# Wakes

In RenderWorld a wake is an additive surface disturbance created by a shape that moves along the water surface. At present there are two options for placing a wake on a water surface:

(1) Provide simple information for a boat hull shape, and have RenderWorld generate the wake;

(2) Read in an existing wake from a file.

In either option, the boat wake has a fixed shape that is permanent in time.

The portion of the ocean file which sets up a wake disturbance is shown below. The `render_disturbances` parameter effects the rendering of the boat area of the wake and is described in section Ocean Disturbances. Reference is made to an additional file (wake.dat) which contains details about the internal structure of the wake.

  `number_disturbances` `1`
  `render_disturbances black`
  `grid_type` `wake`
  `n_points_x` `512`
  `n_points_y` `512`
  `dx` `2.5`
  `dy` `2.5`
  `file` `wake.hdf`
  `generate_file` `no`
  `input_file` `wake.dat`
  `position_x` `0`
  `position_y` `0`
  `direction` `180`
  `velocity` `0`
  `end`

The position (`position_x`, `position_y`) and `direction` of the wake are with respect to the ocean: rotating and/or moving the ocean takes the wake along with it. To generate a new wake from RenderWorld, the file wake.dat has the form:

  `RenderWorld_data_file wake 1 1`
  `generate_file` `yes`
  `filename` `wake.hdf`
  boat rectangle
  `length` `10`
  `width` `3`
  `depth` `1`
  taper `0.1`
  `end`
  ``

The line generate_file yes indicates that the wake will be generated and written to the file wake.hdf in the next line. The length, width, and depth lines provide the overall dimensions of the boat hull, which is modeled as having a rectangular cross section but stream`l`ined hull bottom shape. Finally, taper is a parameter which allows the user to control the tapering of the wake as a function of distance from the hull. Larger values of taper reduce the effective length of the wake.

The option generate_file no means that the wake will be read from the file wake.hdf. When a wake already exists as a file in the correct format, the wake.dat file has the lines:

  `RenderWorld_data_file wake 1 1`
  `generate_file` `no`
  `filename` `wake.hdf`
  `length` `10`
  `width` `3`
  `depth` `1`
  `end`

The wake is stored as a 2D array of float numbers representing surface displacement, in the hdf file format. After the wake has been generated, it can be edited using any image processing software that reads and writes hdf format. In fact, the wake need not be generated by RenderWorld. Any arbitrary 2D array can represent a wake simply by creating it in the hdf floating point format.

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