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

From: C. Rooney (Brock Rooney & Associates Inc.)
Date: Thursday, November 30, 1995

From: C. Rooney (Brock Rooney & Associates  Inc.)
To: RP-ML
Date: Thursday, November 30, 1995
Subject: Re: Attn. Pro/Engineer users!! Attn. Pro/Engineer users!!
On STL generation:

The problems are usually in one of two areas: tessellation quality, and failure 
to match adjacent triangle vertices. Tessellation of trimmed nurb surfaces can
be tricky business.
Most STL generation software is developed one of two ways:
1) Modify the shading software. The cad vendor already has routines to produce
   polygons for display purposes.  Unfortunatly the optimum tessellation
   for Image Generation is somewhat different from the optimum tessellation 
   for RP.  And the triangles from adjacent surfaces still need to be
   matched up.
2) Quick and dirty.  Little thought is given to the tessellation quality. Be 
   glad you got everything tessellated at all.

Brock Rooney & Assoc. customers report that our STL generation products
usually produce better tessellation than does ProE.  This is because our
routines were developed specifically for this application, and generally
work harder at
getting the optimum number of triangles for a particular condition.

On Direct Slicing of Cad Models:

It seems this idea has reared its ugly head again.
So at the risk of boring some:
Direct slicing of the CAD model is a Bad Idea for a Lot of reasons.  A Few are:
1) Performance
   Time to generate a STL file and slice it should be less than 1/10 th the
   time to slice the Cad model.  Take a moderate size CAD file with 500 faces
   and tell your favorite CAD system to section it 5 times.  
   Note the time. Multiply by 1000.  
   In general, intersecting a plane and a surface is a highly iterative 
   procedure, which requires a lot of computations.  The STL Generating and
   Slicing procedures are not iterative, and so run much faster.
   Of course, the cad system could slice a faceted representation, but this
   would produce a similar result to slicing the STL (at best).
2) File size
   For the vast majority of models, the slice data is much larger than the
   STL data. Unless the STL file is REALLY over-tessellated.
   (By the way, experience indicates that a decent STL file will usually be 
   about the same size as the CAD database.)
3) Vertical accuracy
   By manipulating the STL vertices, the slice software can produce a Max 
   vertical error of 1/2 the slice thickness.  In addition, upward and
   downward triangles can be manipulated as needed by the process to produce
   the best part.   Slicing the CAD model produces a Max vertical error equal to
   the slice thickness, since a feature could be just below or just above the
   slice plane.
4) Slice Closure
   In general, the adjacent surfaces in BREP Solid Models do not match exactly. 
   There are cracks and overlaps.  Eventually, the Cutting Plane WILL hit a
   seam, resulting in an incomplete boundary, or duplicate pieces.
   Because the triangles in a valid STL file match EXACTLY, this cannot happen.
   The slice software can ALWAYS produce exact closed boundarys.
5) Slice Accuracy
   Intersecting a plane and a surface has numerical problems when the surface
   normal is nearly perpendicular to the cutting plane. It can be difficult
   to the follow the the intersection curve in these cases, and indeed the
   curve can split into 2 paths.  This eventually WILL happen in your parts.
   There are no such problems slicing a STL file.
6) Slicing is Process dependent; STL generation is not.
   To slice, you need to know at least the build orientation and slice
   thickness.  These are things the CAD modeler does not want to know about.
   This information is not needed to generate a STL file.
(I could go on, but enough for now)

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.

STL is in the top 3 CAD data formats (with DXF and IGES) because of this
simplicity. It is being used in several other areas besides RP.  STEP, on the
other hand, has been in development for longer than this industry has existed.

C. Brock Rooney, Pres., Brock Rooney & Associates Inc.  (Brockware)
       268 George St.  Birmingham Michigan 48009 USA
(810) 645-0236   fax/bbs (810) 645-9020  email brockrooney@delphi.com


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