The Web 3D Service (W3DS) is a portrayal service for three-dimensional geodata such as landscape models, city models, textured building models, vegetation objects, and street furniture. Geodata is delivered as scenes that are comprised of display elements, optimized for efficient real time rendering at high frame rates. 3D Scenes can be interactively displayed and explored by internet browsers with 3D plugins, or loaded into virtual globe applications. The W3DS is capable of handling data sets of a wide range of scales, from full globes down to smaller immobile objects such as lanterns which are still of geographic relevance. It can handle data sets consisting of multiple Levels of Detail for each object, thereby greatly increasing performance without sacrificing quality. The representation of geographic objects my range from very detailed and textured models, to prototypic and generic models used for same types of objects, to abstract boxes or symbols. Scenes are retrieved by queries defining the geographic area, information layers, styles, and further parameters, very similar to the Web Map Service (WMS) interface. The formats used for encoding 3D scenes are designed for limited bandwidth networks like the internet and realistic and efficient real time rendering. They avoid overhead produced by e.g. XML tags, and exploit graphics technologies like display lists, re-using scene graph nodes by defining links, indexed geometry arrays, and software shaders. Due to the used formats the contents delivered by the W3DS is not restricted to static objects, it can also include animations and other visual effects, as well as pre-defined behaviors triggered by user interactions.
The role of a W3DS is similar to the role of a WMS. It does not necessarily provide the original geodata but a view on it. This view is called Scene and consists of display elements representing the geometry, appearance, and behavior of geographic objects. It does not contain attributes and semantic information. In contrast to the WMS, the output of a W3DS is not images, but scene graphs consisting of a tree like structure of nodes, groups, transforms, shapes, materials, and geometries. Since attributes are not part of the scene graph, they must be accessed by additional service operations. The W3DS GetFeatureInfo operation is responsible for accessing additional information on objects and functions equivalent to the WMS.
The W3DS can have its own data store or it can retrieve all data from a WFS, forwarding and processing all data on request. Another possibility is to locally cache the content of a WFS and synchronize all data in order to save network bandwidth and processing time during the handling of requests, thus reducing the response times. In the case of a WFS as original data store, we assume that the WFS supports the CityGML application schema, otherwise clients would not benefit from an intermediate 3D portrayal service. In such a setup, the WFS plays the role of a central data repository, that is maintained and updated by the owner of the data. The W3DS then acts as middleware enabling efficient visualization and streaming to the interactive client.
GML already supports many features for describing three-dimensional objects derived from the ISO 19100 series of international standards, like surfaces, solids, and aggregate types. CityGML adds semantic and structure to the GML schema and defines very precisely how elements of the urban environment can be modeled in a logical and interchangeable way. It is defined as an application profile of GML and contains a spatial model, an appearance model and a thematic model for various types of real world objects. In contrast to usual 2D vector formats (e.g. SVG), CityGML can represent the geometrical properties of objects in a level of detail that is even sufficient for Virtual Reality applications. However, this specification draft was developed in order to leverage the usage of another set of standards focusing on efficient real time visualization techniques and 3D cartographic features, first and foremost on X3D, which is a W3C standard and KML which has been adopted by the OGC. The advantage of using visualization centric exchange formats is on the one hand the more compact encoding allowing a higher throughput in limited bandwidth networks and on the other hand the support of sophisticated appearance, shading, behavior, and animation models. The W3DS shall support X3D as mandatory format.

scope.txt · Last modified: 2010/05/17 17:06 (external edit)