Hey there,

I'm looking at developing some models to be printed on a RepRap/MakerBot type machines. As such I'm wondering how well models with interpenetrating surfaces are processed. For example, if you had a cube sticking out of another cube (not as an extruded face but as a discrete object) would it be processed any differently than the same thing done with an extruded face? Or if you had two cubes perfectly side by side would it be printed as a single continuous object or would skeinforge try and maintain some sort of separation?

Anyone with more experience with how skeinforge deals with the stl files that have multiple objects have any observations to add?

When you export your design as STL for processing by Skeinforge you are using a very primitive format: just triangles describing the faces of the object, with a convention dictating which side of the triangle is inside.

To answer your first qustion: it should not matter how you define your objects at a higher level, the design software 'renders' the faces to triangles regardless of the way you defined the objects parts.

Your second question touches on a notorious problem: when two surfaces touch exactly it is ill defined whether this is a face or merely part of the volume, so the design software may 'do whatever it wants' here. The same problem occurs when subtracting objects that share a face from each other: it is unclear wether a zero-thickness volume should be rendered or just empty space.

So as a rule always have objects overlap a tiny bit (both when adding or when subtracting them) so they don't share such a problematic face.

HTH!

just to clarify; the problem that reinoud describes with coplanar faces exists even in the core of the mathematics, so it cannot be solved except by model designers taking it into account when modelling. a small overlap is essential for valid models

---

-----------------------------------------------
Wooden Mendel
Teacup Firmware

This is what Adrian explains well in the wiki, but somehow CoCreate does not have this problem. I can union shapes with coincident faces and always get manifold results.

---

[www.hydraraptor.blogspot.com]

Every better modeller can deal with that.
AOI is plain broken with coplanar surfaces Blender is just bad with boolean
in genral. Others do it more properly.

Edited 1 time(s). Last edit at 05/02/2010 03:32PM by MarcusWolschon.

---

-------------------------------------------
* homeprototype free 3d design repository
* Blog
* Google+