What a .3mf contains

A .3mf is an OPC package: a ZIP with rules about what goes inside.

Which files are inside?

Bambu Studio, OrcaSlicer and Creality Print also store each object in its own 3D/Objects/*.model file.

What is in the model file?

• <model>
  • <resources>
    • <object id="1"> is a <mesh>: vertices and triangles
    • <object id="2"> is a <components> assembly
      • <component objectid="1" transform="..."/>
    • <basematerials> and any extension resources
  • <build>
    • <item objectid="2" transform="..."/> places object 2 on the plate

<resources> defines objects: a mesh, or an assembly of other objects. <build> places them on the plate with a transform. Each transform is multiplied down through the components to reach the mesh.

How does a placed object become triangles?

Each step multiplies in its own transform before the last one lands as triangles on the plate:

1. take the transformthe one on the <item> in <build>
2. resolve objectidfind that <object> in <resources>
3. recurse an assemblyinto each <component>, multiplying transforms down the tree
4. emit a <mesh>its triangles land under the accumulated transform

An assembly nested three deep accumulates three transforms before its triangles land.

What rules must a valid file follow?

• One <build> per model file.
• No object may contain itself.
• Meshes must be closed: every edge shared by exactly two triangles.
• Units default to millimetres.

Where is it specified?

The 3MF Core Specification. lib3mf is the reference reader preslice uses.