anchor.diff:  //            let c = V2d (V2i (c / pixelSize)) * pixelSize + V2d(pixelSize / 2.0, pixelSize / 2.0)
   //            let tc = c / V2d heightSampler.Size
   
  -            let h = heightSampler.SampleLevel(tc, 0.0).X * heightScale
  +            let h = heightSampler.SampleLevel(tc, 0.0).X * 1.5
               let wp = world + V4d(0.0, 0.0, h, 0.0)
               wp
   
  
positive.code:                 let tc2 = m.P2.tc
  
  
                  let u = 0.5 * (tc1 - tc0)
                  let v = 0.5 * (tc2 - tc0)
  
                  
                  let h0 = heightSampler.SampleGrad(tc0, u, v).X * heightScale
  
negative.code:         )
  
          
          tessGrid 128
              |> Sg.ofIndexedGeometry
              |> Sg.effect [ 
                  Shader.pgmVertex |> toEffect
                  //Shader.pgmHeight |> toEffect
                  Shader.pgmTessControl |> toEffect
                  Shader.pgmTessEval |> toEffect
                  Shader.pgmFragment |> toEffect
  