Thermal Modeling And IR Scene Generation

Thermal Modeling and IR Scene GenerationRandy K. ScogginsUS Army Engineer Waterways Experiment StationAbstractThe Waterways Experiment Station (WES) has for several years been engaged in the devel-opment of numerical models for the simulation of terrain surface temperatures. Models havebeen developed to describe major components of the earth's surface such as grasslands,lands essentially bare of vegetation, such as deserts, and forested areas. These modelscombine first principles and semi -empirical techniques and use local surface meteorologyand material thermal properties for input. Recently, WES has begun work on using thesemodels to provide terrain radiance information needed for production of realistic computergenerated infrared (IR) images. The approaches used in terrain temperature modeling arediscussed, along with the structuring of the codes in a form amenable to scene generationrequirements. Data needed by the models are outlined, and model predictions are comparedto measured temperatures. Various approaches to account for the effects of atmosphericcontaminants which may be applied to computer scene simulation are presented. Computertexture generation techniques based on thermal modeling as applied at WES are outlined.Application of models in an IR scene generation computer code developed by WES is discussedand example imagery presented. Possible extension of current work to other spectralregions and additional efforts for increased realism are also discussed along with possibleapproaches.IntroductionCreation of images by computers using computer data bases has for many years been anarea of intense study. Generally, the reason for scene generation by computers is to 'see'the terrain or object of interest in ways and under conditions difficult or impossible toobtain in any other manner. Much has been accomplished in the production of views simu-lating visible light sensors. Interest has increased in using computers to simulate imagesas would be seen by multispectral systems, primarily near and far IR, such as the work byKornfeld of the Army's Center for Night Vision and Electro Optics (Kornfeld 1985).The US Army Engineer Waterways Experiment Station has begun a program to simulate ther-mal (8 to 14 micrometer) images of major surface types such as agriculture, forest, andgrassland. Mean gray levels for these terrain backgrounds are estimated by a set of com-puter models developed for WES. These models use terrain and environmental data as well asparameters describing specific terrain type materials to calculate temperatures of terrainbackgrounds. Temperatures are related to mean radiance and thus to mean gray level. Vari-ation about the mean gray level is accomplished by a terrain -dependent formula using stan-dard deviations and correlation lengths specific to terrain types. A similar approach isused for structures such as buildings and roadways. Computer- created structure scenes arethen inserted into a complex background scene to produce the final result.Background Surface Temperature ModelsThree fundamental temperature modeling codes for estimating surface temperatures ofmajor terrain components have been developed by WES. Each model makes temperature predic-tions for a basic class of terrain (bare soil, vegetation, etc.). These computer modelsrequire as input the geometric and thermophysical properties of the materials being modeledas well as standard meteorological data representing the time frame of interest. Outputconsists of physical and radiometric temperatures as a function of time for each area beingmodeled. More advanced components of the models modify radiometric temperature predictionsto account for surface roughness and view angle. The initial versions of the programs werecompleted in 1981; since then, several modifications and extensions have been made, andupdates are being integrated into the software to improve the performance of the models.Modifications have also been made to the codes so that temperature predictions for all com-binations of basic surface terrain types and orientations are produced automatically andoutput in a form compatible with the scene generator software.Bare terrain surface temperature modelA first -principles model was developed to calculate surface temperature for nonvegetatedsurfaces (e.g., plowed fields, deserts) by performing an energy budget analysis of the ter-rain surface (Balick et al. 1981). Each major component of energy exchange between thesurface plane (which is assumed to be infinitely thin), the subsurface materials, and the50 /