anchor.diff:  let GRAYSCALE_MODE = true
   let IMAGE_READ_MODE = if GRAYSCALE_MODE then ImreadModes.GrayScale else ImreadModes.Color
   let MAT_TYPE = if GRAYSCALE_MODE then MatType.CV_8UC1 else MatType.CV_8UC3
  -
  -//thresholding images
   let THRESHOLD = 130.
   
   type KeyPolygon = {
  
positive.code:     BottomRight: Point
      BottomLeft: Point }
  
  //inclusive coordinates,
  //N.B. OpenCV exludes second range value for matrix operations which isn't currently accounted for
  let coordsTopAndBottom =
      [|
          (Point(310, 200), Point(275, 214))
  
negative.code:             findChangedKeyImages keyImages previousKeyImages |> List.ofSeq
          else
              []
  
      //vague attempt to avoid spamming with cars and crowds of people
      let activeKeys = if activeKeys.Length < 4 then activeKeys |> List.map fst else []
      activeKeys, keyImages
  