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DESIGN TIPS

Bad design files will throw errors and create models that explode or cannot be solved, here are some tips for importing FOLD or SVG files that work.

Importing FOLD:

The FOLD file format is specified in these docs. This tool imports FOLD v1.0 files with all of the following fields populated:

• vertices_coords
• edges_vertices
• edges_assignment
• faces_vertices

You may specify the target fold angle of each crease using the edges_foldAngle field. Note that fold angle is a number in degrees lying in the range [−180, 180]. The fold angle is positive for valley folds, negative for mountain folds, and zero for flat regions. Accordingly, the sign of edges_foldAngle should match edges_assignment if both are specified. You may also assign a null fold angle to a crease so that it is allowed to move freely - this is especially useful when the fold angle is unknown or the crease pattern is fully constrained and able to be driven by other creases in the system.

If you are unsure whether your FOLD file is valid, you can inspect it using the FOLD Viewer. If you are having trouble, please refer to the FOLD spec.

Importing SVG:

• The SVG importer supports path, line, rect, polygon, and polyline objects with the appropriate stroke color:
• Mountain folds have red stroke - rgb(255, 0, 0), hex #ff0000
• Valley folds have blue stroke - rgb(0, 0, 255), hex #0000ff
• Boundary edges have black stroke - rgb(0, 0, 0), hex #000000 - use this edge type for both the outline of the pattern, and any internal holes.

• The final fold angle of a mountain or valley fold is set by its opacity. For example, 1.0 = 180° (fully folded), 0.5 = 90°, 0 = 0° (flat). Any fold angle between 0° and 180° may be used. The following patterns contain creases with varying final fold angles:

• Before the model can enter the simulator, this tool will automatically triangulate regions in the pattern that form polygons with more than three sides. To control the triangulations, draw lines in yellow - rgb(255, 255, 0), hex #ffff00. In general, patterns are more stable when their triangulation is symmetric and minimizes long, skinny triangles. You can see how the app automatically triangulated your pattern by enabling "Facet Crease" visibility in the Advanced Options > Edges Visibility menu on the left side of the screen. It may not be necessary to add triangulation lines on your pattern.

• Facet creases (yellow triangulation lines) are creases that try to remain flat (0° fold angle) as the pattern folds, but some compliance in the constraints of the simulation will allow them to bend slightly. Drawing in facet creases allows you to control the way the model bends in between mountain/valley folds. This is especially relevant in patterns with curved creases. If you import an svg with bezier curves and select the checkbox "crease pattern contains curves" in the import dialog, we attempt to triangulate it for you using the algorithm described by Simple Simulation of Curved Folds Based on Ruling-aware Triangulation. We recommend trying this first, and add additional facet creases to your svg design where needed.

• Cuts have green stroke - rgb(0, 255, 0), hex #00ff00 - use this edge type to form thin slits in the pattern.

• Undriven creases have magenta stroke - rgb(255, 0, 255), hex #ff00ff - creases of this type will swing freely. Use this crease type when you're not sure what angle some of the creases in your pattern should fold to.

• Line style and stroke width do not matter.
• Remove clipping masks from your file before importing.
• This tool should be able to automatically clean files of slightly misaligned vertices, stray vertices, duplicate lines, and extra vertices falling in the middle of an edge, but it is recommended to remove these errors yourself in order to avoid problems.
• If your simulation is not working, check that the imported pattern looks correct by clicking on the Pattern view in the top nav bar. You can also download a copy of the imported pattern ( File > Save Pattern as SVG ) and view it in a vector editing program of your choice.

For Adobe Illustrator users:

I use Illustrator to create SVGs (though any vector editing program should be fine). Here are some tips I've found for making svgs to import into this tool.

• If you are starting with a pre-made vector file, first release all clipping masks (Select all + Object > Clipping Mask > Release).
• Create geometry using the Line Segment, Rectangle, and Polygon tools.
• Illustrator can help you select all lines of a particular type so that you can edit their color or opacity together. Click the line, then go to Select > Same > Appearance to select all similar lines in the pattern.
• Be sure that CSS Properties is set to Presentation Attributes in the Advanced options.
• Finally hit Save As and select .svg extension. I'm using the default SVG 1.1 settings, but version 1.0 will work as well.

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SIMULATION ERROR

Average vertex error gives a sense of how much the distance constraints in the origami pattern are being violated (i.e. how much the sheet is being stretched). The error at each vertex is evaluated by averaging the percent deviation of all its distance constraints with adjacent vertices. This error is reported as a percent of the total length of the distance constraint to remove scaling effects.

This measurement is equivalent to Cauchy strain or engineering strain of the distance constraints on this system. Increasing the Axial Stiffness will tighten these constraints and lower the error in the simulation.

To visualize the error of each vertex graphically, select Strain Visualization under Mesh Material in the left menu.

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SIMULATION SETTINGS

This app uses a compliant dynamic simulation method to solve for the geometry of an origami pattern at a given fold angle. The simulation sets up several types of constraints: distance constraints prevent the sheet from stretching or compressing, face constraints prevent the sheet from shearing, and angular constraints fold or flatten the sheet. Each of these constraints is weighted by a stiffness - the stiffer the constraint, the better it is enforced in the simulation.

Axial Stiffness is the stiffness of the distance constraints. Increasing axial stiffness will decrease the stretching/compression (strain) in the simulation, but it will also slow down the solver. Face Stiffness is the stiffness of the face constraints, which help the axial constraints prevent deformation of the sheet's surface between the creases.

Fold and facet stiffnesses correspond to two types of angular constraints. Fold Stiffness is the stiffness of the mountain and valley creases in the origami pattern. Facet Stiffness is the stiffness of the triangulated faces between creases in the pattern. Increasing facet stiffness causes the faces between creases to stay very flat as the origami is folded. As facet stiffness becomes very high, this simulation approaches a rigid origami simulation, and models the behavior of a rigid material (such as metal) when folded.

Internally, constraint stiffnesses are scaled by the length of the edge associated with that constraint to determine its geometric stiffness. For Axial constaints, stiffness is divided by length and for angular constraints, stiffness is multiplied by length.

Since this is a dynamic simulation, vertices of the origami move with some notion of acceleration and velocity. In order to keep the system stable and help it converge to a static solution, damping is applied to slow the motion of the vertices. The Damping slider allows you to control the amount of damping present in the simulation. Decreasing damping makes the simulation more "springy". It may be useful to temporarily turn down damping to help the simulation more quickly converge towards its static solution - especially for patterns that take a long time to curl.

A Numerical Integration technique is used to integrate acceleration into velocity and position for each time step of the simulation. Different integration techniques have different associated computational cost, error, and stability. This app allows you to choose between two different integration techniques: Euler Integration is the simplest type of numerical integration (first order) with large associated error, and Verlet Integration is a second order integration technique with lower error and better stability than Euler.

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STRAIN VISUALIZATION

Cauchy strain or engineering strain is a unitless measurement of how much a material is being stretched or compressed under load. The Strain Visualization illustrates the strain across an origami sheet by mapping it to a color from blue (no strain) to red (max strain).

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USER INTERACTION

Toggle this control to enable/disable mouse interaction with the origami model. When enabled, mousing over the model will display a highlighter; clicking and dragging allows you to interact with the model in real time. Very vigorous interactions with the model may cause it to pop into a strange configuration that it can't escape - use the Reset button to start the simulation again from a flat state.

This app uses GPU functions that are not supported by this device/browser, please try again on desktop with Chrome/Firefox.