warp.mesh\_eval\_velocity
=========================

.. function:: warp._src.lang.mesh_eval_velocity(id: uint64, face: int32, bary_u: float32, bary_v: float32) -> vec3f

   .. hlist::
      :columns: 8

      * Kernel
      * Differentiable

   Evaluate the interpolated velocity on a face of the :class:`warp.Mesh` from barycentric coordinates.
   
   Linearly interpolates the face's three per-vertex velocities the same way
   :func:`mesh_eval_position` interpolates positions:
   ``v0 * bary_u + v1 * bary_v + v2 * (1 - bary_u - bary_v)``. Requires the mesh to have been
   constructed with a ``velocities`` array; returns a zero vector otherwise.
   
   :param id: The mesh identifier
   :param face: The face (triangle) index
   :param bary_u: Barycentric weight of the face's first vertex
   :param bary_v: Barycentric weight of the face's second vertex
   
   :returns: The interpolated velocity, or a zero vector if the mesh has no velocities.
   
   .. rubric:: Example
   
   .. testcode::
   
       @wp.kernel
       def vel_at(mesh_id: wp.uint64, out: wp.array[wp.vec3]):
           out[0] = wp.mesh_eval_velocity(mesh_id, 0, 0.25, 0.25)
   
       points = wp.array([[0,0,0],[1,0,0],[1,1,0],[0,1,0],[0,0,1],[1,0,1],[1,1,1],[0,1,1]], dtype=wp.vec3)
       indices = wp.array([0,3,2, 0,2,1,  4,5,6, 4,6,7,  0,1,5, 0,5,4,
                           2,3,7, 2,7,6,  0,4,7, 0,7,3,  1,2,6, 1,6,5], dtype=wp.int32)
       velocities = wp.array(np.tile([0.0, 0.0, 1.0], (8, 1)), dtype=wp.vec3)
       mesh = wp.Mesh(points=points, indices=indices, velocities=velocities)
   
       out = wp.zeros(1, dtype=wp.vec3)
       wp.launch(vel_at, dim=1, inputs=[mesh.id], outputs=[out])
       print(out.numpy()[0])
   
   .. testoutput::
   
       [0. 0. 1.]