# PyTorch Robot Kinematics
- Parallel and differentiable forward kinematics (FK) and Jacobian calculation
- Load robot description from URDF, SDF, and MJCF formats 

# Usage
Clone repository somewhere, then `pip3 install -e .` to install in editable mode.

See `tests` for code samples; some are also shown here.

## Forward Kinematics (FK)
```python
import math
import pytorch_kinematics as pk

# load robot description from URDF and specify end effector link
chain = pk.build_serial_chain_from_urdf(open("kuka_iiwa.urdf").read(), "lbr_iiwa_link_7")
# prints out the (nested) tree of links
print(chain)
# prints out list of joint names
print(chain.get_joint_parameter_names())

# specify joint values (can do so in many forms)
th = [0.0, -math.pi / 4.0, 0.0, math.pi / 2.0, 0.0, math.pi / 4.0, 0.0]
# do forward kinematics and get transform objects; end_only=False gives a dictionary of transforms for all links
ret = chain.forward_kinematics(th, end_only=False)
# look up the transform for a specific link
tg = ret['lbr_iiwa_link_7']
# get transform matrix (1,4,4), then convert to separate position and unit quaternion
m = tg.get_matrix()
pos = m[:, :3, 3]
rot = pk.matrix_to_quaternion(m[:, :3, :3])
```

We can parallelize FK by passing in 2D joint values, and also use CUDA if available
```python
import torch
import pytorch_kinematics as pk

d = "cuda" if torch.cuda.is_available() else "cpu"
dtype = torch.float64

chain = pk.build_serial_chain_from_urdf(open("kuka_iiwa.urdf").read(), "lbr_iiwa_link_7")
chain = chain.to(dtype=dtype, device=d)

N = 1000
th_batch = torch.rand(N, len(chain.get_joint_parameter_names()), dtype=dtype, device=d)

# order of magnitudes faster when doing FK in parallel
# elapsed 0.008678913116455078s for N=1000 when parallel
# (N,4,4) transform matrix; only the one for the end effector is returned since end_only=True by default
tg_batch = chain.forward_kinematics(th_batch)

# elapsed 8.44686508178711s for N=1000 when serial
for i in range(N):
    tg = chain.forward_kinematics(th_batch[i])
```

We can compute gradients through the FK
```python
import torch
import math
import pytorch_kinematics as pk

chain = pk.build_serial_chain_from_urdf(open("kuka_iiwa.urdf").read(), "lbr_iiwa_link_7")

# require gradient through the input joint values
th = torch.tensor([0.0, -math.pi / 4.0, 0.0, math.pi / 2.0, 0.0, math.pi / 4.0, 0.0], requires_grad=True)
tg = chain.forward_kinematics(th)
m = tg.get_matrix()
pos = m[:, :3, 3]
pos.norm().backward()
# now th.grad is populated
```

We can load SDF and MJCF descriptions too, and pass in joint values via a dictionary (unspecified joints get th=0) for non-serial chains
```python
import math
import torch
import pytorch_kinematics as pk

chain = pk.build_chain_from_sdf(open("simple_arm.sdf").read())
ret = chain.forward_kinematics({'arm_elbow_pan_joint': math.pi / 2.0, 'arm_wrist_lift_joint': -0.5})
# recall that we specify joint values and get link transforms
tg = ret['arm_wrist_roll']

# can also do this in parallel
N = 100
ret = chain.forward_kinematics({'arm_elbow_pan_joint': torch.rand(N, 1), 'arm_wrist_lift_joint': torch.rand(N, 1)})
# (N, 4, 4) transform object
tg = ret['arm_wrist_roll']

# building the robot from a MJCF file
chain = pk.build_chain_from_mjcf(open("ant.xml").read())
print(chain)
print(chain.get_joint_parameter_names())
th = {'hip_1': 1.0, 'ankle_1': 1}
ret = chain.forward_kinematics(th)

chain = pk.build_chain_from_mjcf(open("humanoid.xml").read())
print(chain)
print(chain.get_joint_parameter_names())
th = {'left_knee': 0.0, 'right_knee': 0.0}
ret = chain.forward_kinematics(th)
```

## Jacobian calculation
The Jacobian (in the kinematics context) is a matrix describing how the end effector changes with respect to joint value changes
(where ![dx](https://latex.codecogs.com/png.latex?%5Cinline%20%5Cdot%7Bx%7D) is the twist, or stacked velocity and angular velocity):
![jacobian](https://latex.codecogs.com/png.latex?%5Cinline%20%5Cdot%7Bx%7D%3DJ%5Cdot%7Bq%7D) 

For `SerialChain` we provide a differentiable and parallelizable method for computing the Jacobian with respect to the base frame.
```python
import math
import torch
import pytorch_kinematics as pk

# can convert Chain to SerialChain by choosing end effector frame
chain = pk.build_chain_from_sdf(open("simple_arm.sdf").read())
# print(chain) to see the available links for use as end effector
# note that any link can be chosen; it doesn't have to be a link with no children
chain = pk.SerialChain(chain, "arm_wrist_roll_frame")

chain = pk.build_serial_chain_from_urdf(open("kuka_iiwa.urdf").read(), "lbr_iiwa_link_7")
th = torch.tensor([0.0, -math.pi / 4.0, 0.0, math.pi / 2.0, 0.0, math.pi / 4.0, 0.0])
# (1,6,7) tensor, with 7 corresponding to the DOF of the robot
J = chain.jacobian(th)

# get Jacobian in parallel and use CUDA if available
N = 1000
d = "cuda" if torch.cuda.is_available() else "cpu"
dtype = torch.float64

chain = chain.to(dtype=dtype, device=d)
# Jacobian calculation is differentiable
th = torch.rand(N, 7, dtype=dtype, device=d, requires_grad=True)
# (N,6,7)
J = chain.jacobian(th)

# can get Jacobian at a point offset from the end effector (location is specified in EE link frame)
# by default location is at the origin of the EE frame
loc = torch.rand(N, 3, dtype=dtype, device=d)
J = chain.jacobian(th, locations=loc)
```

The Jacobian can be used to do inverse kinematics. See [IK survey](https://www.math.ucsd.edu/~sbuss/ResearchWeb/ikmethods/iksurvey.pdf)
for a survey of ways to do so. Note that IK may be better performed through other means (but doing it through the Jacobian can give an end-to-end differentiable method).

# Credits
- `pytorch_kinematics/transforms` is extracted from [pytorch3d](https://github.com/facebookresearch/pytorch3d) with minor extensions.
This was done instead of including `pytorch3d` as a dependency because it is hard to install and most of its code is unrelated.
  An important difference is that we use left hand multiplied transforms as is convention in robotics (T * pt) instead of their
  right hand multiplied transforms.
- `pytorch_kinematics/urdf_parser_py`, and `pytorch_kinematics/mjcf_parser` is extracted from [kinpy](https://github.com/neka-nat/kinpy), as well as the FK logic.
This repository ports the logic to pytorch, parallelizes it, and provides some extensions.