a 3D printer has to be able to tell what part of the build volume is solid- and what part is air.
But it only has math to do it with, So the one characteristic that can be said to define a solid object, mathematically, is that is has only ONE side… its OUTSIDE.
But 3D modeling apps do not have to model objects as solids… instead- they can also represent forms as SURFACES.
Try to picture the difference between a balloon, and a rock the same shape as the balloon.
The balloon is a thin surface with nothing inside… and that is how modeling apps represent surfaces… even the surface of a solid object is actual only the exterior of the object.
with a balloon, it holds air in because there is no hole between the outside of its surface… and its inside volume.
If a printer can determine that a model surface represents a contiguous surface with no holes into the interior volume- then it can determine that filling that interior volume with resin will form a proper solid. That is- the surface delineates the separation boundary between the Object- and the empty space around the object.
Now- try to imagine a HALF a balloon. Still in the shape of a balloon, but not actually able to hold any air inside, because its now just two sides of a hemispherical surface.
Mathematically, any model with even the smallest hole in it, is still just like that Half balloon… just a curved surface- that does NOT separate an interior volume from and exterior volume.
Looked at from one side- the half balloon looks like it has form… but its surface is infinitely thin and does not fully enclose a separate volume from an exterior space.
So- your scanner can scan the outside of something like a tooth, but even the slightest missing piece of the surface will result in a hole to the ‘inside’ that makes that tooth look like half a balloon to the printer.
It can RENDER on screen just fine, because light can bounce off a surface whether it is solid or not. And in animation or CGI- you don’t really CARE if the model is solid or not. you only need it to render- and so LOTS of software simply does not even Try to maintain the solidity of models.
Your software or scan resulted in ONE part of the model being a qualifed solid. But the tooth part in NOT being a solid and so, unprintable,
You can look to see if your modeling app has an Object Doctor or similar feature for fixing bad geometry in a model and see if it can fix the model that won’t print.
But Preform simply doesn’t have the capability to treat SOME models entirely differently than others. If it will print one model- but not the other, then the Other model is simply not valid solid geometry.
Preform uses the same OpenGL technology to RENDER the model it imports… thus, just like a CGI app- it can SHOW you a surface that is not a solid… but when it doesn’t print at all, that means the model is most probably not a solid.