The goal of the Countermine Computational Testbed Sensor Model Development Program is to design a software simulation for candidate airborne imaging sensors suitable for use in the remote detection of mines. The simulation takes as input several sensor parameters and a time-dependent history of location and orientation of the sensor. A scene sampling module generates an array of query ray origins and directions from the view point through the image plane to obtain radiance values from other testbed modules. Blurring effects, including those from diffraction, aberrations, detector spacing, and digitization are accounted for by using results from the validated NVTherm model. In this way, the sensor system's modulation transfer
function is imposed on the image. In addition, atmosphere effects are
incorporated through the use of external scattering models. If necessary, the resulting radiance image is re-sampled at desired pixel locations. Finally, the detector response characteristics are applied to the radiance image for computation of signal voltages. A noise voltage is then added and the digitization process simulated to produce the final sensor output synthetic image. The model is implemented in C++ using object-oriented programming techniques that allow for flexible extension of the simulation to different types of sensors and geometries. Model design goals, techniques, components, and specific image synthesis algorithms implementations are discussed along with the presentation of example results.
The Bi-directional Reflectance Distribution Function (BRDF) is a
general way to represent reflection from surfaces. However, even when
ignoring variations in location and wavelength, the BRDF is still a
4-dimensional function and the number of BRDF samples necessary for
direct utilization can impose severe computational requirements. To
address these and other problems, analytical BRDF equations have been
developed for computer image simulations. Limitations remain since
they may be empirical, appropriate only for specific surfaces, or
require difficult-to-obtain physical parameters. BRDF shading based on
signal processing methods has recently been presented in computer
graphics that projects any BRDF onto an orthogonal basis set and then
employs a truncated linear series for shading with controlled
accuracy. This paper presents an overview of BRDF shading and
frequency-space representations using orthogonal polynomials as basis
functions. Techniques are then described that utilize the resulting
linear shading equations to factor lighting and geometry components of
a scene for image-based re-lighting. Statistical properties of
images may then be partially computed for fixed views in a pre-process
phase and employed to rapidly compute view properties for arbitrary
lighting distributions and different BRDFs. Techniques are also
described for producing correct blended BRDFS aimed at level-of-detail
frequency-space shading.
Current engineering-level smart munition sensor models emphasize sensor/target interactions with detection and aim-point information being the principal outputs. Background is not treated with the same fidelity. False alarm rate is based on captive flight statistics and is not actually simulated. The lack of a means to evaluate effects of background in an end-to-end simulation mode motivated the development of the WES Smart Munition Thermal Sensor Model. The model consists of a generic set of algorithms used to simulate platform dynamics, scanning geometry, and infrared sensor optics and electronics. Thermal target models of a vehicle developed by Georgia Tech Research Institute are instantiated into a background scene consisting of calibrated thermal imagery. Parameters are set to reflect the flight dynamics, scanning, optics and electronics of a specified munition, and the output voltage is processed though an appended target acquisition algorithm. A hypothetical smart munition with a thermal sensor (simple flying spot detector) is configured and flown over high-resolution thermal imagery obtained from selected locations to demonstrate effects of varied terrain and environmental conditions.
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