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235-242.Pdf (449.2Kb) Eurographics/ IEEE-VGTC Symposium on Visualization (2007) Ken Museth, Torsten Möller, and Anders Ynnerman (Editors) Hardware-accelerated Stippling of Surfaces derived from Medical Volume Data Alexandra Baer Christian Tietjen Ragnar Bade Bernhard Preim Department of Simulation and Graphics Otto-von-Guericke University of Magdeburg, Germany {abaer|tietjen|rbade|preim}@isg.cs.uni-magdeburg.de Abstract We present a fast hardware-accelerated stippling method which does not require any preprocessing for placing points on surfaces. The surfaces are automatically parameterized in order to apply stippling textures without ma- jor distortions. The mapping process is guided by a decomposition of the space in cubes. Seamless scaling with a constant density of points is realized by subdividing and summarizing cubes. Our mip-map technique enables ar- bitrarily scaling with one texture. Different shading tones and scales are facilitated by adhering to the constraints of tonal art maps. With our stippling technique, it is feasible to encode all scaling and brightness levels within one self-similar texture. Our method is applied to surfaces extracted from (segmented) medical volume data. The speed of the stippling process enables stippling for several complex objects simultaneously. We consider applica- tion scenarios in intervention planning (neck and liver surgery planning). In these scenarios, object recognition (shape perception) is supported by adding stippling to semi-transparently shaded objects which are displayed as context information. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism -Color, shading, shadowing, and texture 1. Introduction from breathing, pulsation or patient movement. Without a well-defined curvature field, hatching may be misleading by The effective visualization of complex anatomic surfaces is a emphasizing erroneous features of the surfaces. Stippling, challenging task since nested surfaces and complex geome- in principle, requires high-quality surface models. However, tries are involved. With conventional surface and volume since stippling does not produce visually striking primitives rendering, it is often not possible to convey object shapes and such as lines, this technique is better suited for visualizing depth relations precisely. This gave rise to sparse visual rep- surface models with moderate or average quality. Compared resentations, such as hatching and stippling [VKG05]. Saito to hatching, stippling is more adequate for objects which do and Takahashi [ST90] as well as Interrante and Kim [IK01] not exhibit ridges, valleys or other regions of high curvature. showed that depth perception of medical surface models can be effectively enhanced by hatching along curvature direc- Stippling is a rendering technique, where small dots are tions. Hatching requires high-quality smooth surface mod- used to convey shape and shading. The points’ density and els which are very difficult to create automatically based on their location on the surface affect the appearance of a stip- clinical patient data exhibiting noise and minor artifacts, e.g. pled object. While densely distributed dots achieve the im- © The Eurographics Association 2007. 236 Baer et. al / Hardware-accelerated Stippling of Surfaces derived from Medical Volume Data pression of dark shading, sparsely covered regions seem like even because the object moves over a sequence of frames. brighter shaded areas. The points are distributed randomly, This "shower door" effect hampers the use of this technique but the spacing among the points is regular without any lin- in animation sequences, in which particles should move ear or regular pattern. along with the object. We present an effective hardware-accelerated stippling In object space approaches, NPR primitives are associ- method. Compared with existing methods, our approach has ated with actual locations on the model’s surface to achieve the following advantages: frame-coherence. Meier [Mei96] used painterly strokes and Cornish et al. [CRL01] developed a view-dependent parti- 1. It is frame-coherent and therefore suitable for animations cle system in which the distribution and density of strokes is and interactive exploration. All operations for texture co- regulated by a hierarchical clustering algorithm. Similarly, ordinate generation, access and mapping are supported Pastor and Strothotte [PFS03] proposed a concept to gener- by graphics hardware. ate interactive stippling animations using graphics hardware. 