anchor.diff:          let heightSampler =
               sampler2d {
                   texture uniform?HeightFieldTexture
  -                filter Filter.MinMagLinear
  +                filter Filter.Anisotropic
                   addressU WrapMode.Clamp
                   addressV WrapMode.Clamp
               }
   
           let planeSize = 10.0
           let heightScale = 0.5
  -        let pixelSize = 2.0
  +        let pixelSize = 5.0
   
           [<ReflectedDefinition>]
           let sampleHeight (world : V4d)  =
  
positive.code:                 //return { innerLevel = [|1.0; 1.0|]; outerLevel = [| 1.0; 1.0; 1.0; 1.0 |]} 
                  return { innerLevel = [|i0; i1|]; outerLevel = [| t0; t1; t2; t3 |]}  
              }
  
          let hTessEval (m : Patch4<Vertex>) =
              tessEval {
                  let c = m.TessCoord.XY
  
                  let p0 = m.P0.wp * (1.0 - c.X) + m.P1.wp * c.X
                  let p1 = m.P2.wp * (1.0 - c.X) + m.P3.wp * c.X
                  let wp = p0 * (1.0 - c.Y) + p1 * c.Y
  
                  let tc0 = m.P0.tc * (1.0 - c.X) + m.P1.tc * c.X
                  let tc1 = m.P2.tc * (1.0 - c.X) + m.P3.tc * c.X
                  let tc = tc0 * (1.0 - c.Y) + tc1 * c.Y
  
                  let wp = sampleHeight2 wp tc
  
                  return {
                      pos = uniform.ViewProjTrafo * wp
  
negative.code:         )
  
          
          tessGrid 128
              |> Sg.ofIndexedGeometry
              |> Sg.effect [
                  DefaultSurfaces.trafo |> toEffect 
                  Shader.hTessControl |> toEffect
                  Shader.hTessEval |> toEffect
                  //Shader.hGeometry |> toEffect
                  DefaultSurfaces.constantColor C4f.White |> toEffect
                  DefaultSurfaces.diffuseTexture |> toEffect
  //                DefaultSurfaces.simpleLighting |> toEffect 
  