Interactive Expressive Illustration of 3D City Scenes Bin Pan ∗ , Xiang Chen ∗ , Xiaoming Guo † , Wei Chen ∗ and Qunsheng Peng ∗ ∗ State Key Lab of CAD&CG, Zhejiang University, Hangzhou, China 310027 † College of Sciences, Liaoning Shihua University, Fushun, China 113001 Abstract—While many approaches have been developed to visualize 3D city scenes, most of them exhibit the visualization results in a uniform rendering style. This paper presents an expressive rendering approach for visualizing large-scale 3D city scenes with various rendering styles integrated in a seamless way. Each view is actually a combination of the photorealistic rendering, the nonphotorealistic rendering, and the line drawing, so as to highlight the information that is interesting for the users and de-emphasize the other that is less important. At run-time, the users are allowed to specify their interested locations with pre-determined 3D landmarks. Our system automatically computes the salience of each location and visualize the entire scene with emphasis in the area of interests. The GPU-based implementation enables real-time performance, and demonstrates outstanding practicality. Keywords-City Scenes; Expressive Rendering; Salience; I. I NTRODUCTION Due to the rapid development of computer hardware and the progress in (semi-) automatic data acquisition, it is now possible to create large-scale 3D city scenes at reasonable costs. An increasing number of applications and systems incorporate virtual 3D city scenes as essential system components such as urban planning and redevel- opment, facility management, etc. Usually, large-scale 3D city scenes are characterized by a large number of objects with different types, structures and hierarchies, yielding a high degree of visual detail. Thus, they transport a huge amount of information, which frequently causes perceptional and cognitive problems for the users such as heavy visual clutter and information overload. This observation reveals a fundamental problem of visualization of complex 3D city scenes, namely how to access information efficiently and effectively for users. The 3D representation of a 3D city scene serves as a medium to convey spatial-related information in a compre- hensive way. The requirements on virtual 3D city scenes vary between different applications. In the context of tourism, entertainment, or public participation, a high degree of photorealism is required. A rendering is an abstraction that favors, preserves, or even emphasizes some qualities while sacrificing, suppressing, or omitting other characteris- tics that are not the focus of attention[1]. This paper presents our preliminary efforts to effectively present information based on the users interaction. Note that most of the current virtual 3D environment employ a single rendering style, such as photorealistic (PR) or non- photorealistic (NPR) for all buildings in the 3D city scene. It is our belief that an expressive rendering technique would clarify meaningful structures in the 3D city scene. However, the decision of interesting regions and structures depends on the users’ demand. In our system, the models that are important for the users are rendered at full details using PR, and the area surrounding the important buildings is rendered by NPR. Regions far from the focal point are presented merely by line drawing method. By combining different rendering styles with different weighting and proximity schemes, our approach favors abstractive visualization of large-scale scenes in a smooth and characteristic-preserving way (Figure 1). II. RELATED WORK A 3D city scene is a three-dimensional representation of a city or an urban environment. Nowadays, virtual 3D city scenes allow for visually integrating huge quantum of urban information within a single framework. To facilitate comprehensive exploration of a city scene, artists have developed a set of principles by which they adjust rendering qualities such as the amount of color and contrast in order to emphasize some areas of an illustration and de-emphasize other areas. Focus + Context Visualization:Focus + context visual- ization has been well studied in the past years[2] [3] [4]. In [5], a set of tools were presented to enable simultaneous exploration of a virtual world from two different viewpoints. One viewport is used to display the surrounding environment and the other is interactively adjusted to display the user concerned area. In [6], Semantic Depth of Field (SDOF) is utilized for information visualization and to blur different parts of the depicted scene in dependence on their rele- vance. Objects of interest were depicted sharply in SDOF, whereas the context of the visualization is blurred. In [4], Matthias et al. proposed 3D generalization lenses. It is a visualization technique for virtual 3D city scenes that combined different levels of structural abstraction. Focus area within lens volumes are shown in full details while less important details in the surrounding area are excluded. Tassilo Glander et al. [3] presented a novel concept for the real-time depiction of landmarks which emphasizes these 3D objects by improving their visibility with respect to their 2011 12th International Conference on Computer-Aided Design and Computer Graphics 978-0-7695-4497-7/11 $26.00 © 2011 IEEE DOI 10.1109/CAD/Graphics.2011.61 406