What is a .amf file?

The right idea, at the wrong time

AMF was standardised by ASTM in 2011 to replace STL, and it fixed STL's failings comprehensively. Units, colour, materials, part names, constellations of positioned instances, and two things nothing else has:

• Curved triangles. An AMF triangle can carry normal vectors or tangent

data at its corners, so a curved surface can be represented accurately with far fewer triangles than STL needs. A cylinder can be smooth instead of faceted.

• Graded and lattice materials, expressed as formulas over the volume, for

parts whose composition varies through the body.

It is a better designed format than 3MF in several respects, and it lost anyway. 3MF arrived in 2015 with Microsoft, HP, Autodesk, Dassault, Prusa and Ultimaker behind it and shipped in the slicers people use, and that settled it.

What is in the file

AMF is XML, optionally zip-compressed with the same .amf extension:

<amf unit="millimeter" version="1.1">

<object id="1"> <mesh> <vertices> <vertex><coordinates><x>0</x><y>0</y><z>0</z></coordinates></vertex> ... </vertices> <volume> <triangle><v1>0</v1><v2>1</v2><v3>2</v3></triangle> ... </volume> </mesh> </object>

</amf>

The unit attribute is the important one, and it accepts millimetre, centimetre, metre, inch, foot and micron.

Volumes are the neat structural idea: one mesh's vertices can be shared by several volumes, each with its own material. A two-material part shares one vertex list instead of duplicating the interface between them.

Units, done right

Unlike STL, an AMF says what its numbers mean. This site reads the declaration and converts correctly, so an AMF in inches becomes a correctly sized millimetre STL without anyone doing arithmetic.

Where you will meet AMF

• Slic3r and PrusaSlicer read and write it
• Meshmixer, Netfabb and Autodesk Within support it
• Some academic and research toolchains adopted it for its lattice support
• A handful of print services still list it

Cura and Bambu Studio prefer 3MF. Simplify3D never supported AMF well.

Should you use it?

For new work, no. Use 3MF. It does nearly everything AMF does, every current slicer reads it, and the tooling is better maintained.

Keep AMF in mind for one situation: when you need curved triangles or graded materials and the receiving software supports them. Nothing else in common use expresses those.

Compressed or not, same extension

An AMF may be plain XML or a zip archive containing that XML, and both are named .amf. That catches tools which decide by extension: a compressed AMF handed to a reader expecting text produces a parse error full of binary gibberish.

Compression matters more here than it sounds, because XML this verbose is enormous uncompressed. A model that is 4 MB as binary STL can be 30 MB as plain AMF and 3 MB zipped. This site accepts either form, deciding from the file's first bytes rather than its name.

Constellations

AMF's answer to placing many copies of one object is the constellation: a list of instances, each with a position and rotation, referring to shared geometry. A plate of forty identical parts is one mesh plus forty placements.

It is a good design, and it is also the part of AMF with the weakest support. Several readers ignore constellations entirely and show you one part at the origin. This site reads the source geometry and reports that instance placement was not applied, instead of showing one copy and calling the file converted.

Reading AMF here

This site reads both the plain XML and the zip-compressed form, honours the declared unit, and keeps per-volume colours and materials. Constellations, AMF's instancing mechanism, are reported rather than silently flattened, because only the source geometry is read.