Added constrained uniform sampling of angles for SE3, SO3, and Quater…

…nions (#139)

spatialmath/base/quaternions.py

@@ -14,11 +14,14 @@
 import math
 import numpy as np
 import spatialmath.base as smb
+from spatialmath.base.argcheck import getunit
 from spatialmath.base.types import *
+import scipy.interpolate as interpolate
+from typing import Optional
+from functools import lru_cache
 
 _eps = np.finfo(np.float64).eps
 
-
 def qeye() -> QuaternionArray:
     """
     Create an identity quaternion
@@ -843,29 +846,112 @@ def qslerp(
         return q0
 
 
-def qrand() -> UnitQuaternionArray:
+def _compute_cdf_sin_squared(theta: float):
     """
-    Random unit-quaternion
+    Computes the CDF for the distribution of angular magnitude for uniformly sampled rotations.
+    
+    :arg theta: angular magnitude
+    :rtype: float
+    :return: cdf of a given angular magnitude
+    :rtype: float
+
+    Helper function for uniform sampling of rotations with constrained angular magnitude. 
+    This function returns the integral of the pdf of angular magnitudes (2/pi * sin^2(theta/2)).
+    """
+    return (theta - np.sin(theta)) / np.pi
 
+@lru_cache(maxsize=1)
+def _generate_inv_cdf_sin_squared_interp(num_interpolation_points: int = 256) -> interpolate.interp1d:
+    """
+    Computes an interpolation function for the inverse CDF of the distribution of angular magnitude.
+    
+    :arg num_interpolation_points: number of points to use in the interpolation function
+    :rtype: int
+    :return: interpolation function for the inverse cdf of a given angular magnitude
+    :rtype: interpolate.interp1d
+
+    Helper function for uniform sampling of rotations with constrained angular magnitude. 
+    This function returns interpolation function for the inverse of the integral of the 
+    pdf of angular magnitudes (2/pi * sin^2(theta/2)), which is not analytically defined.
+    """
+    cdf_sin_squared_interp_angles = np.linspace(0, np.pi, num_interpolation_points)
+    cdf_sin_squared_interp_values = _compute_cdf_sin_squared(cdf_sin_squared_interp_angles)
+    return interpolate.interp1d(cdf_sin_squared_interp_values, cdf_sin_squared_interp_angles)
+
+def _compute_inv_cdf_sin_squared(x: ArrayLike, num_interpolation_points: int = 256) -> ArrayLike:
+    """
+    Computes the inverse CDF of the distribution of angular magnitude.
+    
+    :arg x: value for cdf of angular magnitudes
+    :rtype: ArrayLike
+    :arg num_interpolation_points: number of points to use in the interpolation function
+    :rtype: int
+    :return: angular magnitude associate with cdf value
+    :rtype: ArrayLike
+
+    Helper function for uniform sampling of rotations with constrained angular magnitude. 
+    This function returns the angle associated with the cdf value derived form integral of 
+    the pdf of angular magnitudes (2/pi * sin^2(theta/2)), which is not analytically defined.
+    """
+    inv_cdf_sin_squared_interp = _generate_inv_cdf_sin_squared_interp(num_interpolation_points)
+    return inv_cdf_sin_squared_interp(x)
+
+def qrand(theta_range:Optional[ArrayLike2] = None, unit: str = "rad", num_interpolation_points: int = 256) -> UnitQuaternionArray:
+    """
+    Random unit-quaternion
+    
+    :arg theta_range: angular magnitude range [min,max], defaults to None.
+    :type xrange: 2-element sequence, optional
+    :arg unit: angular units: 'rad' [default], or 'deg'
+    :type unit: str
+    :arg num_interpolation_points: number of points to use in the interpolation function
+    :rtype: int
+    :arg num_interpolation_points: number of points to use in the interpolation function
+    :rtype: int
     :return: random unit-quaternion
     :rtype: ndarray(4)
 
-    Computes a uniformly distributed random unit-quaternion which can be
-    considered equivalent to a random SO(3) rotation.
+    Computes a uniformly distributed random unit-quaternion, with in a maximum 
+    angular magnitude, which can be considered equivalent to a random SO(3) rotation.
 