2. Our multi-cube mapping algorithm minimizes texture They consider each vertex as a potential location of one stip- distortions and integrates different shading tones and pling point. The amount of stippling points, according to scales in one texture while satisfying the constraints of shading tones and scaling, will be achieved by subdivision Tonal Art Maps [PHWF01]. and simplification of the object geometry. As the amount of 3. The performance is independent of the number of stip- points depends on the number of vertices, time consuming pling points. Moreover, no preprocessing is necessary to preprocessing steps are necessary. The point distribution is realize a stippling rendering for any object. realized with a point hierarchy based on the polygonal mesh In this paper, we describe our stippling approach and and a point relaxation. present scenarios, in which stippling is applied for inter- Lu et al. [LME∗02] introduced an interactive volume vi- vention planning purposes. In contrast to medical education, sualization that simulates stippling drawings, thus combin- where the author may, in principle, work for a long time to ing traditional volume and NPR illustration techniques. Each create a few expressive visualizations, intervention planning volume is illustrated with an appropriate number of stip- is a routine task, which has to be accomplished very fast. pling points initially generated. Treavett and Chen [TC00] Therefore, automatic adjustment of parameters and render- and Sousa et al. [SESS05] apply NPR techniques to scien- ing speed are also considered. Stippling should be used for tific and medical datasets to improve comprehensibility of context objects which serve as orientation aids and not for medical education and surgical training examples. the focus objects, e. g. a tumor, which are immediately rel- evant for an intervention. Therefore, accuracy is not an im- Texture-based methods combine aspects of image- and portant issue. The major source of uncertainty for these visu- object-based approaches. Textures with stippling points or alizations is the segmentation process, not the stippling ap- hatching strokes are mapped to the surface of the in- proach. put model. The first stippled renditions were presented by Winkenbach and Salesin [WS94] for parametric surfaces us- ing randomness to distribute the points on the texture and 2. Related Work therefore on the surface of a model. They also developed prioritized stroke textures to apply a hatching pattern in ob- Image-, object- and texture-based methods are the main ap- ∗ proaches to illustrate objects with stippling or other render- ject space. Klein et al. [KLK 00] used Art Maps and Praun ing styles. Deussen et al. [DHvOS00] and Secord [Sec02] et al. [PHWF01] introduced Tonal Art Maps (TAMs) and produce high-quality stippling images based on greyscale lapped textures. TAMs are a sequence of hatch images rep- images as input. Both use distribution methods based on resenting different tones and mip-map levels. Strokes within Voronoi diagrams and relaxation to achieve a regular posi- the images are scaled to have an appropriate stroke size and tioning of stippling points that avoids nearly all visual pat- density at all resolutions. They are organized to maintain co- terns. While the presented images are visually attractive, herence across scale and tones. Yuan et al. [YNZC05] intro- these image-based approaches are restricted to single rendi- duced a 2D geometry-image processing method to generate tions. The stippling point distribution obtained in one frame stippling renditions. Points are directly rendered and there- is not guaranteed to correspond with the one obtained in the fore the frame rate is roughly inversely proportional to the next frame. Secord et al. [SHS02] propose a method where amount of generated stippling points. frame-coherence is pursued on the image plane, not in ob- ject space. Stippling points or hatching strokes (see Eissele 3. Theoretical Background and Overview et al. [EWE04]) are attached to the model’s surface. This ap- proach is not frame-coherent in object space and the non- Image space methods are normally fast, but since the points photorealistic rendering (NPR) primitives (e.g. points for are placed in image space, undesirable effects (e. g."shower stippling or strokes for hatching) will move across the ob- door" effect) may appear. Object space methods are frame ject’s surface during an animation by shading changes or coherent but commonly require time-consuming preprocess- © The Eurographics Association 2007. Baer et. al / Hardware-accelerated Stippling of Surfaces derived from Medical Volume Data 237 ing steps. Texture-based methods are as fast as image- • the texture design including the representation of various based techniques, frame-coherent like object-based ap- shading tones by the density of stippling points and proaches and furthermore predominantly geometry indepen- • the texture mapping process based on a parametrization
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