structure Ndc = 
struct
  (* ndc = normalised device coordinates *)

  fun ltrbToVertex (left, top, right, bottom) =
    #[ left, bottom
     , right, bottom
     , left, top

     , left, top
     , right, bottom
     , right, top
     ]

  fun ltrbToVertexRgb (startX, startY, endX, endY, r, g, b) =
    #[ startX, endY, r, g, b
     , endX, endY, r, g, b
     , startX, startY, r, g, b

     , startX, startY, r, g, b
     , endX, endY, r, g, b
     , endX, startY, r, g, b
     ]

  fun fromPixelX (xpos, windowWidth, windowHeight) =
    let
      val halfWidth = Real32.fromInt windowWidth / 2.0
    in
      (xpos - halfWidth) / halfWidth
    end

  fun fromPixelY (ypos, windowWidth, windowHeight) =
    let
      val halfHeight = Real32.fromInt windowHeight / 2.0
    in
      ~((ypos - halfHeight) / halfHeight)
    end
end