     .. runblock:: pycon
 
         >>> from spatialmath.base import qrand, qprint
         >>> qprint(qrand())
     """
-    u = np.random.uniform(low=0, high=1, size=3)  # get 3 random numbers in [0,1]
-    return np.r_[
-        math.sqrt(1 - u[0]) * math.sin(2 * math.pi * u[1]),
-        math.sqrt(1 - u[0]) * math.cos(2 * math.pi * u[1]),
-        math.sqrt(u[0]) * math.sin(2 * math.pi * u[2]),
-        math.sqrt(u[0]) * math.cos(2 * math.pi * u[2]),
-    ]
+    if theta_range is not None:
+        theta_range = getunit(theta_range, unit)
+
+        if(theta_range[0] < 0 or theta_range[1] > np.pi or theta_range[0] > theta_range[1]):
+            ValueError('Invalid angular range. Must be within the range[0, pi].'
+            + f' Recieved {theta_range}.')
+
+        # Sample axis and angle independently, respecting the CDF of the 
+        # angular magnitude under uniform sampling. 
+
+        # Sample angle using inverse transform sampling based on CDF 
+        # of the angular distribution (2/pi * sin^2(theta/2))
+        theta = _compute_inv_cdf_sin_squared(
+            np.random.uniform(
+                low=_compute_cdf_sin_squared(theta_range[0]), 
+                high=_compute_cdf_sin_squared(theta_range[1]),
+            ),
+            num_interpolation_points=num_interpolation_points,
+        )
+        # Sample axis uniformly using 3D normal distributed
+        v = np.random.randn(3) 
+        v /= np.linalg.norm(v)
 
+        return np.r_[math.cos(theta / 2), (math.sin(theta / 2) * v)]
+    else:
+        u = np.random.uniform(low=0, high=1, size=3)  # get 3 random numbers in [0,1]
+        return np.r_[
+            math.sqrt(1 - u[0]) * math.sin(2 * math.pi * u[1]),
+            math.sqrt(1 - u[0]) * math.cos(2 * math.pi * u[1]),
+            math.sqrt(u[0]) * math.sin(2 * math.pi * u[2]),
+            math.sqrt(u[0]) * math.cos(2 * math.pi * u[2]),
+        ]
+
 
 def qmatrix(q: ArrayLike4) -> R4x4:
     """

spatialmath/pose3d.py

@@ -34,7 +34,7 @@
 
 from spatialmath.twist import Twist3
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 if TYPE_CHECKING:
     from spatialmath.quaternion import UnitQuaternion
 
@@ -455,12 +455,16 @@ def Rz(cls, theta, unit: str = "rad") -> Self:
         return cls([smb.rotz(x, unit=unit) for x in smb.getvector(theta)], check=False)
 
     @classmethod
-    def Rand(cls, N: int = 1) -> Self:
+    def Rand(cls, N: int = 1, *, theta_range:Optional[ArrayLike2] = None, unit: str = "rad") -> Self:
         """
         Construct a new SO(3) from random rotation
 
         :param N: number of random rotations
         :type N: int
+        :param theta_range: angular magnitude range [min,max], defaults to None.
+        :type xrange: 2-element sequence, optional
+        :param unit: angular units: 'rad' [default], or 'deg'
+        :type unit: str
         :return: SO(3) rotation matrix
         :rtype: SO3 instance
 
@@ -477,7 +481,7 @@ def Rand(cls, N: int = 1) -> Self:
 
         :seealso: :func:`spatialmath.quaternion.UnitQuaternion.Rand`
         """
-        return cls([smb.q2r(smb.qrand()) for _ in range(0, N)], check=False)
+        return cls([smb.q2r(smb.qrand(theta_range=theta_range, unit=unit)) for _ in range(0, N)], check=False)
 
     @overload
     @classmethod
@@ -1517,6 +1521,8 @@ def Rand(
         xrange: Optional[ArrayLike2] = (-1, 1),
         yrange: Optional[ArrayLike2] = (-1, 1),
         zrange: Optional[ArrayLike2] = (-1, 1),
+        theta_range:Optional[ArrayLike2] = None,
+        unit: str = "rad",
     ) -> SE3:  # pylint: disable=arguments-differ
         """
         Create a random SE(3)
@@ -1527,6 +1533,10 @@ def Rand(
         :type yrange: 2-element sequence, optional
         :param zrange: z-axis range [min,max], defaults to [-1, 1]
         :type zrange: 2-element sequence, optional
+        :param theta_range: angular magnitude range [min,max], defaults to None -> [0,pi].
+        :type xrange: 2-element sequence, optional
+        :param unit: angular units: 'rad' [default], or 'deg'
+        :type unit: str
         :param N: number of random transforms
         :type N: int
         :return: SE(3) matrix
@@ -1557,7 +1567,7 @@ def Rand(
         Z = np.random.uniform(
             low=zrange[0], high=zrange[1], size=N
         )  # random values in the range
-        R = SO3.Rand(N=N)
+        R = SO3.Rand(N=N, theta_range=theta_range, unit=unit)
         return cls(
             [smb.transl(x, y, z) @ smb.r2t(r.A) for (x, y, z, r) in zip(X, Y, Z, R)],
             check=False,

spatialmath/quaternion.py

@@ -1225,12 +1225,16 @@ def Rz(cls, angles: ArrayLike, unit: Optional[str] = "rad") -> UnitQuaternion:
         )
 
