Specificy yaw in funciton name, better doc string.

Author: myeatman-bdai

spatialmath/pose3d.py

@@ -1162,21 +1162,33 @@ def inv(self) -> SE3:
         else:
             return SE3([smb.trinv(x) for x in self.A], check=False)
 
-    def SE2(self) -> SE2:
+    def yaw_SE2(self, order: str = "zyx") -> SE2:
         """
-        Create SE(2) from SE(3)
+        Create SE(2) from SE(3) yaw angle.
 
-        :return: SE(2) with same rotation as the yaw angle using the 'zyx' convention,
-            and translation in x,y
+        :param order: angle sequence order, default to 'zyx'
+        :type order: str
+        :return: SE(2) with same rotation as the yaw angle using the roll-pitch-yaw convention,
+            and translation in x,y.
         :rtype: SE2 instance
 
-        "Flattens" 3D rigid-body motion to 2D, along the z axis.
+        Roll-pitch-yaw corresponds to successive rotations about the axes specified by ``order``:
+
+            - ``'zyx'`` [default], rotate by yaw about the z-axis, then by pitch about the new y-axis,
+              then by roll about the new x-axis.  Convention for a mobile robot with x-axis forward
+              and y-axis sideways.
+            - ``'xyz'``, rotate by yaw about the x-axis, then by pitch about the new y-axis,
+              then by roll about the new z-axis. Convention for a robot gripper with z-axis forward
+              and y-axis between the gripper fingers.
+            - ``'yxz'``, rotate by yaw about the y-axis, then by pitch about the new x-axis,
+              then by roll about the new z-axis. Convention for a camera with z-axis parallel
+              to the optic axis and x-axis parallel to the pixel rows.
 
         """
         if len(self) == 1:
-            return SE2(self.x, self.y, self.rpy()[2])
+            return SE2(self.x, self.y, self.rpy(order = order)[2])
         else:
-            return SE2([e.SE2() for e in self])
+            return SE2([e.yaw_SE2() for e in self])
 
     def delta(self, X2: Optional[SE3] = None) -> R6:
         r"""

tests/test_pose3d.py

@@ -662,13 +662,13 @@ def test_functions(self):
 
         # conversion to SE2
         poseSE3 = SE3.Tx(3.3) * SE3.Rz(1.5)
-        poseSE2 = poseSE3.SE2()
+        poseSE2 = poseSE3.yaw_SE2()
         nt.assert_almost_equal(poseSE3.R[0:1,0:1], poseSE2.R[0:1,0:1])
         nt.assert_equal(poseSE3.x , poseSE2.x)
         nt.assert_equal(poseSE3.y , poseSE2.y)
 
         posesSE3 = SE3([poseSE3, poseSE3])
-        posesSE2 = posesSE3.SE2()
+        posesSE2 = posesSE3.yaw_SE2()
         nt.assert_equal(len(posesSE2), 2)
 
     def test_functions_vect(self):