
Line-Art Rendering of 3D-Models Christian R¨ossl Leif Kobbelt Max-Planck-Institute for Computer Sciences Stuhlsatzenhausweg 85, 66133 Saarbrucken,¨ Germany {roessl,kobbelt}mpi-sb.mpg.de Abstract arts for centuries. Well known examples are silhouette drawings where just the contours of an object are out- We present an interactive system for computer aided lined or more sophisticated techniques like engraved generation of line art drawings to illustrate 3D mod- copper plates that have formerly been used for print- els that are given as triangulated surfaces. In a pre- ing or pen-and-ink illustrations. processing step an enhanced 2D view of the scene is Apart from the advantage of abstraction or artis- computed by sampling for every pixel the shading, the tic features line drawings also provide some technical normal vectors and the principal directions obtained benefits: they can be stored resolution independent or from discrete curvature analysis. Then streamlines are provide level of detail like features for different resolu- traced in the 2D direction fields and are used to de- tions. Besides, such pictures can easily be reproduced fine line strokes. In order to reduce noise artifacts the on any monochrome media. user may interactively select sparse reference lines and the system will automatically fill in additional strokes. By exploiting the special structure of the streamlines an intuitive and simple tone mapping algorithm can be derived to generate the final rendering. 1. Introduction In contrast to photorealistic images based on physical simulation of lighting and shading, non- photorealistic rendering techniques offer various ad- vantages for printed illustrations. The higher level of abstraction that they provide helps to put emphasis on the crucial information of an illustration. Abstraction mechanisms available to designers include levels of de- tail and focus attention to stress the important parts of Figure 1. Hand-made line art drawing have a drawing or enriching an image with additional visual been used for centuries. Our example is from information and effects. 1778. Such illustrations provide a high den- Typical applications for the use of non-photorealism sity of clearly perceptible information while are e.g. architectural sketches, illustrations in medical also satifying aesthetic standards. text books or technical manuals, and cartoons. Many of these applications prefer a very specific technique: line rendering. Here, a picture consists of monochrome In this paper we present a tool that assists an artist lines or strokes only. An artist may chose between dif- in manufacturing a line drawing from a 3D model. The ferent line styles or vary line attributes like length, geometry of the object is given as a triangle mesh. This thickness or waviness. There is a broad spectrum allows us to handle a huge class of models ranging from of drawing styles, that have been developed in fine technical constructions in CAD to real world data that is obtained from a range scanner. The system frees the of techniques that are pixel based and produce discrete user from drawing individual lines or strokes. Instead images. [14] introduce the concept of G-buffers that curvature data is first sampled from the object and provide additional information per pixel such as depth then used to semi-automatically generate lines. Our value, normal vector or the parameter value of a para- rendering style produces long single or cross hatched metric surface. Non-photorealistic images that resem- lines of varying thickness that are especially appropri- ble line drawings can then be generated by applying ate for technical drawings. well known image processing operators. In contrast, [9] uses a modified ray-tracing algorithm for emulating 2. Related Work engraved copper plates. With processing pixel data it is also possible to take advantage of graphics hardware: [3] generate copper plate like images from intersection Over the last years various computer based tech- lines of OpenGL clipping planes with the object. niques for producing line drawings semi-automatically The approach that is presented in this paper exam- or even automatically have been developed. There are ines the geometry of a model by approximating the two basic approaches: The first one is image based. principal directions in every vertex of the given tri- Here, the user is assisted in converting a digital grey angle mesh. Then this data is interpolated across tri- scale image into a line drawing without having to draw angles and sampled per pixel. Here, the G-buffer con- individual strokes. [12] define a potential field over an cept is used, and the graphics hardware is exploited. image and draw equipotential lines of varying density. Once all data is grabbed from image buffers the user In [15] predefined stroke textures are used to map the may modify these buffers and an additional (”sten- tone of the reference image and to produce a pen-and- cil”) buffer and interactively place continuous stream- ink illustration. This method is