Correct FOV_DIST and MPPX calculations

Correct FOV_DIST and MPPX calculations

Derivation of FOV_DIST for a 1600x1200 monitor
with a vertical viewing angle of 30 degrees:

        ---  +._
         |   |  '-._     angle
  height |   |      '-._/
  (px)   |   |         ('-._
        ---  +--------------+ (o) eye

             |--------------|
               FOV_DIST (px)

  height   = 1200 px
  angle    =   30 deg
  fov_dist = cot(angle)*height = 2078.46

Derivation of angle for window of a given height:

  angle    = atan(height/FOV_DIST)

Derivation of resolution in meters per pixel
at a given distance:

        ---  +._
         |   |  '-._
  length |   |      '-._
  (m)    |   |          '-._
        ---  +--------------+ (o) eye

             |--------------|
               dist (m)

  length   = tan(angle) * dist
  mppx     = length / height
           = (tan(atan(height/FOV_DIST)) * dist) / height
           = dist/FOV_DIST