Whitecaps

Whitecaps pose new challenges to rendering efficiency. In the earth's oceans, whitecaps exist for 4 to 8 seconds after their initial onset. If you want an ocean surface that has a steady whitecap activity, the ocean surface simulation must begin 4 to 8 seconds earlier in simulated world time than the frames you want to render. Thus there can be a significant amount of computational time taken up evolving the ocean surface and associated whitecaps to a steady state. In addition, creating and tracking whitecaps over the entire ocean would be unnecessarily burdensome, since most of the ocean is not seen by the camera. To reduce the load, the user specifies the area of the ocean surface on which whitecaps are to be simulated. If the camera is moving substantially over the surface, you must be careful to chose enough area for the entire range of camera motion.

At present, the RenderWorld Engine generates 2D whitecap layer which rides on the ocean surface. Bubbling foam and splash are not yet implemented. The present implementation works well for conditions in which the viewer is not concerned with the detailed internal structure of the whitecaps.

To put whitecaps into the ocean surface, replace the line in the ocean file

whitecaps no

with the two lines

whitecaps yes

whitecap_file whitecaps.dat

The file whitecaps.dat contains the information about the whitecap life cycle, frequency, foam and optical properties. An example of the file is

RenderWorld_data_file whitecaps 2 2

sampling_region

center 25 0

length 80 80

interval 0.075 0.075

max_distance 4000

reference_time 0.0

evolution_time 0.041666667

ramp_up_interval 4.0

ramp_up_increment 0.1

formation_time 0.2

decay_time 3.0

threshold 0.4 0.5

reflectivity 0.15 0.55

smearing_scales 1.0 1.0 0.20

acceleration_offset 0.2

whitecap_file input

file_name whitecap_data.dat

add_small_scale no

diffusion no

end

When a rendering job begins, the whitecap field on the ocean surface must be created dynamically. Whitecaps have a typical lifetime of about 4-8 seconds (although this is user adjustable), and so the ocean and whitecaps fields must be simulated for approximately that length of physical time in order to build of the whitecaps from nothing to a steady state. The time at which the user wants this steady state to exist is given by the reference_time parameter. In this case, the reference_time is 0, so the RenderWorld Engine begins the ocean/whitecap simulation at a time with a negative value. The amount of time before reference_time at which the simulation begins is given by ramp_up_interval, in this case 4 seconds. Note that during this initial ramp up to steady state, the ocean surface and whitecaps evolve, but no image rendering is done. During the ramp up period, the fundamental time step of the whitecap evolution process is given by ramp_up_increment. After the ramp up has been completed, the time interval used for whitecap evolution is on the line labeled evolution_time. The reason for allowing two different time steps is for efficiency: Over time intervals of image rendering, the whitecaps should evolve smoothly from frame to frame, and so evolution_time should be equal to or smaller than the time between frames. During the ramp up however, longer time steps should be taken because the ramp up could take substantial amounts of pre-rendering overhead time if the ramp up evolved at the frame rate, and because whitecaps typically do not have any fast temporal behavior and longer time steps do not sacrifice quality.

In order to understand the next six parameters, the model of whitecap dynamics must be discussed. Whitecaps have a lifecycle which begins when the ocean surface reaches a dynamically favorable condition, at which point a whitecap grows in intensity and size for a short period of time, then decays over a much longer period. The RenderWorld Engine gives the user the ability to control the dynamical condition for onset, the growth rate and intensity, and the decay profile. In addition, there is an option for adding simple texturing to the internal structure of the whitecap.

Once a whitecap begins to grow, it continues growth for a period of time labeled as formation_time. At the end of this time, the whitecap has grown to its maximum size and brightness, and decays over decay_time seconds. A whitecap begins growth when the ocean surface acceleration at any given spot on the surface exists a threshold given by the first value in the threshold line. During growth, a whitecap grows in diffuse (spectrally white) reflectivity, and reaches its maximum reflectivity when the local acceleration reaches the second value in the threshold line. The values of the minimum and maximum reflectivity are given in the reflectivity line.

The two lines, smearing_scales and acceleration_offset, allow you to alter the definition of acceleration used to control whitecap growth. The first and second values in smearing_scales define the size of the area around a point to average the acceleration, and the third number is the interval of time over which to average. This averaging over space and time controls the effective size of a patch of whitecaps on the ocean surface, and how sensitive it is to rapid and small-scale ocean structure. The value of acceleration_offset moves the physical time of the acceleration calculation back. With an acceleration_offset of 0, whitecaps typically begin formation on the tops of the waves. By increasing the value of acceleration_offset, the onset criteria are satisfied earlier in time, so that the whitecap begins forming on the front size of a wave. A negative value for acceleration_offset would cause the whitecaps to typically begin forming on the backsides of the waves.

Because of the overhead time spent ramping up the whitecap field before rendering begins, the whitecap field at the reference_time can be written to a file and read back again in a later rendering job. Thus whitecap_file has three options. The option whitecap_file input reads the whitecap field from the file given on the line file_name whitecap_data.dat. This field is used at the reference_time. The option whitecap_file output creates a new whitecap field at the reference_time and writes it to the file specified. The third option, whitecap_file ignore does not do any file I/O. In this third option, the file_name line must be removed.

The last option in the whitecap input file is whether to add small-scale texture fluctuations to the whitecaps. The three options are no, yes, or texture. If yes is picked, then replace add_small_scale no with the following:

add_small_scale yes

size 64 64

spacing 0.075 0.075

seed 19334

power_law_length 1.0

power_law 0.5

mixing_time 0.125

log_normal_width 0.4

end

If add_small_scale texture is specified then texture maps are used to color the whitecaps. The texture maps are specified in the exact same way as the ocean surface texture maps. A sample set of whitecap texture input parameters is shown below

add_small_scale texture

RenderWorld_data_file texture 2 2

surface_type ocean

number_of_textures 1

location 0. 0.

orientation 0.

size 100. 100.

periodicity 1

motion 0

new_picture 0

file_name whitecap_texture.rgb

priority 1

bands 3

image_above yes

multiplyReflectance 0

alphaAbove 0

min_radiance_above 0 0 0

max_radiance_above 30 30 30

image_below no

endTexture

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