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 (left, top, right, bottom, r, g, b) =
    #[ left, bottom, r, g, b
     , right, bottom, r, g, b
     , left, top, r, g, b

     , left, top, r, g, b
     , right, bottom, r, g, b
     , right, top, r, g, b
     ]

  fun fromPixelX (xpos, windowWidth, windowHeight) =
    let
      val halfWidth = Real32.fromInt (windowWidth div 2)
      val xpos = xpos - halfWidth
    in
      if windowWidth > windowHeight then
        let
          val difference = windowWidth - windowHeight
          val offset = Real32.fromInt (difference div 2)
        in
          xpos / (halfWidth - offset)
        end
      else
        xpos / halfWidth
    end

  fun fromPixelY (ypos, windowWidth, windowHeight) =
    let
      val halfHeight = Real32.fromInt (windowHeight div 2)
      val ypos = ~(ypos - halfHeight)
    in
      if windowHeight > windowWidth then
        let
          val difference = windowHeight - windowWidth
          val offset = Real32.fromInt (difference div 2)
        in
          ypos / (halfHeight - offset)
        end
      else
        ypos / halfHeight
    end
end