   Tangents for heightmap
  Im working on a water shader, that displaces the vertices of a plane in the vertex shader, via a few height maps running in different directions animated over time.But with the verices varying in height over time, the tangents are going to vary and need to be calculated each frame. But how can i do this if i dont have access to the other 2 vertices in the triangle?   
  as you're animating the vertices probably in worldspace, tangentsspace is also in worldspace, so you can  calculate the height for the vertex left/right/top/bottom of the current vertex, although that might rise the computatoin amount quite a lot.depending on the formular you're using to calculate the height, you could calculate the tangentspace using the derivered formular.   