Directing Gaze in 3D Models with Stylized Focus
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Eurographics Symposium on Rendering (2006) Tomas Akenine-Möller and Wolfgang Heidrich (Editors) Directing Gaze in 3D Models with Stylized Focus F. Cole1, D. DeCarlo2, A. Finkelstein1, K. Kin1, K. Morley1, and A. Santella2 1Princeton University 2Rutgers University Abstract We present an interactive system for placing emphasis in stylized renderings of 3D models. The artist chooses a camera position, an area of interest, and a rendering style for the scene. The system then automatically renders the scene with emphasis in the area of interest, an effect we call “stylized focus.” Stylized focus draws the viewer’s gaze to the emphasized area, through local variations in shading effects such as color saturation and contrast as well as line qualities such as texture and density. We introduce a novel algorithm for local control of line density that exhibits a degree of temporal coherence suitable for animation. Animating the area of emphasis produces an effect we call the “stylized focus pull.” Finally, an eye-tracking experiment verifies that the emphasis does indeed draw the viewer’s gaze to the area of interest. 1. Introduction from 2D sources. One novel form of dynamic imagery presented in this paper is the stylized focus pull. In live action Artists have over the centuries developed a set of principles cinematography, a focus pull is used to subtly or overtly by which they adjust rendering qualities such as line draw the attention of the audience from one part the scene to density and contrast in order to emphasize some areas of another, by dynamically adjusting which parts of the scene an illustration and de-emphasize other areas. This ability are in focus. Our system provides a similar effect using the is one of the cornerstones of abstraction, allowing the tools of abstraction. artist to focus the viewer’s attention on what is important while eliding unnecessary detail. Researchers advocating One challenge in creating abstractions from 3D models non-photorealistic rendering (NPR) techniques often cite is the need for local control of level of detail (LOD). the ability to direct the viewer’s attention as a benefit of Controlling LOD has two main purposes: first, to reduce NPR [GG01,SS02]. Few systems, however, have offered this visual artifacts that occur when many features of the ability automatically and with local control. DeCarlo and model project to a small area in the image; second, to Santella [DS02, SD04] developed a system to automatically aid in directing the viewer’s gaze by reducing detail in create abstract renderings from photographs, based on where de-emphasized regions of the image. In both cases, it is people tend to look in the photos. However, to date no vital to maintain temporal coherence for dynamic imagery. equivalent work has been performed for 3D rendering. We Coherence is a thorny challenge for LOD because details present an interactive system for directing a viewer’s gaze in that suddenly appear or disappear will visually “pop.” stylized imagery rendered from 3D models (Figure 1). This paper offers several contributions to the literature. There are several benefits to basing such a system First, our system is the first to provide temporally coherent on 3D models. First, 3D is a natural domain for many control of emphasis for interactive 3D NPR applications. applications that can benefit from our method, such as Second, in support of such applications we describe a rendering of architectural models. Second, a 3D system can new algorithm for local control of line LOD. Third, we adjust rendering qualities in ways that might be difficult or present the concept of the stylized focus pull wherein impossible using purely 2D methods. Indeed, we experiment emphasis is animated between two areas using the tools with such effects (Section 3) and evaluate their effectiveness of abstraction. Finally, we present the results of several (Section 4). Finally, it is easier to create certain kinds user studies demonstrating that the visual effects for 3D of dynamic or animated imagery from 3D models than renderings described herein do indeed cause the viewer to c The Eurographics Association 2006. F. Cole, et al. / Directing Gaze in 3D Models with Stylized Focus Kosara et al. [KMH01] also explored abstraction and emphasis for 3D models. They extended the conventional depth of field blur effect by blurring semantically irrelevant objects, regardless of their distance to the camera. The viewer was expected to direct attention to the remaining sharp objects in the scene. The authors noted, however, that the blurriness looks unnatural and can be irritating to the eye. Isenberg et al. [IMS00] proposed a more general formulation for assigning emphasis, but used a simple rendering scheme based only on line weight. In contrast to both these papers, our system integrates control of many rendering parameters for both lines and color that work in concert to provide an effective emphasis effect, as shown in our