Interactive Volume Illustration Zoltan´ Nagy, Jens Schneider, Rudiger¨ Westermann Scientific Visualization & Imaging Group University of Technology Aachen Abstract In this paper our emphasis is on proposing in- teractive approaches to non-photorealistic volume In this paper we describe non-photorealistic render- illustration. Our first technique creates hatching ing techniques for volumetric data sets. First, we fields coinciding with the principal curvature di- outline an automatic approach that generates line rections along selected volume structures. Creat- drawings to illustrate such data sets and to aug- ing the hatching field and rendering the strokes is ment traditional volume rendering techniques. For performed in two separate passes: In the first pass a number of seed points that are placed appropri- higher order differential characteristics of the vol- ately to represent selected volume structures curva- umetric data field are computed and encoded in a ture lines are traced and encoded by a sparse set of hierarchical data structure. At run time, based on control points. These curves are finally drawn as some user-defined importance criterion a represen- hatching strokes modulated by anisotropic lighting tative set of hatching strokes is computed, each of and transparency. Second, in addition to line-art which is effectively encoded by a connected group drawings we present efficient implementations of of line segments. Strokes are finally displayed as volume toon-shading and silhouette rendering using colored and shaded line strips employing OpenGL fragment shader hardware. All techniques together functionality and the anisotropic model proposed by allow us to interactively illustrate volumetric data Banks [1]. sets and to enhance important features using non- Our choice of techniques was mainly driven by photorealistic rendering techniques. two requirements. First, in our opinion line draw- ings should not be integrated into the volume rep- 1 Introduction resentation itself as proposed by [17] thus pro- hibiting flexible and efficient modification of the In this paper we consider volume illustrations as a strokes appearance and arrangement. Second, tech- class of non-photorealistic rendering (NPR) tech- niques should not rely on any polygonal representa- niques with the particular characteristic to empha- tion thus limiting its potential use to geometric ob- size important attributes or parts of an object and to jects. Rather than that we aim at proposing meth- communicate relevant information to the viewer in ods that are capable of interactively illustrating ar- the most effective way. It is quite interesting that bitrary structures in volumetric data sets without the this characterization exactly matches the demands need to generate intermediate surface representa- on visualization techniques in general. In scien- tions at run time. Moreover we want to demonstrate tific visualization the grant challenge is to convey that interactive non-photorealistic rendering is pos- the relevant information in the most comprehensible sible even for large-scale volumetric data sets thus but not necessarily in the most realistic way. With spawning a promising new direction in scientific vi- regard to that observation NPR techniques seem to sualization. be the predestined rendering tool in visualization In addition to volume hatching we present applications, and it comes to no surprise that NPR non-photorealistic volume rendering techniques techniques have already attracted the visualization via three-dimensional texture maps and fragment community during the last couple of years. shader hardware. Dedicated shaders have been de- ¡ veloped that enable iso-surface toon-shading, sil- University of Technology, Aachen, Seffenter Weg 23, 52056 Aachen, Germany, Phone:+49-(0)241-80-28920, Fax:+49-(0)241- houette rendering and view-dependent opacity en- 80-22241, e-mail: [email protected] hancement at interactive rates in one single render- VMV 2002 Erlangen, Germany, November 20–22, 2002 ing pass. high resolution data sets. 3D curves on free-form surfaces and parametric The remainder of this paper is organized as fol- or implicit surfaces were introduced to emphasize lows. In Chapter 2 we review related work with the geometric properties of 3D models. Winkenbach focus on NPR techniques that are related to our ap- and Salesin [38] also presented an object-based ap- proach. The procedure to compute principal curva- proach, in which oriented strokes were utilized to ture directions in volumetric data sets is described simulating different tones. Markosian et al. [25] in Chapter 3. Chapter 4 is dedicated to interactive employed line drawings for interactive NPR ren- NPR techniques for volumetric data sets. Future di- dering. The focus in this work, however, was on rections and alternative issues are subject of Chap- enhancing silhouettes and cusps by only placing a ter 5. sparse set of appropriately selected strokes. Finally, Praun et al. [28] employed lapped multi-textures to achieve real-time hatching. They extended on the 2 Related work concept of