Displacement map vertices

[pask]

I have e heightMap and i load it into a texture , create a meterial and set the material displacementMap = heightMapTexture , apply the material and planeGeometry to a mesh and add the to the scene. The question is how can i access the values ot the position vertices , i tried looking at the material,texture and plane geometry properties but found nothing. Also can the displacement map be applied to geometry buffer?