improved in [16] in a lines. Instead of generating all strokes from stream- way that the stroke textures are generated automati- lines, the system lets the user chose a few reference cally from both a set of reference strokes and an inter- lines and generates strokes by interpolation between actively modifiable direction field. these curves. This guarantees that only good, visually The second approach takes into account the 3D ge- appealing strokes that are not disturbed by noise are ometry of a scene. Here, an early step was the use of rendered. For the final Postscript image the silhouette haloed lines [1] that give an impression of depth and that has been grabbed from an image buffer and con- the use of different line styles for outline and shad- verted to a set of polygons is added. ing [4]. [19] utilizes special stroke textures to render high quality pen-and-ink illustrations from polygonal models. These stroke textures are created for differ- 3. Overview of the algorithm ent materials and resemble hand draw strokes. During shading the strokes are mapped to image space to- In this section we give a short overview over the gether with the corresponding polygons and are used various stages of our algorithm. The details are ex- for tone mapping then. This system was extended to plained in the subsequent sections. The input data is process free-form surfaces in [20] where isoparametric given by an arbitrary triangle mesh viewed from a curves are used for rendering, following [5]. Such curves given perspective and the output is a line art image look especially well for surfaces of revolution. Elber of the object. For the preprocessing of the surface, we also employs lines of curvature of parametric surfaces have to assume that the mesh is mostly manifold. Non- and contour lines of implicit surfaces [6]. By precalcu- manifold artifacts can be handled as long as they do lating these stokes and applying a simple shader even not cover a significant part of the image. interactiveness can be achieved [7]. The first phase of the algorithm preprocesses the Most recently, [21] generate a discrete direction field mesh data. For every vertex of the mesh intrinsic geo- over a triangulated (subdivision) surface. Originally metric attributes like the normal vector and the prin- the principal directions are calculated via discrete cur- cipal curvature directions are computed. This infor- vature analysis. In order to reduce noise and to get a mation is needed to determine the stroke placement pleasant rendering this field is globally optimized in a and orientation in the later phases. The strokelines second step by minimizing some energy functional. For or hatches of our line art renderings will follow the the final picture a silhouette drawing is combined with (projected) principal curvature directions on the given hatches from the filtered principal directions. surface, since those directions characterize the under- The previously mentioned approaches that respect lying geometry very well and carry a maximum shape the geometry of an object are all able to produce ”real” information. The user can control the geometric de- stokes, e.g. in Postscript. Besides, there is another class tail considering a smoothed version [8] of the mesh for curvature analysis. Although the geometric attributes From these we can derive normal and principal curva- are estimated for the vertices only, we can (linearly) ture directions. For surface points in the interior of the interpolate them across the triangles to obtain a set of triangular faces these attributes can be computed by continuous attribute fields that are defined everywhere barycentric interpolation of the values in the corners. on the given surface. This interpolation step can be done very elegantly The 3D-object is then rendered together with its by the underlying rasterization engine which maps the attributes into a so-called enhanced frame buffer sim- 3D mesh to the frame buffer: vector or scalar valued ilar to a G-buffer. For every pixel this frame buffer attributes can be encoded as RGB colors and assigned contains lots of information about the visible surface to the mesh vertices. Rendering the object without point. The pixel data includes the local shading (grey any shading then generates a frame buffer pixel matrix value) which later determines the local stroke den- which stores the interpolated values. Arbitrary preci- sity. Additionally, the normal vector is stored for every sion can be obtained by appropriate scaling and multi- pixel which provides the necessary information for con- pass rendering. For vector valued attributes a conclud- tour extraction. For the orientation of the strokelines, ing normalization step might be necessary. We render the projected principal curvature directions are also each attribute field separately. In the end we obtain a stored. set of images, one of them containing the (grey-scale) In the second phase the enhanced pixel informa- shaded view of the object.
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