Turn 3D models into sewing patterns for simulation and sewing with this add-on
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Step 0: Installation
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Download the latest version above.

Save the archive somewhere on your PC. In Blender, go to Edit > ⚙️ Preferences > Add-ons > Install...
and select the .zip file.

Make sure to enable the Add-on too
Step 1: Preparing your mesh
Bad geometry - consider remeshing
Your input mesh should meet some requirements. It does not have to be perfect, but it should be kind of clean. It should be manifold, make sure the normals are correct, and it should not be too dense.
Step 2: Adding seams
It's time to add our seams. As you've probably guessed, the "sewing pattern" created by the add-on is really just a UV layout, but in 3D. So, for a good sewing pattern, we need a good UV layout.

Go ahead and add seams around your mesh wherever you see fit. Some seams will wrap nicely around the existing geometry, as is the case in the Suzanne example shown here.

This won't always be the case, sometimes you need to make additional cuts using tools like Knife K or Connect vertex path J . This will often leave you with bad geometry, as this cut creates tiny edges and triangles.
You can use the Edge > Clean up knife cut operator, which is included in this add-on. to clean fix this. Select the edge chain you wish to clean up (usually the part you just cut), and run the operator. Increase the "Min edge length" untill all the small triangles are gone. If the edge chain is crooked, you can relax it. You can also relax neighboring vertices.

On the left you can see where I added seams to
the model. (tip: drag to rotate)
Step 3: Unfold
Step 4: Simulate
Step 5: Export pattern
Step 6: Sewing
Tutorial on YouTube
Unwrap the model as you normally would. It's important that your Seams always match your UV's!. This is because the seams are used for cutting, but the UV's are used for unfolding! You can get an idea of how well your unwrap works by visualizing stretching.

UV Editor > Sidebar > View > Overlays > Stretching > Type: Area .
If there are places that show up too green, it will apply too much tension to the fabric. Where would you add seams based on the result on the right?
Here comes the interesting part. If you're happy with the unwrap, it's time for unfolding.
It's best to have your object scale applied at this point!
Object > Seams to Sewing Pattern > Seams to Sewing Pattern
You'll be presented with a couple of options.
Unwrap You can choose to perform an unwrap before executing the operator, or you can keep your existing UV's.
Remesh Check this if you want to remesh the individual islands after the unfolding. Additionally, This is a good idea for cloth simulation. Target number of triangles refers to the number of output triangles. It won't be exactly this number, but close.

Note the edges that appear between the islands. These will be used for sewing later!
Object > Seams to Sewing Pattern > Quick Cloth Sim can help you set up a simple cloth sim.

Cloth simulation can be finicky. There's many settings to tweak, but I will highlight just a few important ones:

Shape > Sewing and Max Sewing Force:
Enable sewing to collapse the empty edges between the islands.

Pressure and Target Volume:
Enable pressure to "inflate" the object. Target Volume can be used in addition to this. The original volume of the mesh, before unfolding, is stored under Object Properties > Custom Properties > S2S_InitalVolume. Copy / paste this value.

Physical Properties > Air Viscosity:
Can be increased if the cloth seams to collapse in on itself.

If the object has features that don't want to "pop out", keyframe the "Pressure" to something high at frame 0, and to a more reasonable ammount later.

On the left you can see the result of a cloth simulation @ 20.000 triangles.
If you're just interested in simulation, you can stop here and enjoy your model. If you want to hold the model in your hands, let's continue!
We can export our sewing pattern with Object > Seams to Sewing Pattern > Export Sewing Pattern.
This produces a .svg file, with dimensions in millimeters that should correspond with the side of your 3D model.
Please note: this operation should be called after unfolding the mesh!
To help you line up your pieces when sewing, markers are exported as well. They work as followed:
Every sewing line (the empty edges bridging the mesh islands) that's marked as seam will result in a marker. You can add these manually, or you can let the plugin generate them for you, by setting the Alignment Marker option to Autodetect + Seam.

I use Inkscape to put my islands on A4 pages afterwards.
Transfer the printed pattern onto a sheet of fabric.
I'm using some old cotton curtain as a test.

If you have a sewing machine, you can use it for the big parts. Smaller parts will have to be sewn by hand.
On the left is what it looks like when the sewing is finished. I left a small hole in the back for stuffing, and to turn the piece inside out. On the right is what the piece looks
like when inverted.
All that remains is stuffing the piece with some whool and sewing it shut.

On the right is a 3D scan of the finished piece. Looks a lot like the simulation, right?

If this has your interest, scroll back up, install, and try for yourself.

Have fun!