I'm trying to write an importer addon for blender using the python api. And the documentation has been less than "helpful" so far. In threejs you have a page like this: https://threejs.org/docs/index.html#api/objects/SkinnedMesh

for ( var i = 0; i < geometry.vertices.length; i ++ ) {

    // Imaginary functions to calculate the indices and weights
    // This part will need to be changed depending your skeleton and model
    var skinIndex = calculateSkinIndex( geometry.vertices, i );
    var skinWeight = calculateSkinWeight( geometry.vertices, i );

    // Ease between each bone
    geometry.skinIndices.push( new THREE.Vector4( skinIndex, skinIndex + 1, 0, 0 ) );
    geometry.skinWeights.push( new THREE.Vector4( 1 - skinWeight, skinWeight, 0, 0 ) );


I'm trying to find the equivalent code or examples for the Blender Python API. After I create a mesh, how do I then go about setting weights and indices for each index?

mesh = bpy.data.meshes.new('Mesh')
mesh.from_pydata(self.verts, [], self.faces)
mesh_object = bpy.data.objects.new('Mesh_Object', mesh)
# How to add in vertex weights and indices? 
  • $\begingroup$ Related $\endgroup$
    – batFINGER
    Jul 22, 2018 at 9:16
  • $\begingroup$ The related link only shows weights. How do you know which bone each vertex is being weighted to without assigning an index? Is the group number the index? Do you assign a vertex group and that's bone 0? And then another group and that's bone 1? $\endgroup$ Jul 22, 2018 at 10:47
  • $\begingroup$ docs.blender.org/manual/en/dev/rigging/armatures/skinning/… $\endgroup$
    – batFINGER
    Jul 22, 2018 at 12:47
  • $\begingroup$ And that's an explanation of how to assign weights using the UI. It doesn't explain any of the syntax or data structures needed in the python api. $\endgroup$ Jul 22, 2018 at 13:45

1 Answer 1


Okay, after posting on the blender forums, I was able to find an answer. To add vertex weights to a mesh you have to do the following:

  1. Create a Mesh
  2. Create an Armature
  3. Create a list of vertex groups (the index of the vertex group is the bone index)
  4. Set the parent of the mesh to the Armature
  5. Add an Armature modifier to the mesh (so it know to deform by vertex group)

The code is kind of long, but it's one function so hopefully it's easier to trace through that the entire import plugin.

# Create Armature

armature = bpy.data.armatures.new('Armature')
armature_object=bpy.data.objects.new('Armature_object', armature)
armature.draw_type = "STICK"

bpy.context.scene.objects.active = armature_object


print("Creating bones")

for bone in self.bones:
    id = bone['id']
    pid = bone['parent_id']
    mtx = bone['matrix']

    joint = armature.edit_bones.new("bone_%03d" % id)
    joint.head = Vector( (0,0,0) )

    if pid != -1:
        joint.parent = armature.edit_bones[pid]
        joint.head = joint.parent.tail

    tail = Vector( (mtx[12], mtx[14], mtx[13]) )
    joint.tail = joint.head + tail

# Create Mesh

bm = bmesh.new()
mesh = bpy.data.meshes.new('Mesh')

for vertex in self.verts:
    pos = vertex['pos']
    x = pos[0]
    y = pos[1]
    z = pos[2]
    vert = bm.verts.new( (x, z, y) ) 


for group in self.faces:
    for i in range(0, group['faceCount']):
        a = bm.verts[group['indices'][i * 3 + 0]['index']]
        b = bm.verts[group['indices'][i * 3 + 1]['index']]
        c = bm.verts[group['indices'][i * 3 + 2]['index']]
        face = bm.faces.new( (a, b, c) )
        face.material_index = group['materialIndex']
        # face.loop bmloop sequence to add uv


mesh_object = bpy.data.objects.new('Mesh_Object', mesh)

# For Vertex Weights, first we create a group for each bone
for i in range(0, len(self.bones)):
    mesh_object.vertex_groups.new("bone_%03d" % i)

#Loop over all of the vertices
for vertex in self.verts:
    index = vertex['index']
    indices = vertex['indices']
    weights = vertex['weights']
    mesh_object.vertex_groups[indices[0]].add([index], 1.0, "REPLACE")


mesh_object.parent = armature_object
modifier = mesh_object.modifiers.new(type='ARMATURE', name="Armature")
modifier.object = armature_object

bpy.context.scene.objects.active = mesh_object

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