Keywords: clouds

Summary

This demo provides a working example of the fast clouds model in DIRSIG, which allows the user to import a cloud field using the regular grid mechanism, assign volumetric materials to it and use the "fast" cloud radiometry solver. This feature to model clouds is explicitly called the "fast" cloud model because the radiative transfer within the cloud uses a direct solution for first order scattered solar radiance and approximates multiple scattering contributions. It is intended primarily for producing the effects of large-scale clouds on a scene (particularly shadowing), but not for accurate, detailed radiometry of the clouds themselves.

Details

The cloud field used in this demo was generated by the Cloud Scene Simulation Model (CSSM), which was developed by TASC for the Air Force in the late 80’s and early 90’s. The model simulates multi-dimensional cloud water density fields for input to radiative transfer models and scene generation systems. However, CSSM has not been actively developed for some time and obtaining access to the model is difficult.

Important
 RIT cannot redistribute CSSM and does not have any up-to-date information regarding how to obtaining access to the model.

The output of CSSM was converted into a regular grid and a set of abosprtion and scattering optical properties from the Optical Properties of Aerosols and Clouds (OPAC) software package were assigned to it.

The use of CSSM to generate cloud fields for DIRSIG is not a requirement. Other tools capable of producing such data can be utilized with this approach. The only requirement for DIRSIG is to produce the input voxelized grid and correctly associate the cloud optical properties to that 3D structure.

Important Files

The key set of files in this scene are contained in a bundled object stored in geometry/bundles/cloud_cum30 folder. A listing of the files in that bundle folder are listed below:

cloud_cum30
cloud_cum30/cloud.glist
cloud_cum30/cloud.mat
cloud_cum30/cucc00.abs
cloud_cum30/cucc00.grid
cloud_cum30/cucc00.pha
cloud_cum30/cucc00.sct

The cloud is instantiated into an <object> in the cloud_cum30/cloud.glist file shown below. Note that the instantiation is primarily composed of a material label (ID) that links to a material entry that specifies the geometric and optical properties of the cloud:

<geometrylist>
  <object search_paths="local">
    <localmaterials>cloud.mat</localmaterials>
    <basegeometry>
      <atmospherics>
        <cloud_matid>4321</cloud_matid>
      </atmospherics>
    </basegeometry>
    <staticinstance/>
  </object>
</geometrylist>

Since this example uses an object bundle, the local search path option is used with the local material file cloud_cum30/cloud.mat shown below. As stated earlier, the material entry includes the optical properties files (see the ABS_FILE, SCT_FILE and PHA_FILE variables) and geometry (see the CNC_FILE, INSERT_POINT and SCALING variables):

MATERIAL_ENTRY {
    NAME = cucc
    ID = 4321
    RAD_SOLVER_NAME = Cloud
    RAD_SOLVER {
        ABS_FILE = cucc00.abs
        SCT_FILE = cucc00.sct
        PHA_FILE = cucc00.pha
        CNC_FILE = cucc00.grid
        INSERT_POINT = -25000 -25000 1000
        SCALING = 50 50 50
    }
}

The voxel cloud grid contained in cucc00.grid is 1000 x 1000 x 41 elements. The SCALING values scaled this field to 50,000 x 50,000 meters and the -25,000 x -25,000 INSERT_POINT shifts this grid to be centered about the XY origin (0,0).

The cloud_cum30/cloud.glist file must be directly instanced into the scene, like shown in the geometry lists included in the demo.scene file excerpt below:

  <geometrylist enabled="true">
    <geometrylistinclude name="Ground" enabled="true">ground.glist</geometrylistinclude>
    <geometrylistinclude name="Clouds" enabled="true">bundles/cloud_cum30/cloud.glist</geometrylistinclude>
  </geometrylist>
Important
 The cloud bundle cannot be instanced. That means it cannot appear in an intermediate GLIST file (for example, in something like a geometry/everything.glist file) even if the instance is static with not offsets, scaling or rotation.

Setup

This section includes any step-by-step instructions for running and visualizing the simulation.

To run the simulation, perform the following steps:

  1. Run the DIRSIG demo.sim file

  2. Load the resulting demo.img radiance file in the image viewer.

Results

Output of the demo showing the cloud field over a uniform terrain.

images/demo.png