FOLD [spec, viewer, API]

FOLD (Flexible Origami List Datastructure) is a file format (with extension
.fold) for describing origami models: crease patterns, mountain-valley
patterns, folded states, etc. Mainly, a FOLD file can store a mesh with
vertices, edges, faces, and links between them, with optional
2D or 3D geometry, plus the topological stacking order of faces that
overlap geometrically.
A mesh can also easily store additional user-defined data.
One FOLD file can even store multiple such meshes in "frames"
(but this feature is not yet supported in any code).

This repository both documents the FOLD format (which is still in early
stages so its definition is evolving) and provides web software tools
and JavaScript libraries to aid in manipulation of FOLD files.
FOLD is built upon
JSON (JavaScript Object Notation)
so parsers are available in essentially all
programming languages. Once parsed, the format also serves as the typical
data structure you'll want to represent foldings in your software.
Our libraries also help build useful redundant data structures for
navigating the mesh.

FOLD is similar in spirit to the
OBJ format (and other similar
formats) for storing 3D meshes; its main distinguishing features are
easy parsing, easy extensibility, the ability to disambiguate
overlapping faces with stacking order, and the ability to define edges
and thus edge properties (such as mountain-valley assignments) and
arbitrary polyhedral complexes.
(Without edges, OBJ cannot distinguish between two faces sharing two
consecutive vertices from faces sharing an edge.)
In addition, FOLD can support linkages (with 1D edges but no 2D faces).

FOLD Format Documentation

FOLD Software Tools

Here is software supporting the FOLD format, built both within this project
and by other people:

FOLD viewer
loads and display a given .fold file, useful for visualization and testing
Amanda Ghassaei's
Origami Simulator
supports FOLD input and output
Tomohiro Tachi's
Freeform Origami
(simulator and design tool)
supports FOLD input and output
Robby Kraft's Rabbit Ear
is based on FOLD. It includes a
file converter
(FOLD display, SVG import/export, and flat folding).
ORIPA: Origami Pattern Editor
v1.10+ supports FOLD import/export (in addition to its OPX file format).
Crease Pattern Editor
supports FOLD export
Rhino Grasshopper FOLD plugin
converts between Rhino meshes and FOLD format/data

FOLD JavaScript Library

For simple web apps, add this tag to your HTML:
<script src="https://edemaine.github.io/fold/dist/fold.js"></script>
(or save a local copy of
dist/fold.js
and use that).
Then, if you add FOLD = require('fold') to your JavaScript/CoffeeScript code,
you can access the library via FOLD.moduleName.functionName, e.g.,
FOLD.filter.collapseNearbyVertices.

For Node apps, just npm install --save fold;
then add FOLD = require('fold') to your JavaScript/CoffeeScript code;
then access the library via FOLD.moduleName.functionName, e.g.,
FOLD.filter.collapseNearbyVertices.

The FOLD library API
documents the available modules and functions for manipulating FOLD objects.
If you have a .fold file, first parse it with JSON.parse(fileContents)
to get a FOLD object.

The JavaScript library also provides a command-line interface called
fold-convert. To use it, make sure you have Node.js
installed, and then run npm install -g fold from the command line.
Then you can use the following features:

fold-convert -o .fold *.opx: Convert ORIPA .opx files to .fold.
fold-convert --flat-fold -o B.fold A.fold:
Flat fold crease pattern A into folded geometry B