title: Streamline usage for drawing trajectory
body: <p>My project is based on finding trajectory of objects registered by camera. I have already obtained a 3d matrix (2 dimensions are for image scene, 3rd axis is a time arrow) where I put the points which represent a following position of an object. Each point has an index by which I differ objects (points with value "3" refers only to a trajectory of object with index "3"). Now I would like to draw a line that would connect the following (in time axis) points.</p>
  
  <p>I found out that streamline(x, y, z, u, v, w, sx, sy, sz) function would suit me perfectly. However I can't define the proper u, v, w matrix. I thought that if I want a streamline for transition from point A(xa, ya, za) to point B(xb, yb, zb) then I need to calculate vector AB = [xb-xa, yb-ya, zb-za] and put it to my u, v, w: u(xa, ya, za) = xb-xa; v(xa, ya, za) = yb-ya; w(xa, ya, za) = zb-za;</p>
  
  <p>and the rest of u, v, w for points with no trajectory remain 0. But the result of streamline is empty space. Could anyone give me some advice ?</p>