Speaking as an RP vendor, bad STL files are still a big problem. Our
customer support department sees them all the time, and have gotten very
good at identifying problems such as intersecting surfaces (caused by a
failure to fully union the solid), even without seeing the file. For
more subtle problems, we like to use a third party product such as
Magics RP or STL View to confirm STL problems and to prevent
finger-pointing exercises ("Your software is bad" "No, your file is
Some of the toughest problems to spot are what we call "stray triangles"
- an extra triangle or two stuffed in behind a wall or inside the part.
We see maybe 3-4 of these types of problems a year, and it can be very
time consuming to figure out just what's going on. As a graphics
programmer, my guess is that the stray triangles are caused by errors in
the CAD systems and not by the users. By the time the files get to us,
it's difficult to know which CAD system was used to create them, and in
fact they may have passed through several different systems so it's hard
to say which ones are better. Some of the worst STL files don't come
from CAD systems at all but from scanned data, but maybe that's another
Principal Software Engineer
Sanders Prototype, Inc.
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