Stefan Schlechtweg

A developer's guide to silhouette algorithms for polygonal models

Silhouettes play an important role in shape recognition because they provide one of the main cues for figure-to-ground distinction. However, since silhouettes are view dependent, they need to be determined for every frame of an animation. Finding an efficient way to accomplish this is nontrivial. Indeed, a variety of different algorithms exist that compute silhouettes for geometric objects. This article provides a guideline for developers who need to choose between one of these algorithms for his or her application. We restrict ourselves to discussing only those algorithms that apply to polygonal models, because these are the most commonly used object representations in modern computer graphics. Thus, we can use all algorithms discussed to take a polygonal mesh as input and compute the visible part of the silhouette as output. Some algorithms, however, can also help compute the silhouette only, without additional visibility culling. The silhouettes representation might vary depending on the algorithm class - that is, the silhouette might take the form of a pixel matrix or a set of analytic stroke descriptions.Generating object silhouettes lies at the heart of nonphotorealism and is needed for many styles of rendering. This article describes, categorizes, discusses, and recommends algorithms for computin...

ArcTrees: visualizing relations in hierarchical data

In this paper we present, ARCTREES, a novel way of visualizing hierarchical and non-hierarchical relations within one interactive visualization. Such a visualization is challenging because it must display hierarchical information in a way that the user can keep his or her mental map of the data set and include relational information without causing misinterpretation. We propose a hierarchical view derived from traditional Treemaps and augment this view with an arc diagram to depict relations. In addition, we present interaction methods that allow the exploration of the data set using Focus+Context techniques for navigation. The development was motivated by a need for understanding relations in structured documents but it is also useful in many other application domains such as project management and calendars.

RenderBots—Multi-Agent Systems for Direct Image Generation

The term stroke‐based rendering collectively describes techniques where images are generated from elements that are usually larger than a pixel. These techniques lend themselves well for rendering artistic styles such as stippling and hatching. This paper presents a novel approach for stroke‐based rendering that exploits multi‐agent systems. RenderBots are individual agents each of which in general represents one stroke. They form a multi‐agent system and undergo a simulation to distribute themselves in the environment. The environment consists of a source image and possibly additional G‐buffers. The final image is created when the simulation is finished by having each RenderBot execute its painting function. RenderBot classes differ in their physical behavior as well as their way of painting so that different styles can be created in a very flexible way.

High Quality Hatching

Hatching lines are often used in line illustrations to convey tone and texture of a surface. In this paper we present methods to generate hatching lines from polygonal meshes and render them in high quality either at interactive rates for on‐screen display or for reproduction in print. Our approach is based on local curvature information that is integrated to form streamlines on the surface of the mesh. We use a new algorithm that provides an even distribution of these lines. A special processing of these streamlines ensures high quality line rendering for both intended output media later on. While the streamlines are generated in a preprocessing stage, hatching lines are rendered either for vector‐based printer output or on‐screen display, the latter allowing for interaction in terms of changing the view parameters or manipulating the entire line shading model at run‐time using a virtual machine.

Creating non-photorealistic images the designer's way

We present a novel way for quickly and easily designing non-photorealistic images based on elementary operations which are linked together to create a variety of visual effects. Rather than mimicking a visual effect that an artist has already produced, we instead mimick the process undergone for the artist to produce that image. Compared to traditional approaches, this opens the possibility to have the images created by users with no programming skills. We describe a modular system that makes these elementary operatios available to the user. A specially designed user interface allows for an easy and intuitive combination of these operations to create an image. Visual feedback is provided to the user at any time and for any stage in the process.

Enhancing slice-based visualizations of medical volume data

Slice-based visualizations of CT and MRI data are frequently used for diagnosis, intervention planning and intraoperative navigation since they allow a precise analysis and localization. We present new techniques to enhance the visualization of cross sectional medical image data. Our work is focussed on intervention planning and intraoperative navigation. We address the following problems of slice-based visualization in these areas: the lack of a graphical overview on the positions of anatomic structures, the localization of a target structure and the display of safety zones around pathologic structures. To improve the overview, we introduce LIFTCHARTs, attached as vertical bars to a slice-based visualization. For localizing target structures, we introduce halos. These techniques restrict the occlusion of the original data to a minimum and avoid any modification of the original data. To demonstrate the usability of these visualization techniques, we show two application scenarios in which the techniques come into operation.

The intelligent pen: toward a uniform treatment of electronic documents

This paper introduces the intelligent pen metaphor, an approach to uniformly treat interaction with electronic documents, especially for marking and annotating a document. This enables an easier handling of electronic documents also for novice and inexperienced users. Interaction tasks are combined in one metaphor that is well known to the user from his or her daily work with paper documents. We also present a prototypical implementation that exploits an alternative user interface to offer additional support for these users. All information added to the document by a user is stored together with the original text allowing an easy document exchange and thus having influence on collaborative work. The main contribution of this paper is the support of interactive learning processes. This pertains to active reading as well as document navigation.

An approach to visualizing transparency in computer-generated line drawings

This paper builds on the principles of depicting transparency in hand-made line drawings, and develops a method to generate similar, but computer-generated, line drawings in a two-step process. In the first step, a brightness image is generated, which is used as a basis for the line drawing created in the second step. Three different methods derived from traditional drawing techniques are shown.

Knowledge-supported graphical illustration of texts

We introduce a new method to automatically and dynamically illustrate arbitrary texts from a predefined application domain. We demonstrate this method with two experimental systems (Text Illustrator and Agi3le) which are designed to illustrate anatomy textbooks. Both systems exploit a symbolic representation of the content of structured geometric models. In addition, the approach taken by the Agi3le-system is based on an ontology providing a formal representation of important concepts within the application domain as well as a thesaurus containing alternative linguistic and visual realizations for entities within the formal domain representation. The presented method is text-driven, i.e., an automated analysis of the morphologic, syntactic and semantic structures of noun phrases reveals the key concepts of a text portion to be illustrated. The specific relevance of entities within the formal representation is determined by a spreading activation approach. This allows to derive important parameters for a non-photorealistic rendering process: the selection of suitable geometric models, camera positions and presentation variables for individual geometric objects. Part-whole relations are considered to assign visual representations to elements of the formal domain representation. Presentation variables for objects in the 3D rendering are chosen to reflect the estimated relevance of their denotation. As a result, expressive non-photorealistic illustrations which are focussed on the key concepts of individually selected text passages are generated automatically. Finally, we present methods to integrate user interaction within both media, the text and the computer-generated illustration, in order to adjust the presentation to individual information seeking goals.

miniCAVE: a fully immersive display system using consumer hardware

We present the design and construction of a small scale CAVE that employs active stereo using shutter glasses for viewing as well as for projection. It is controlled by a single server with four graphics boards. The main use of this device is educational, students should be given a possibility to see their programs run in an immersive setting. Further applications include VR design studies and the development of new VR interaction techniques using the Nintendo Wii controller as input device.