-ML
Date: Thursday, November 30, 1995
Subject: Re: Attn. Pro/Engineer users!! Attn. Pro/Engineer users!!
... 6 good advantages of STL files deleted
> STL format follows the KISS dictum of good engineering design.
> Any beginning computer programmer can create a program to read binary
> stl files in less than 20 lines of code. Alternative CAD standards 
> (perhaps STEP) are much more complex.

This one might be the most important of all.  With this simplicity comes
universal support and portability.  As far as I know, all true 3-D CAD
systems can produce an STL file.  You can be guaranteed that if you send
an STL file to an RP house, they can handle it no matter what system
created it.  But try sending a Pro/E or IDEAS model to an RP house. 
Unless they happen to have the same system that was used to create the 
file, they won't be able to use it.

Even an IGES file differs greatly depending on how it was produced.  An 
IGES file can contain so many different data structures, the quality of 
the model depends very much on the implementation.  However, an STL file 
has ONE data structure, the facet (triangle).

(Actually the key behind the STL format is the triangulated mesh.  The
particular STL standard is somewhat lame, e.g. why include the normals?,
but there are several similar formats for representing a triangulated mesh
and most share the same advantages.)

The only disadvantage of the STL file that I see is the inability to
exactly represent curved surfaces.  However, well-written software can
easily process files with >200,000 facets in a reasonable amount of time,
and with that many facets, almost any part can be represented with
significantly greater accuracy than any RP system is capable of. 

I suspect that the STL format has a long life ahead of it, and I don't 
see anything replacing it any time soon.

Tony.


Date: 01 Dec 95 11:20:02 EST
From: "Peter H. Gien" <71773.3150@compuserve.com>
Subject: Re: Attn. Pro/Engineer users!! Attn. Pro/Engineer users!!
Tony wrote:

>>The only disadvantage of the STL file that I see is the inability to
exactly represent curved surfaces.  However, well-written software can
easily process files with >200,000 facets in a reasonable amount of time,

While I agree in general that the simplicity of STL files is what makes them
popular, I would like to add that modifying the files has proven to be a
stumbling block. For example, reading 200,000 triangles into Pro/E will bring it
to it's knees. Same is true with most other CAD systems. I have found, however,
that there are several CAD systems that support Polyhedral models, since that is
what an STL file is.

For example, 3D/EYE can read a large STL file without any problem. It also
repairs normals correctly. Having read the file into 3D/EYE, one can convert it
to say an OBJ or DXF file. Both of these formats will produce an exact one to
one match of the triangles. Then read the file into Caligari TrueSpace2 and you
will be able to refine the mesh, push and pull surfaces and perform many other
manipulations. Another wonderful program for manipulating mesh files is 3DStudio
from AutoDesk although it is a little pricey.

If you confine your CAD outlook to traditional mechanical design software, then
you will find a dearth of tools for manipulating meshes. But if you look beyond
CAD, and start digging into the rendering and animation tools you will be
pleasantly surprised. A good source for this 3DDesign magazine, published by
Miller-Freeman.

Peter Gien
POGO International, Inc.