Re: STL file format

From: Michael Brindley
To: Chuck Kirschman (Clemson  University)
Cc: RP-ML
Date: Sunday, September 25, 1994
Subject: Re: STL file format 
Chuck Kirschman writes:
> 3 is not true.  The SLA requires that the part be in positive CAD space,
> but the file doesn't care.  You get an entire float to do with as you

The file doesn't care about the part being in the first octant.  The
official spec says that it must be in the first octant.  Thus, any
file containing an object with a zero or negative coord does not 
follow the spec.  Any software reading such a file is therefor
quite within its rights to reject the file as invalid - possibly
damaged or to act in some odd fashion (the old GIGO principle).

> wish, and you can have the part completely in negative space.  The reason
> for the requirement for the SLA is that multiple parts need to be built
> in relative proximity to each other, so the machine cannot arbitrarily
> move them into the positive space.  And the positive space is required

The machine does not arbitrarily move them; the people have to create
STL files that have the various parts positioned in space relative 
to one another to properly fill up the build volume with multiple objects.
As long as the parts are positioned with in a region of space which
fits inside the build volume of the machine, why would the contoller
software care that the objects are in a certain exact spot?  It is
trivial for the software to move all those objects to the desired
location for building (maintaining the positions of the objects
relative to each other).

> Another problem with the STL format is that it does not specify what
> happens when multiple objects are containted in the same file.  Luckily,
> the defacto use is that there is a comment at every object start, and
> almost everyone seems to agree on this.  This makes it possible to 
> simply concatenate multiple STL files.

Interesting ... a defacto extension to the STL file format.

> One other thing to think about is that there are typically one half as
> many point as facets, so each point is listed in the file multiple times.
> Several smaller files based on meshing have been proposed, and I'd like
> to see one of them come into use.

I haven't seen those other file formats.  Do you have any 
references?  The only one I've seen discussed is Rock and Wozny's
RPI format which is exceeding complex.  I could waste many electrons
typing about what is wrong with the STL file format!  I have my own
ideas about what would make a good format.

  --> Mike Brindley