code: function (viewportWidth, viewportHeight, distance) {
                  if (viewportWidth <= 0) {
                      throw new ArgumentError(Logger.logMessage(Logger.LEVEL_SEVERE, "WWMath", "perspectivePixelSize",
                          "invalidWidth"));
                  }
  
                  if (viewportHeight <= 0) {
                      throw new ArgumentError(Logger.logMessage(Logger.LEVEL_SEVERE, "WWMath", "perspectivePixelSize",
                          "invalidHeight"));
                  }
  
                  if (distance < 0) {
                      throw new ArgumentError(Logger.logMessage(Logger.LEVEL_SEVERE, "WWMath", "perspectivePixelSize",
                          "The specified distance is negative."));
                  }
  
                  var frustumHeight = WWMath.perspectiveFrustumRectangle(viewportWidth, viewportHeight, distance).height;
                  return frustumHeight / viewportHeight;
              }
docs: Computes the vertical size of a pixel in model coordinates at a given distance from the eye point in a
  perspective projection. This returns zero if the specified distance is zero. The returned size is
  undefined if the distance is less than zero.
  <p/>
  This method assumes the model of a screen composed of rectangular pixels, where pixel coordinates denote
  infinitely thin space between pixels. The units of the returned size are in model coordinates per pixel
  (usually meters per pixel).
  
  @param {Number} viewportWidth The viewport width, in screen coordinates.
  @param {Number} viewportHeight The viewport height, in screen coordinates.
  @param {Number} distance The distance from the perspective eye point at which to determine pixel size, in
  model coordinates.
  @returns {Number} The pixel size at the specified distance from the eye point, in model coordinates per
  pixel.
  @throws {ArgumentError} If the specified width or height is less than or equal to zero, or if the
  specified distance is negative.