stl's and such

From: Elaine Hunt (Clemson University)
Date: Monday, September 26, 1994

From: Elaine Hunt (Clemson  University)
To: RP-ML
Date: Monday, September 26, 1994
Subject: stl's and such
>Chuck Kirschman wrote:
>>>2 is noteworthy.  I believe that we (the RP community) should take
>>>advantage of the 80 characters of header information to state this
>>>and other information, such as if it is a non-closed solid (support),
>>>which CAD pkg, and so on.
>
>>Dick Newton agreed:
>>I agree, as long as these "extensions" are well documented, usefull and
>>agreed to by the rapid prototyping community as a whole (ie not just
>>useful to one particular RP vendors process). They shouldn't break any
>>existing software either. (I believe Helisys currently uses part of the
>>header to indicate if the file has been sorted and pseudo facets have
>>been removed. 3D systems uses part of the header to indicate if it has
>>been run thru 3D verify.)
>>
Are RP vendors that are out there"listening" ready to share how the use the
header with the user  community?
>>
>>>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.
>>
>>Not everyone agrees on this. Helisys and Imageware will not handle
>>concatentated STL files. Packages such as 3D systems View and partman
>>and Solid Concepts STLTRANS will destroy this extra header information if
>>you read the file in, manipulate the file, and write it out.


Can we find common ground on which to agree and make the file useful to
all>vendors?
>
>
>>>Several smaller files based on meshing have been proposed, and I'd like
>>>to see one of them come into use.

Is an RP vendor working on this and who has input into its development?
>
>
>>A list of point coordinates at the start of the file, followed by a list
>>of triangles (or polygons) that reference those points. It might also have
>>the benefit of making software vendors check to see if their stl files
>>are well connected.
>
Good idea.. Anybody using it?????
>>
>>What is the purpose of the normals?
>
>> running a file  thru their software will totally mess up the normals and
>>makes the file  unusable on other (ie SLA, DTM) machines).
>
Interesting comments especially for those contemplating multiple RP system
in-house.
>
>
>>Also, what is the purpose of the two bytes of pad on the end of the
>>triangles?  Is this wasted space? It makes every other triangle
>>un-aligned on 32 bit machines that require 4 byte boundry alignment
>>on structures.
>>
>>Why do CAD vendors output files that are not well connected, (ie
>>each triangle shares all its edges with other triangles and each edge
>>is only connected to one other edge)? A closed volume that meets these
>>connectivity requirements MUST have an EVEN number of triangular
>>facets. Why do I see stl files that the CAD vendors insist are "good"
>>with ODD numbers of facets?
>>
>>The STL file format does work but it does have it's problems. Maybe this
>>forum could work to clarify and document some of these inconsistencies.


YES   maybe the nap is over and we can find meaning for our existence.....

Elaine


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