add example to visualize floating base velocity

examples/floating-base-velocity-viewer.py

@@ -0,0 +1,92 @@
+import pinocchio as pin
+import numpy as np
+import hppfcl
+
+from pinocchio.visualize import MeshcatVisualizer
+from time import sleep
+
+def create_pin_cube_model(j0='freeflyer'):
+    model = pin.Model()
+
+    if j0 == 'freeflyer':
+        j0 = pin.JointModelFreeFlyer()
+    elif j0 == 'spherical':
+        j0 = pin.JointModelSpherical()
+    elif j0 == 'sphericalzyx':
+        j0 = pin.JointModelSphericalZYX()
+    else:
+        raise ValueError("Unknown joint type")
+
+    jointCube = model.addJoint(0, j0, pin.SE3.Identity(), "joint0")
+    M = pin.SE3(np.eye(3), np.matrix([0.0, 0.0, 0.0]).T)
+    model.appendBodyToJoint(jointCube, pin.Inertia.FromBox(1, 0.8, 0.4, 0.2), M)
+    return model
+
+def create_pin_geometry_cube_model(model):
+    jointCube = model.getFrameId("joint0")
+    geom_model = pin.GeometryModel()
+    cube_shape = hppfcl.Box(0.8, 0.4, 0.2)  # x, y, z
+    cube = pin.GeometryObject(
+        "cube_shape", 0, jointCube, cube_shape, pin.SE3.Identity()
+    )
+    cube.meshColor = np.array([1.0, 0.1, 0.1, 0.5])
+    geom_model.addGeometryObject(cube)
+    return geom_model
+
+def create_model(joint0_type='freeflyer'):
+    model = create_pin_cube_model(joint0_type)
+    geom_model = create_pin_geometry_cube_model(model)
+    return model, geom_model
+
+def pin_step(model, vizer, v_index_increment, dt = 0.1, enable_gravity=True):
+
+    if not enable_gravity:
+        model.gravity.setZero()
+
+    data = model.createData()
+
+    tau = np.zeros(model.nv)
+    q = pin.neutral(model)
+    v = np.zeros(model.nv)
+    v[v_index_increment] += 1.
+
+    vizer.display(q)
+    print(f"{q=}, {v=}")
+
+    sleep(1)
+
+    a = pin.aba(model, data, q, v, tau)
+    v_next = v + a * dt
+    q_next = pin.integrate(model, q, v_next * dt)
+
+    print(f"{a=}, {q_next=}, {v_next=}")
+    return q_next, v_next
+
+
+if __name__ == "__main__":
+
+    list_joint0 = ['freeflyer', 'sphericalzyx', 'spherical']
+
+    for joint0_name in list_joint0:
+
+        print(f"\njoint0_name = {joint0_name}")
+
+        model, geom_model = create_model(joint0_name)
+        print(f"{model.nq=}")
+        print(f"{model.nv=}")
+
+        q = pin.neutral(model)
+        print(f"neutral q configuration: {q=}")
+
+        vizer = MeshcatVisualizer(model, geom_model, geom_model)
+        vizer.initViewer(open=True, loadModel=True)
+        vizer.display(q)
+
+        sleep(2)
+
+        for i in range(model.nv):
+            print(f"v[{i}] += 1")
+            q_next, v_next = pin_step(model, vizer, i, dt=0.2, enable_gravity=False)
+            vizer.display(q_next)
+            sleep(2)
+