Using .skn skin file as model texture

[modpy44]

Does anyone have an idea how to convert a .dae model into .skn and use it inside mujoco xml model ?

[yuvaltassa]

Do you use Python?
I can post a Python snippet for creating an SKN file, though you would have to parse the DAE into Python yourself.
That said, increasing MuJoCo's asset import capabilities is high on our list, we hope MuJoCo will be able to do this natively soon.

[modpy44]

@yuvaltassa that would be great!

[yuvaltassa]

First of all, to see the output of this process, take a look at our skinned dog model (see corresponding video here to see what a MuJoCo skin can do for you).

The first thing I needed to do was figure out the vertex bone weights, this an nvertices x nbody matrix. Up to you how you do that. Once I had the weights, I first created an XML-based skin using pymjcf, but that's more of a debugging step and a convenient way to access all the relevant data. You can skip that if you like.

The snippet that actually saves all the data to an SKN file looks like

f = open('dog_skin.skn', 'w+b')
nvert = skin.vertex.size//3
f.write(struct.pack('4i', nvert, nvert, skin.face.size//3, physics.model.nbody-1))
f.write(struct.pack(str(skin.vertex.size)+'f', *skin.vertex))
f.write(struct.pack(str(2*nvert)+'f', *physics.model.mesh_texcoord.flatten()))
f.write(struct.pack(str(skin.face.size)+'i', *skin.face))
for bone in skin.bone:
  name_length = len(bone.body)
  assert name_length <= 40
  f.write(struct.pack(str(name_length)+'c', *[s.encode() for s in bone.body]))
  f.write((40-name_length)*b'\x00')
  f.write(struct.pack('3f', *bone.bindpos))
  f.write(struct.pack('4f', *bone.bindquat))
  f.write(struct.pack('i', bone.vertid.size))
  f.write(struct.pack(str(bone.vertid.size)+'i', *bone.vertid))
  f.write(struct.pack(str(bone.vertid.size)+'f', *bone.vertweight))
f.close()

Hope this helps, I'd recommend making a very simple test case first.

[modpy44]

Thanks @yuvaltassa