Keywords: sky, modtran, mirror, cloud masks
Summary
This scene contains a mirrored hemisphere that reflects what a MODTRAN-driven sky irradiance field looks like over a roughly 8 hour and 20 minute time period and demonstrates the addition of cloud "masks" to the NewAtmosphere plugin.
Details
Much like the Skyview1 demo, a mirrored hemisphere is used to visualize the sky irradiance pattern predicted by a MODTRAN-driven NewAtmosphere model generated by the atm_builder. The sensor uses the Raw capture method to create a spectral radiance image cube for the Vis/NIR region of the spectrum. It also incorporates the use of cloud fraction "masks" that describe a quick and simplified model of how clouds affect the downwelled irradiance. The fraction maps (individual images of the fractional cloud content of the sky dome) are used in conjunction with a simplified spectral model of cloud irradiance parametrized by a scalar ratio of the cloud to open sky for a blue (490nm) wavelength. For more details, see the corresponding entry in the online documentation. It is only intended as a plausible approximation in the visible and NIR.
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Important
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The cloud mask does not model physical (volumetric clouds), only a variation in the downwelled irradiance from the atmosphere. They do not cast shadows nor are they visible from a top down sensor. They are primarily intended as a way to study angular variation in downwelled irradiance at the surface (for example, the impact of clouds from a roughened sea surface) |
Important Files
This section highlights key files important to the simulation.
The Scene
The scene geometry is contained in the geometry folder. The ground,
arrows and letters are described by polygon geometry in various OBJ
files that are instanced in geometry/demo.glist. This glist file
also contains as SPHERE geometry primitive that is half buried
in the ground. The simple materials for the ground, arrows and
letters are not worthy of discussion. The North and East letters are placed
vertically outside the field of view so they are seen in the reflection.
Like the Skyview1 demo, the mirror material assigned to the hemisphere describes a perfect reflector with no loss of energy at the surface
The Atmosphere
The atmosphere uses the NewAtmosphere radiative transfer model to access
MODTRAN for the atmospheric path radiances and transmissions. The
MODTRAN model is driven by the mls_rural_23km.tp5 tape5 file,
which uses the mid-latitude summer (MLS) atmospheric profile and
rural aerosol model with a 23 km visibility. The atmospheric
database is stored in the file classic_mls_rural_23km.adb.hdf
and is generated using the atm_builder tool.
The Platform
The demo.platform file describes a simple 320 x 240 (QVGA) camera
that is configured with the Raw capture method to create a spectral
radiance cube from 0.400 to 0.800 microns at 0.010 micron spectral
sampling. The platform was placed at a low view angle (high
declination angle) and just North of East in order to see the
reflection of the sun rise at the start. The clock rate of the
sensor is set to take an instantaneous shot every 15 minutes, creating
a series of images over the day.
The Simulation
This demo uses the JSON based simulation file (".jsim") which includes the setup for the "NewAtmosphere" plugin and the additional cloud masks entry (see the online documentation for more information about both formats and available options, particularly how to modify the NewAtmosphere setup to use your MODTRAN profile).
The Masks
All cloud masks used for the demo are contained in the "masks" subfolder. A contact sheet is provided for reference:
Results
RGB visualizations of the spectral image cubes (demo-t0000-t00**.img)
generated by the simulation can be created using the 0.65, 0.55 and
0.45 micron channels (bands 25, 15, and 5 ) using the image_tool:
image_tool convert --gains=1e4 --biases=0 --bands=25,15,5 demo-t*.imgThe resultant series of images can be combined into a movie using ffmpeg or a similar tool.
