三维建模之Models vs. 3D Symbols

Models vs. 3D Symbols

Well, a week has passed and we’re sure that all of you are anxiously awaiting the answer to last week’s question about the multipatch. Those of you that picked answer 5 were correct. The multipatch is a type of shapefile and it consists of a number of surface patches where each surface patch describes a surface. The multipatch will become more important to you as you begin to grow your expertise in 3D scene generation. We will come back to the multipatch later in this newsletter and will include further discussion of it in future issues.

In the previous issue we interspersed the terms “models” and “3D Symbols” very carefully. Now it is time to make the distinction between these two terms. The ArcScene and ArcGlobe environments that come with 3D Analyst look very much like the environments found in today’s expensive simulators. Some commercial flight simulators are so real that pilots log the time spent in them in the same way they log their actual time in the air. To the people who construct these simulators, the buildings, trees, signs, vehicles, etc. that appear in these environments are all referred to as “models.” The analogy is that they are like the models one might purchase in a toy store or hobby shop. They are a representation of a real object. As GIS users, you will quickly recognize that this is not the same type of model we have been using for years to analyze or summarize things like soil loss due to erosion, wildlife habitat definition or traffic flow analysis. Adding to this confusion is the release of the ModelBuilder tool in ArcGIS, version 9. This tool allows you to create new processes and see them in a nice linear interface. It does not make “models” of houses or trees. The logical way to avoid the confusion that was sure to arise from the overuse of the word model was to use a different term. In ArcScene and ArcGlobe, buildings, trees, etc. are referred to as 3D Symbols. However, the software we will use to make custom 3D Symbols is used by the simulator and architectural communities, and those software packages will still call the houses and trees you make “models.” An easy way to keep this straight is to tell yourself that when you convert a 3rd party model for use in ArcScene or ArcGlobe, it becomes a 3D Symbol.

Importing Existing Symbols from non-GIS Applications

The 3D Analyst extension comes with a fairly large library of 3D symbols. It includes several hundred trees, a couple dozen vehicles, numerous buildings, signs and other landscape objects. Fortunately for the GIS user community, ESRI has provided the capability to import existing models from the simulator community and similar technologies. In many instances you may be able to find an object you are looking for on the web in a format that will allow you to convert it to a 3D Symbol. To import a simulator model to the ESRI 3D Symbol format, the model must be in OpenFlight (.flt), 3D Studio (.3ds) or VRML format.

A sample of ESRI 3D symbols available with version 9.

Making Your Own Symbols

If you are building a 3D scene from your very accurate GIS data layers, are you willing to select a generic house symbol from the standard library to represent your house? I suspect the answer is no. Green Mountain GeoGraphics, Ltd. is located in Vermont where the sugaring season is winding down and folks have been tapping their maple trees. The library of trees that comes with 3D Analyst has one sugar maple tree. I could build my town center using only that one maple tree symbol but it probably wouldn’t look very realistic. So what do we do if the library of existing symbols does not meet our needs? The answer is that we make our own symbols. Unfortunately, ESRI does not provide the software necessary to make your own custom 3D Symbols. You will have to acquire some third party software to accomplish this task. Fortunately, there are several inexpensive software solutions that will that will let you build the 3D environment you need. In the coming newsletters we will discuss and outline the steps necessary to make your own custom symbols. We will also present the software packages that can make this process easy and fun.

Positioning 3D Symbols

All 3D Symbols are tied to a point location. 3D Symbols can be used to display a point feature class or can simply added to a 3D scene as a true symbol in the way you might add a graphic to your 2D map. For example, a point feature class may be used to store the location of buildings or street trees. In this case, the centroids of the 3D Symbol are registered to the selected point feature. It is doubtful that a point feature class would be used to represent cars in a parking lot or people on a sidewalk, as these features are not static nor are they linked to a specific location on the ground. These components of the scene would most likely be placed on the landscape as a true symbol and not be tied to a GIS layer.

Another option is to convert your custom 3D Symbol to a multipatch. Once it is converted to this format the custom model ceases to be a dumb symbol and can instead be added to a geodatabase. Now your building can store attributes and know its true location on the landscape. At the 9.0 release, it appears that very little of this capability will be exposed to the user in an easy-to-use interface. In a later issue, we will attempt to show more of this functionality.

Update: For those currently under maintenance, the latest rumor is that all users should have ArcGIS, version 9 in hand by the end of May.