This lecture concerns two related areas - Computer Aided Design, and Geographic Information Systems. What they have in common is that humans know something complex and graphical, and they need to be able to store and use that knowledge. In the past we used a pencil and a drawing table to solve such problems - by making pictures or maps. Pencils have been replaced by computers, for most purposes. It's still the person holding the pencil or mouse who can mess up, but now we have HELP from many software vendors - who want to mess up right along with us.
Engineering and Architecture (e. g. AutoCAD)
Realtime Simulation (e. g. MultiGen)
Animation (e. g. SoftImage, Maya, 3DStudio Max)
Each category has specialized versions. For instance there are many sub-specialties in engineering, with CAD programs for electrical circuitry, programming of Numerically Controlled (NC) milling machines, stereolithography, etc.
Query 1: Define NC milling and stereolithography. How would you decide which to use, in a given circumstance? For instance, we want to build a factory that produces a carburetor for a simple lawnmower engine, using a minimum amount of labor.
The key issues to consider when selecting a CAD system are
CAD systems have been around since the early 1960's, when they ran on mainframe computers such as the IBM 7090. These early systems were oriented toward light-pens, numeric input of dimensional data, and plotters with moving paper or pens. CATIA is one such system which has been a mainstay of the automobile and aerospace industries for over 20 years.
AutoCAD was originally a 2d program. It has been extended into 3d but is not a particularly "user friendly" way to work in 3d. Its file format is called DXF.
A common problem when trying to make 3d models from AutoCAD models is that it is necessary to designate an 'inside' and an 'outside' for polygons. This is usually done by entering vertices in a counterclockwise order seen from the outside. But AutoCAD users don't normally think that way, and so DXF models produce junk when converted into other 3d formats like VRML.
Another problem is that AutoCAD modelers use lots of polygons because they're essentially free in a plotter environment. (Sure it takes longer to plot the picture but then you Xerox it a thousand times or something....) Also, AutoCAD models are not hierarchical. Every vertex is a free standing point in a flat file.
... was developed to support realtime modeling, e. g. for flight simulators. See www.multigen.com. It uses the Flight format (also defined by Multigen's makers), which supports a scene graph hierarchy. It does a very good job of allowing the user to manage Level of Detail in models, which is essential for realtime applications. Newer versions of MultiGen, such as Creator, were designed for game construction and run on PCs.
Flight models can be rendered by Silicon Graphics' Performer software, as well as by many other Image Generator (hardware and software) systems.
Query 3: Explain Level of Detail and the following terms. Refer to your notes from my lecture as needed.
CAD for Animation
Two features distinguish animation-oriented CAD systems:
1) Extended features for high-quality modeling and rendering, and
2) Systems for describing motion and change
High quality rendering is based on features such as splines, lighting models, procedural texture models, and materials models.
Query 24.4: Define each of the above terms.
To describe motion, we normally use keyframes and various means of in-betweening. For instance, we might use a set of parallel graphs showing that as time goes from 0:00 to 0:10 (a ten second animation), X varies from 0 to 30 and back to 0; Y varies uniformly from 0 to 30, and the size S of the object diminishes from 100% to 50%. The initial values of X, Y and S are shown at the left end of the time graphs.
Query 24.5: Assuming y-up and x-right, describe the motion of a ball controlled by the above animation description.
One neat thing about spline surfaces is that you can select any desired level of granularity (smallness of polygons) and then derive a polygonal version of the surface from the spline description. If you have realtime hardware that uses polygons, this means that a spline model is a kind of a generator of polygon models, e. g. the various levels of detail you might need in your video game.
Query 6: Describe the following GIS concepts in your own terms. Give one or more examples of the kinds of things that are represented with each technique.
vector feature description
raster feature description
Query 7: Describe an overlay or spatial join, and provide an example of its usefulness.
The dominant GIS is called Arc/Info, produced by the Environmental Systems Research Institute (ESRI). We have a license of its simple minded cousin, ArcView. ArcInfo is of the same vintage as AutoCAD and thus has a command-line oriented interface, whereas ArcView is inherently mouse-oriented.
Query 8: The ESRI site above describes a number of tools and products that can be used by programmers who want to build map-based software. Probably most of you have used at least one such product, such as the free maps of anywhere (in the USA anyhow) that are provided by Mapquest.com.
Which of the ESRI tools do you consider is the most likely product that could have been used by Mapquest.com to write the software which is being used to produce the maps you can get online at www.mapquest.com?
The Army got interested in realtime simulation in 1984 when Col. Jack Thorpe at DARPA convinced them by some simple video game demos, that it would be possible to network together some commercial off the shelf (COTS) hardware and built a team training simulator for armor (tanks). The SIMNET system was wildly successful, and spawned the whole networked simulation industry which is still a mainstay of IST's research and development efforts. And tanks need a lot more detailed terrain than aircraft do.
Considering terrain leads to several interesting topics. We will discuss in class
* Manual and automatic terrain preparation from GIS data
* Phototexture-based simulation systems
* Continuous Level of Detail systems
* Dynamic Terrain
* Standards. In particular we will discuss SEDRIS - which stands (approximately) for Standard Environmental Data Representation and Interchange System
Numerous terms will come up which you should note and prepare to describe on your midterm exam. Here are some Queries to help you organize your thoughts.
Query 9. How much data space (in mb) would be required to store 50,000 square km of terrain data represented in 125m grid elevation posts at a vertical resolution of 1 cm?
Query 10. What is microterrain?
Query 11. If an image generation system does not use geospecific phototexture, then must the terrain simply be colored (e. g. generic green for vegetation, blue for water?) What other alternatives exist?
Query 12. Why would Continuous Level of Detail be a desirable feature? What specific CLOD feature was mentioned in conjunction with Special Operations Forces helicopter pilots?
Query 13. What is SEDRIS and who worries about such things?