RE: stl files

From: Anshuman Razdan (razdan@asu.edu)
Date: Fri Nov 17 2000 - 09:51:44 EET


Mike
        Every so often, as Ian Gibson would say, some one comes along and reopens
the STL saga. Its late so my response may be a bit tired and sharp...

a) You dont need a normal. If your vertices are in correct order (Counter
clockwise) then normal can be computed..

b) You are talking about the least efficient file format here. if a vertex
is shared by 10 triangles its in the file 10 times....so reduncdancy
redundancy redundancy -- actually cause of many errors -- cant say it enough
number of times....

Good look in ur discovery process.

AR

>> -----Original Message-----
>> From: owner-rp-ml@ltk.hut.fi
>> [mailto:owner-rp-ml@ltk.hut.fi]On Behalf Of
>> Doty Mike
>> Sent: Thursday, November 16, 2000 11:28 PM
>> To: NECOINC@aol.com; rp-ml@bart.lpt.fi; Marshall@Ennex.com
>> Subject: Re: stl files
>>
>>
>> Very interesting write-up on stl format. I don't understand
>> why the normal needs 3 coordinates, since it is only used to
>> indicate which side of the face is inside the enclosed
>> volume of the solid and which side is outside. It seems
>> that the shape could be almost as accurately stored in a
>> file 1/3 the size by using the normal coordinates as a
>> fourth point to describe the vertex of a tetrahedron,
>> projected along a normal vector at the center of the other
>> three points. the direction of the surface normal needs
>> only a single sign bit. the magnitude of the normal would
>> be 0 if the face lies on a plane, otherwise it would lie on
>> the curvature of the surface of the shape. Which side of
>> the tetrahedral surface is interior to the solid and which
>> side is outside is determined by the sign bit according to
>> the right-hand rule applied to the three points.
>>
>> Mike Doty
>>
>> For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/

For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/



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