v3.0 python api for setting the scale of InstanceOnPoints

Question

I'm relatively new to blender and scripting in blender so I am missing some basic concepts. I'm working with Blender 3.0.

I'm trying to import some position and radius information for many particles (~18k). After a bit of stumbling around I've found that importing a vertex cloud and creating sphere instances on the vertices is a very fast way to load my geometry.

My problem is that I can't figure out how to scale the instances - I know from playing around with geometry nodes in the gui that a 'random value' node can output per-instance scaling vectors. However I want to input a list of scaling vectors to my InstanceOnPoints geometry node.

When I naively try to do this by giving my node's input['Scale'] a numpy array, it throws an error (seemingly it wants just a length 3 list to turn into a single vector).

I'm having a lot of trouble searching for this information in the docs or elsewhere, so any help is appreciated.

My script:

import bpy
import bmesh
import numpy as np
import mathutils

# load some data with each row [x,y,z,radius,type]
fdir = '/Users/bryan/Downloads/frame_0.npy'
pdata = np.load(fdir)
scale = 0.1
box = np.array([200, 200, 600])*scale

# for each particle type
for typ in [0,1]:
    # select some subset of particles
    cond = (pdata[:,2]<10)*(pdata[:,4]==typ)
    pos = pdata[cond,0:3]*scale + np.array([[0,0,box[2]/2]])

    # these are the scale vectors I want to apply
    scales = pdata[cond,3].reshape(-1,1)*scale*np.ones((len(pos),3))
    
    # make vertex cloud
    obj_name = "points_t{t}".format(t=typ)
    mesh_data = bpy.data.meshes.new(obj_name+"_data")
    pos_obj = bpy.data.objects.new(obj_name, mesh_data)
    bpy.context.collection.objects.link(pos_obj)
    mesh_data.from_pydata(pos, [], [])

    # add the geometry nodes
    gm = pos_obj.modifiers.new('GN', 'NODES')
    geom_in = gm.node_group.nodes.get('Group Input')
    geom_out = gm.node_group.nodes.get('Group Output')
    points = gm.node_group.nodes.new('GeometryNodeMeshToPoints')
    sphere = gm.node_group.nodes.new('GeometryNodeMeshIcoSphere')
    instances = gm.node_group.nodes.new('GeometryNodeInstanceOnPoints')
    
    sphere.inputs['Subdivisions'].default_value = 2
     
    gm.node_group.links.new(geom_in.outputs['Geometry'], points.inputs['Mesh'])
    gm.node_group.links.new(points.outputs['Points'], instances.inputs['Points'])
    gm.node_group.links.new(sphere.outputs['Mesh'], instances.inputs['Instance'])
    gm.node_group.links.new(instances.outputs['Instances'], geom_out.inputs['Geometry'])
    
    instances.inputs['Scale'].default_value = scales # this causes an error

Answer 1

Per-instance scaling on a point cloud can be accomplished by setting the radius attribute of the point cloud, and adding a InputRadius node. The following lines need to be added to the original script:

pos_obj.data.attributes.new(name='radius', type='FLOAT', domain='POINT')
pos_obj.data.attributes['radius'].data.foreach_set('value', radii) # where radii is a list of radii of the same length as the number of points

rnode = gm.node_group.nodes.new('GeometryNodeInputRadius')
    
gm.node_group.links.new(rnode.outputs['Radius'], instances.inputs['Scale'])