Use a streamtube trace to visualize flow in a vector field. Specify a vector field using 6 1D arrays of equal length, 3 position arrays x, y and z and 3 vector component arrays u, v, and w. By default, the tubes’ starting positions will be cut from the vector field’s x-z plane at its minimum y value. To specify your own starting position, use attributes starts.x, starts.y and starts.z. The color is encoded by the norm of (u, v, w), and the local radius by the divergence of (u, v, w).