One of the highlights of the Jerusalem Archaeological Park's Ethan and Marla Davidson Center is a real-time visual simulation model of the Herodian Temple Mount on display in the Center's interactive classroom. This model was constructed for the Israel Antiquities Authority by Lisa M. Snyder of the Urban Simulation Team at UCLA (Bill Jepson, Director) following the reconstruction plan of Jerusalem Archaeological Park Director of Excavations Ronny Reich.

Real-time models are very different from the computer animations that are common today. Animations use a sequence of pre-determined and pre-rendered images displayed at 24 frames per second to create the illusion of movement along a fixed path. This process is very time intensive. Beyond the time required to build the model, computer time must be allotted to draw, or render, the individual frames of the animation. Depending on the complexity of the scene, these individual frames may take anywhere from 20 minutes to two hours to render. The resulting animation is very limited in terms of user interaction, comparable to a rented movie on video - you can play it, you can rewind it, and you can play it again, but little beyond that.

In contrast, the individual frames of a real-time simulation model are rendered at the moment of interaction, allowing the user complete freedom to explore the modeled environment. This experience is roughly comparable to a military flight training simulation and requires a very powerful computer. To display the same 24 frames a second as an animation, the computer running the real-time simulation must render a frame every 41 milliseconds. This is 28,800 times faster than the computer that takes 20 minutes to render an individual animation frame. In the Center, the model of the Herodian Temple Mount runs on a Silicon Graphics Onyx2 Infinite Reality 2 that generates close to 30 frames a second (which equals a frame every 33 milliseconds - 36,000 times faster than the computer that takes 20 minutes to render an individual animation frame).

This rendering speed differential requires that real-time models be constructed and rendered in a very data efficient manner. For this, the Center's model of the Herodian Temple Mount relies on two software components. The first is the program used to create the model's three-dimensional geometry. The Center's model was built using MultiGen, a commercially available program. MultiGen is a polygonal surface modeler, which means that only the visible surfaces (or shell) of a structure are constructed. Multi-sided polygons and three-sided triangles are the building blocks of all three-dimensional forms created in MultiGen. The second software component is the software that is used to interact with the model. The Center uses 'uSim,' a software package that is proprietary to the Urban Simulation Team at UCLA. This program constantly evaluates the user's movements through the modeled environment and instructs the computer to render the appropriate frames to create a seamless simulation experience. Computing speed and power are critical for uSim because of the complexity of the visual information being rendered. For the Herodian Temple Mount model, uSim has 33 milliseconds to evaluate and render an average of 45,000 triangles per frame. At the most data intensive point of the model, uSim evaluates and renders slightly over 70,000 triangles per frame.

Real-time models present many advantages over animations. The most compelling is the ability of the user to interact with the environment and control their exploration. Where animations provide a single point of view, there is complete freedom of movement within a real-time model. Users can explore the environment at their will and make choices about what they experience. Real-time models are also very easy to update with new information. The onerous rendering requirements of animations often mean that a clip, once completed, is the final product and rarely updated. In contrast, changes to a real-time model can be implemented easily and are included in the simulation experience the next time the model is loaded. For the archaeologists at the Temple Mount site, this means that new excavation data can be incorporated into the real-time model and shown almost instantaneously to visitors at the Ethan and Marla Davidson Center.

The following pages describe the technical processes used to create the Center's real-time simulation model of the Herodian Temple Mount. The archaeological information and historic sources used for the reconstruction are mentioned in the following pages only as they impact the construction of the computer model. Overlap between a defense of the reconstruction and the construction of the model, however, is inevitable as the two processes continually informed one another during the two years of the project.