art maps described in [20] by using a set of oriented stroke textures each of which represents In the past a number of different approaches have different tones. In visualization NPR techniques been carried out to simulate the imagery generated have been considered to substitute or to enhance tra- by artists and technical writers in an automatic and ditional rendering techniques. Saito [32] described computer-assisted way. Since in our current work a point-based rendering system to allow for fast pre- we mainly concentrate on the simulation of line viewing of volumetric data sets. Expressive textures drawings, we will only refer to those attempts that have been used in [30, 18] to improve the under- are directly related to ours. For an excellent in- standing of the shape of complex structures. Inter- troduction to and a comprehensive survey of NPR rante [17] developed techniques to enhanced spa- techniques in general let us refer to the book of tial and shape information of transparent surfaces in Gooch and Gooch [12] and to the web-page main- volumetric data sets by constructing patterns of thin tained by Reynolds [29], where many related online opaque lines. In order to accelerate volume render- resources are given. ing and to enhance the insight into complex struc- An image-based system for generating computer- tures Csebf´ alvi et al. [3] proposed a contour based aided pen-and-ink illustrations using oriented stroke visualization technique. Kirby et al. [19] described textures was proposed in [33, 34]. In the final ver- a technique closely related to oil painting, which sion stroke textures conveying color, tone and ori- effectively enhances the information content in 2D entation were generated automatically from a set of images by using multiple layers covering different representative strokes and a user-controlled direc- kinds of features. An image-based approach for tion field. A digital engraving system based on lines simulating pen-and-ink drawings to augment vol- coinciding with potential fields in the image was ume rendering was introduced by Treavett and Chen presented in [27]. Rossl¨ and Kobbelt [31] described [36]. Ebert and Rheingans [7] proposed a modi- a semi-automatic image-based technique for gener- fied volume rendering pipeline that is amenable to ating technical illustrations of 3D models, where the a variety of different NPR techniques, which can be user has to manually select partitions in image space used to further enhance traditional rendering meth- exhibiting coherent curvature direction to generate ods. The visualization of tensor data by means of appropriate hatching fields. A real-time rendering brush strokes was illustrated by Laidlaw et al. [21]. system was described in Lake et al. [22], where Here, a brushing technique was used to stress direc- a palette of textures representing different tones by tional information and to guide the user towards the different pencil strokes is precomputed and used to orientation of electrocardial fields. cover the object. However, due to the employed viewport mapping to generate texture coordinates from viewport coordinates, the textures seem to stay 3 Preprocess above the object thus limiting the techniques suit- ability for animations. We now start with a description of the preprocess In [5] attributed lines in object space were used to that has to be performed to generate accurate princi- augment traditional renderings with regard to user- pal curvature directions in three-dimensional scalar defined importance of information. In [39, 8, 9, 15] data fields. 666 3.1 Discrete curvature estimation this is not a problem if hatches are generated as pro- posed in [17] by integrating backward and forward Although principal curvature directions can be de- along the curvature field path lines. Then, along rived from the extremal values of a quadratic form each path the flip can be performed based on the [6], this technique is impractical in the current current direction of the stroke. scenario due to its numerical complexity. Monga Global methods to continuously orient the entire et al.[26] derived a considerably faster method field, however, fail in general. This is because in for computing the principal directions in three- many real data sets we find curvature fields contain- dimensional scalar fields. First, an orthonormal ing regions in which curvature streams with oppo- basis for is computed explicitly. Therefore, let £ ¡ site direction meet each other, but which smoothly t 2 2 G = F = (g0, g1, g2) and ¢ = g0 + g1. Then an merge into each other in some other region. As a orthonormal matrix P can be given that rotates the matter of fact, in these cases global methods like first basis vector into the direction of G: sweep-planes or region growing successively flip ¥ ¤ g g g 0 g1 2 ¨ 0 the directions back and forth but do not converge. ¦ ¦ ¦ ¦ § G § G ¨ g g g g G On the other hand, optimization schemes such as 1 0 2 ¨ 1 ¦ ¦ © ¦ ¦ § P = § = , h, f G G ¨ § G conjugate gradients as employed in [28] for surface g2 ¦ ¦ ¦ ¦ G 0 G hatching are far too expensive in three dimensions.