The SCENE section of the configuration file is where the user specifies the ``scene'' that they wish to simulate. This includes items such as the name of the file containing the 3D facetized scene and the material database file associated with it, the day and time for the simulation, and the scene's location on the Earth. Below is the section from our example CFG file:
SCENE {
GDB_FILENAME = example.gdb
GDB_UNITS = METERS
ODB_FILENAME = example.odb
MATERIAL_FILENAME = ./example.mat
DATE = 6 24 1996
GMT_TIME = 17.50
LOCAL_TIME = 5.0
LATITUDE = 43.00
LONGITUDE = 78.00
GROUND_ALTITUDE = 0.218
}
Table 32-2 describes each of the variables that may appear in the SCENE section of the DIRSIG configuration file.
Table 32-2. List of the variables that can be set in the section.
| Variable Name | Usage | Description |
| GDB_FILENAME | Required * | Name of the geometric database used in the simulation |
| GDB_UNITS | Required | Unit of measure for the GDB coordinates |
| ODB_FILENAME | Required * | Name of the object database used in the simulation |
| MATERIAL_FILENAME | Required | Name of the material database file to be used with this GDB |
| DATE | Required | Date of the simulation |
| GMT_TIME | Required * | Greenwich Mean Time (GMT) of the simulation |
| LOCAL_TIME | Required * | Local time of the simulation |
| GMT_OFFSET | Required | Hour offset from GMT at the scene (West of GMT is positive) |
| LATITUDE | Required | Latitude of the scene being simulated in decimal degrees |
| LONGITUDE | Required | Longitude of the scene being simulated in decimal degrees |
| GROUND_ALTITUDE | Required | Altitude [km] above sea level (ASL) corresponding to 0.0 in the GDB |
*The GDB_FILENAME and ODB_FILENAME variables are used to define the geometric contents of the scene in the form of facetized elements. The user may provide a geometric database file via the GDB_FILENAME variable, an object or scene database file via the ODB_FILENAME variable or both. But, at least one of these variables must be assigned a legitimate file.
*The LOCAL_TIME and GMT_TIME are mutually exclusive variables.
The facetized scene that will be simulated must be stored in a DIRSIG geometric database (GDB) file. The format of this file is described in Geometric Database File. The name of the desired geometric database file is then assigned to the GDB_FILENAME variable. Release 3.1 did not have the GDB_PATH variable (in the PATHS section) and assumed that files not specified with an absolute path (referenced from the root directory '/' ) were relative to the current directory. This is no longer the case. You should use the same filename conventions outlined in the Input configurations file format section.
The most common problem that may occur regarding this change is that CFG files with the following lines used to work and now do not:
SCENE {
GDB_FILENAME = example.gdb
}
The file example.gdb would have been looked for in the current directory using Release 3.1. When using Release 3.1.1, DIRSIG will most likely issue an error that the GDB file cannot be found. To resolve this, the user should either:
Set the GDB_PATH variable in the PATHS section to '.' (the current directory), or
Change the filename to a relative or absolute path to the file. For example if example.cfg is in the current directory, the filename must be changed to ./example.cfg in the CFG file.
Since the facet database could have been constructed using any number of tools and at a variety of scales, the geometric units may vary from file to file. The GDB_UNITS variable allows the user to specify that the GDB coordinates should be interpreted as one of the following common measurement units:
CENTIMETERS
METERS
INCHES
FEET
A recent addition to the DIRSIG scene geometry inputs is the Object Database (ODB) file. The format of this file is described in Object Database File. This file is a flexible scene geometry file that allows the user to easily locate and orient objects in the scene. The object database file allows the user to import one or more GDB files (usually containing a single object) and locate these ``sub-scenes'' within the scene. In the future, other geometry file formats will be supported (Wavefront .obj, Prism/MuSES .tdf, etc.). The object database file allows a single geometry item (vehicle, building, etc.) to be located in the scene in more than one location without incurring the memory expense of another physical copy. In the future, the object database file will also allow users to simulate movement of an object during the simulation to produce geometric distortions (distorted vehicles traveling under a scanning system) and thermal artifacts (thermal scaring).
The MATERIAL_FILENAME variable indicates the DIRSIG Material Database file (see Material Database File) that relates the Material IDs assigned to facets in the GDB to optical and thermodynamic properties. This file is looked for in the directory specified by the MATERIAL_PATH variable in the PATHS section. In this example, this file will be looked for in the current directory (since it was specified with a relative filename).
The date is specified using a space delimited string using a mm, dd and yyyy convention. This date is the GMT date, which means that the date must be carefully selected when modeling a late evening acquisition. For example, to simulate 11:00 pm EDT on June 1, the user needs to set the date to June 2, since that is the date in Greenwich at the time of the simulation.
All three of the time variables must be assigned decimal 24-hour clock values. For example, 3:15 pm is denoted as 15.25. The GMT_OFFSET sign convention used requires time zones West of Greenwich to be positive (``Greenwich time is +N hours ahead of this time zone''). For example, GMT in the Eastern Time zone is +5 hours during Eastern Standard Time (EST) and +4 hours during Eastern Daylight Time (EDT).
All of the variables in the section must be set for the simulation to run, with the exception of the GMT_TIME and LOCAL_TIME variables. Only one of these two variables must set to the desired value, and the other will be computed using the GMT_OFFSET value. In the example CFG file, the time was specified using the GMT_TIME and GMT_OFFSET. This same simulation time could have been specified using LOCAL_TIME and GMT_OFFSET variables as follows:
SCENE {
GDB_FILENAME = rooftop.gdb
GDB_UNITS = METERS
ODB_FILENAME = rooftop.odb
MATERIAL_FILENAME = ./rooftop.mat
DATE = 6 24 1996
GMT_TIME = 12.50
LOCAL_TIME = 5.0
LATITUDE = 43.00
LONGITUDE = 78.00
GROUND_ALTITUDE = 0.218
}
The location of the scene's center on the Earth's surface is specified by the LATITUDE and LONGITUDE variables. These values are assumed to be in decimal degrees. Input values that are in degrees, minutes, seconds, etc. must be converted to decimal measurements. DIRSIG assumes locations West of the Prime Meridian as a positive LONGITUDE value. The GROUND_ALTITUDE translates the zero altitude in the facet database (Z-axis = 0) into an altitude above sea level (ASL).