anchor.diff:                  let tc2 = m.P2.tc
   
   
  -                let u = 0.5 * (tc1 - tc0)
  -                let v = 0.5 * (tc2 - tc0)
  +                let u = 1.0 * (tc1 - tc0)
  +                let v = 1.0 * (tc2 - tc0)
   
                   
                   let h0 = heightSampler.SampleGrad(tc0, u, v).X * heightScale
  
positive.code: 
          let pgmTessControl (m : Patch4<Vertex>) =
              tessControl {
                  let p0 = sampleHeight m.P0.wp
                  let p1 = sampleHeight m.P1.wp
                  let p2 = sampleHeight m.P2.wp
                  let p3 = sampleHeight m.P3.wp
  
                  let size = V2d heightSampler.Size
  
negative.code:     let tessGrid (s : int) =
          let s = max 2 (s + 1)
          let positions =
              let offset = -10.0
              let step = 20.0 / float (s - 1)
              [|
                  for y in 0 .. s-1 do
                      for x in 0 .. s-1 do
  