study. One crucial aspect of our method is control over level of detail (LOD). Most LOD work has focused on reducing detail for efficiency while affecting perception as little as possible [LWC∗02]. In contrast, the goal in NPR is to change the impression of the image by reducing unnecessary detail. In NPR, unnecessary detail often takes the form of overly dense or cluttered lines. Most line density control schemes are specialized to the type of lines being drawn. For example, Deussen and Strothotte [DS00] proposed a method to control the drawing of tree leaves and branches. While specialized, their method is effective and provides good temporal coherence. For rendering architectural models in a pen-and-ink style, the method of Winkenbach and Salesin [WS94] reduces line clutter using “indication”: rather than drawing a complicated texture over an entire surface, only a few patches marked by the user are drawn with high detail. Their method also Figure 1: Placing emphasis. In the upper image the viewer’s provides local control over line density, primarily for the gaze is drawn to the upper-left part of the structure, whereas purpose of controlling tone. However, such hand-crafted in the lower image (with the same camera pose) emphasis is line textures are not amenable to rendering with temporal placed in the lower-front balcony. A combination of visual coherence. The system of Strothotte et al. [SPR∗94] offers cues give rise to this effect, including line quality and a similar interface – with similar benefits and limitations – density, color saturation, and contrast. to that of Winkenbach and Salesin with the explicit goal of directing the attention of the viewer. Praun et al. [PHWF01] introduced a temporally-coherent hatching method based look at the emphasized regions. Our examples are largely on blended textures that provides control of line density motivated by architectural rendering, however the potential for reproducing tone. These methods are intended to deal applications for this research are much broader, including with clutter and shading, not abstraction; they are effective medical and technical illustration, games, interactive story at simplifying repetitive or stochastic textures, but not at books, and education. abstracting larger structures. Jeong et al. [JNLM05, NJLM05] developed a method 2. Related work for abstraction based on a series of representations of a 3D model, with varying complexity (created by simplifying DeCarlo and Santella [DS02] used an eye tracker to identify the original model). Their algorithm rendered the model regions in photographs that viewers tended to focus on, and with varying LOD, depending on importance or distance. generated abstract renderings of these photographs with the However, such methods do not provide explicit local control other, less interesting regions reduced in detail. They then of line density; rather it emerges as a by-product of used eye tracking to confirm [SD04] that this abstraction was mesh simplification. Furthermore, the scheme based on effective at guiding viewers’ gazes. We apply this idea to simplification does not perform well on “boxy” meshes 3D models, using similar abstraction techniques to focus the such as architectural models. A method introduced by viewer on particular regions of the model. Wilson et al. [WM04] does offer local control of line LOD c The Eurographics Association 2006. F. Cole, et al. / Directing Gaze in 3D Models with Stylized Focus based on line priority, like the method described herein. The method of Grabli et al. [GDS04] operates similarly to that of Wilson et al., but adds a more sophisticated measure of line density. These two methods only drop strokes in areas of high density. In contrast, the simplification work of Barla et al. [BTS05] simplifies strokes by replacing groups of strokes with fitted, simplified versions. However, none of these methods addressed temporal coherence, inhibiting their use in an interactive setting. One of the contributions of our paper is the introduction of a prioritization algorithm designed for local control of line LOD. This algorithm works with boxy models and enjoys temporal coherence appropriate for rendering dynamic scenes. There are several strategies to assess the effectiveness of NPR algorithms. One approach is to survey users to gather their subjective impressions. Schumann et al. [SSRL96], for example, demonstrate that architects prefer sketchy renderings to depict the preliminary nature of a design. A second approach measures performance in some task. For example, Gooch et al. [GRG04] compare response times for recognition of faces in photographs and artistic renderings. Heiser et al. [HPA∗04] use a concrete task, and evaluate ∗ automatically generated assembly instructions [APH 03] Figure 2: Comparing focus models. Top to bottom: 2D focal by recording construction times for assembly of a piece point, 3D focal point, and focal plane. Left: shading level of furniture. A third approach is available when we are indicates focus; inside yellow is 100%; outside red is 0%.