Contents
Clouds are placed in a scene file with object
type: cloud. The position and orientation given in the keytime data is for the geometric center of the bounding box in which the cloud is contained.
A typical input file containing cloud data is:
RenderWorld_data_file volume 1 1
nx 128
ny 128
nz 16
dx 2
dy 2
dz 1
number_of_bands 3
band_id 1 2 3
absorption 0.01 0.01 0.01
scattering_coefficient .05 .05 .05
extinction_coefficient .06 .06 .06
phase_function_file phase_functions.dat
density_threshold 0.0
scattering_flag multiple
integration_step 1.0
transmission_cut 0.00001
integration_threshold 0.0
end
RenderWorld_data_file cloud 1 1
liquid_water_content binary cloudlwc.dat
small_scale_fluctuation dynamic
nx 64
ny 64
nz 32
scale 0.125
small_scale_file ss.dat
generate_file yes
fluctuation 1.0
power_law 2.
length 1000.
seed 11011
reference_time 0.
update_time 0.25
sigma 0.5
end
end
The first part of this file, between
RenderWorld_data_file volume 1 1 and the first end, dictates the physical and optical properties common to all three volumetric objects: cloud, blobs, and spray. The volume is contained in a rectangular box which is divided into rectangular cells. The number of cells in each direction is nx, ny, and nz. Each rectangular cell has physical dimensions dx, dy, and dz in units of meters.
The optical behavior of the volumetric object is governed primarily by the three parameters absorption, scattering_coefficient, and extinction_coefficient. These parameters control the attenuation of sunlight and diffuse light, and the scattering of light within the volume toward the camera. Raising the absorption coefficent increases the opacity of the cloud, raising the extinction coefficienct increases the strength of the shadows in the cloud, and raising the scattering coefficient increases the overall brightness of the cloud. The other quantity affecting the scattered light is the phase function, which is contained in a separate file and designated by the line
phase_function_file phase_functions.dat .
The phase function determines the amount of light scattered by a piece
of cloud material in each possible direction. It is a color-dependent quantity,
and is responsible for producing optical effects like rainbows and glories.
Different types of clouds can have different phase functions.
The file phase_functions.dat is a text file
with the phase function for each color.
Editing the values in it will alter the scattering behavior and can
lead to exotic effects.The
density_threshold parameter acts at rendering time to reduce any cloud density value less than this threshold to the value zero. Cells with zero density are transparent and do not scatter light.
Physically, clouds can scatter light many times (hundreds!) before the light reaches the camera. The line
scattering_flag multiple provides a simple model of these many scattering events. The alternative choice that can be made is scattering_flag single uses a simpler single scattering event model.
The final three volumetric parameters provide the ability to trade between rendering time and image numerical accuracy. In all three cases, increasing the numerical value of the parameter shortens rendering time and increases the likelihood of having numerical artifacts. Each of these parameters controls the accuracy of numerical integrations that are conducted through the volume of the cloud.
The first parameter,
integration_step, allows you to control the fineness with which numerical integrations are carried out in the rendering. The value of integration_step is in units of the cell size.
The second rendering parameter,
transmission_cut, is concerns the fact that the numerical integrations in the rendering include the transmission through the cloud. This threshold parameter stops the integration when the transmission reduces to this value.The last of these parameters, the values of the integrated quantities are example throughout the cloud volume. Using
integration_threshold, regions in which the integration quantities are relatively small are excluded from the numerical integration.
The second half of the cloud input file contains parameters that are specific to the cloud rendering problem. The cloud is a fixed distribution of density (called liquid water content), which is read from a pre-existing file specified in the line for
liquid_water_content . The term binary is necessary but there are no other options for it. The pre-existing file is the output of the blob and spray evolutions. Thus, even for a static cloud that is custom built uses the spray and/or blob to grow the cloud density distribution. See those sections for more details on sprays and blobs.
The cloud density is a fixed spatial distribution of cloud material, which because of RAM or computational limitations in running sprays and blobs, may not be large enough in terms of total extent compared to fine structure to meet various needs. One approach to solving this problem is to think of the cloud density as the larger scales of cloud structure, with smaller fine detail that is generated during rendering time. If no small-scale structure is needed, then the parameter
small_scale_fluctuation should be set to no, and all of the subsequent lines through the first end eliminated.
When small-scale structure is desired, there are two types to choose from
small_scale_fluctuation dynamic generates small-scale structure that changes in time, and small_scale_fluctuation yes generates small-scales that do not change in time. The example at the top of this section is for the first case, and will be used to describe how small-scale fluctuations are applied. The second case is presented at the end.When small-scale fluctuations are desired, an additional 3D box is created to hold the fluctuations. The number of cells in each direction of this box is given by the
nx, ny, and nz after the small_scale_fluctuation line. The physical size of the small-scale cells is equal to scale times the cell size of the cloud density. The small-scale fluctuations act as a multiplicative change in the density, and are periodically replicated throughout the cloud volume. Once generated, the small-scale fluctuations can be saved in the file small_scale_file, or read back in from the file by specifying generate_file no .The positive-valued parameter
fluctuation controls the magnitude of the fluctuations. Since the fluctuations are log-normal, a value above approximately 0.6 can break up a dense cloud into a sequence of apparently separate smaller clouds with the volume.Although the fluctuations are random (the random number series is based on the
seed parameter), they are correlated in space and time. The power_law controls the rate of fall-off of the correlation with distance (similar to a fractal dimension), and the length is the largest spatial length scale of fluctuations.
The
reference_time is the physical time corresponding to the first generated volume of random fluctuations. As a simulation proceeds over time, the fluctuations are changed periodically at update_time intervals. The parameter sigma controls the rate at which the correlations in the fluctuations decay over time. Larger values of sigma cause the fluctuations to vary rapidly.
For small-scale fluctuations that do not need to evolve over time, the cloud-specific portion of the cloud file is shown below:
small_scale_fluctuation yes
nx 64
ny 64
nz 32
scale 0.125
small_scale_file ss.dat
generate_file yes
fluctuation 1.0
length 1000.
seed 11011
end
The parameters here have the same meaning as for the dynamic fluctuations.