     @classmethod
-    def Rand(cls, N: int = 1) -> UnitQuaternion:
+    def Rand(cls, N: int = 1, *, theta_range:Optional[ArrayLike2] = None, unit: str = "rad") -> UnitQuaternion:
         """
         Construct a new random unit quaternion
 
         :param N: number of random rotations
         :type N: int
+        :param theta_range: angular magnitude range [min,max], defaults to None -> [0,pi].
+        :type xrange: 2-element sequence, optional
+        :param unit: angular units: 'rad' [default], or 'deg'
+        :type unit: str
         :return: random unit-quaternion
         :rtype: UnitQuaternion instance
 
@@ -1248,7 +1252,7 @@ def Rand(cls, N: int = 1) -> UnitQuaternion:
 
         :seealso: :meth:`UnitQuaternion.Rand`
         """
-        return cls([smb.qrand() for i in range(0, N)], check=False)
+        return cls([smb.qrand(theta_range=theta_range, unit=unit) for i in range(0, N)], check=False)
 
     @classmethod
     def Eul(cls, *angles: List[float], unit: Optional[str] = "rad") -> UnitQuaternion:

tests/test_pose3d.py

@@ -72,11 +72,19 @@ def test_constructor(self):
         array_compare(R, np.eye(3))
         self.assertIsInstance(R, SO3)
 
+        np.random.seed(32)
         # random
         R = SO3.Rand()
         nt.assert_equal(len(R), 1)
         self.assertIsInstance(R, SO3)
 
+        # random constrained
+        R = SO3.Rand(theta_range=(0.1, 0.7))
+        self.assertIsInstance(R, SO3)
+        self.assertEqual(R.A.shape, (3, 3))
+        self.assertLessEqual(R.angvec()[0], 0.7)
+        self.assertGreaterEqual(R.angvec()[0], 0.1)
+
         # copy constructor
         R = SO3.Rx(pi / 2)
         R2 = SO3(R)
@@ -816,12 +824,13 @@ def test_constructor(self):
         array_compare(R, np.eye(4))
         self.assertIsInstance(R, SE3)
 
+        np.random.seed(65)
         # random
         R = SE3.Rand()
         nt.assert_equal(len(R), 1)
         self.assertIsInstance(R, SE3)
 
-        # random
+        # random 
         T = SE3.Rand()
         R = T.R
         t = T.t
@@ -847,6 +856,13 @@ def test_constructor(self):
         nt.assert_equal(TT.y, ones * t[1])
         nt.assert_equal(TT.z, ones * t[2])
 
+        # random constrained
+        T = SE3.Rand(theta_range=(0.1, 0.7))
+        self.assertIsInstance(T, SE3)
+        self.assertEqual(T.A.shape, (4, 4))
+        self.assertLessEqual(T.angvec()[0], 0.7)
+        self.assertGreaterEqual(T.angvec()[0], 0.1)
+
         # copy constructor
         R = SE3.Rx(pi / 2)
         R2 = SE3(R)

tests/test_quaternion.py

@@ -48,6 +48,19 @@ def test_constructor_variants(self):
         nt.assert_array_almost_equal(
             UnitQuaternion.Rz(-90, "deg").vec, np.r_[1, 0, 0, -1] / math.sqrt(2)
         )
+
+        np.random.seed(73)
+        q = UnitQuaternion.Rand(theta_range=(0.1, 0.7))
+        self.assertIsInstance(q, UnitQuaternion)
+        self.assertLessEqual(q.angvec()[0], 0.7)
+        self.assertGreaterEqual(q.angvec()[0], 0.1)
+
+
+        q = UnitQuaternion.Rand(theta_range=(0.1, 0.7))
+        self.assertIsInstance(q, UnitQuaternion)
+        self.assertLessEqual(q.angvec()[0], 0.7)
+        self.assertGreaterEqual(q.angvec()[0], 0.1)
+
 
     def test_constructor(self):
         qcompare(UnitQuaternion(), [1, 0, 0, 0])