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Physically-Based Interactive Bi-Scale Material Design



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Physically Based Interactive Bi Scale Material Design Hongzhi Wu Julie Dorsey Holly Rushmeier Computer Graphics Group Yale University Large Scale View Small Scale View Editing Sliders b a c d e Figure 1 Bi scale material design large scale appearance changes produced by editing of small scale geometry b c color d and BRDFs e using our interactive system a which quickly updates the appearance of the large scale object after small scale edits The small scale details rendered with a Lambertian BRDF for a better visualization are shown in the bottom right corner of b e In d the color of the small scale side faces is adjusted to yellow In e the small scale material is changed from a measured silver metallic paint2 BRDF to a Lambertian model Abstract We present the first physically based interactive system to facilitate the appearance design at different scales consistently through manipulations of both small scale geometry and materials The core of our system is a novel reflectance filtering algorithm which rapidly computes the large scale appearance from small scale details by exploiting the low rank structures of the Bidirectional Visible Normal Distribution Function and pre rotated BRDFs in the matrix formulation of our rendering problem Our algorithm is three orders of magnitude faster than a ground truth method We demonstrate various editing results of different small scale geometry with analytical and measured BRDFs In addition we show the applications of our system to physical realization of appearance as well as modeling of real world materials using very sparse measurements CR Categories I 3 7 Computer Graphics Three Dimensional Graphics and Realism Color shading shadowing and texture Keywords rank matrix Links 1 bi scale material editing reflectance filtering lowDL PDF W EB V IDEO Introduction The appearance of materials can vary considerably when viewed at different scales For example individual grains of sand or fab ric threads that are visible on close view merge into a material described by a single reflectance function when viewed from a distance Physically speaking the large scale appearance is uniquely determined by averaging the look of small scale details Bruneton and Neyret 2011 Therefore it would be desirable to build an editing system for interactive appearance design at different scales by manipulating small scale structures This could be useful in applications like building exterior design where the user edits the looks of a building at different view distances Existing interactive material editing systems e g Ben Artzi et al 2006 Pellacini and Lawrence 2007 focus on adjusting material appearance only at a single scale On the other hand previous work Westin et al 1992 Gondek et al 1994 which computes realistic large scale appearance by simulating light interactions in small scale details is too slow to provide interactive feedback Although converting small scale structures to large scale appearance is essentially performing reflectance filtering related techniques Bruneton and Neyret 2011 are not suitable for our purpose due to the lack of support for general geometry and materials Han et al 2007 or costly computational overhead Wu et al 2009 This paper presents to our knowledge the first physically based interactive bi scale material editing system which manipulates smallscale geometry and Bidirectional Reflectance Distribution Functions BRDFs to facilitate appearance design at two different scales consistently The user can freely change both small scale geometry and materials then our system quickly computes the largescale appearance to provide interactive visual feedback As illustrated in Fig 1 various small scale edits can have dramatic effects on appearance We achieve an acceleration rate of over 5000 1 when compared with a ground truth method similar to Westin et al 1992 implemented on modern hardware The key to the performance of our system is a novel reflectance filtering algorithm which efficiently processes the Bidirectional Visible Normal Distribution Function BVNDF and pre rotated BRDFs derived from the changing small scale details We observe and exploit the lowrank structures in both quantities to accelerate the large scale appearance computation using Singular Value Decomposition SVD combined with the random projection method Vempala 2004 Our system can also guide the physical realization of bi scale appearance since the small scale details are explicitly modeled In addition real world materials can be approximately modeled by mimicking the small scale details and then fine tuning the largescale appearance using very few photographs We believe that our system can be useful in many applications including building exterior design outdoor advertisements physical realization of appearance as well as rapid material modeling in visual effects industry pute the large scale appearance by an expensive simulation of light interactions Heidrich et al 2000 computes a large scale BRDF which takes indirect illumination into account using precomputed small scale visibility Recently Zhao et al 2011 produces highly realistic large scale appearance of fabric by modeling detailed small scale structures from micro CT imaging The above methods are not suitable for editing where the small scale details are changed interactively In summary the major contributions of this paper are Reflectance Filtering Normal map filtering techniques Tan et al 2005 Han et al 2007 do not consider shadowing and masking effects and they only support limited types of analytical BRDFs Wu et al 2009 proposed a reflectance filtering algorithm which handles general geometry and materials However their method tightly couples the processing of the small scale geometry with materials whenever either one changes an expensive precomputation must be performed which can take as long as several hours We propose a novel form of interactive material design at two scales through the manipulation of both small scale geometry and materials Our method is physically based so that the large scale appearance is consistent with the small scale details We bridge the gap between interactive editing and previous work on bi scale material modeling Westin et al 1992 by using a novel reflectance filtering algorithm that rapidly computes large scale appearance from changing small scale details Our system facilitates the design of physically realizable materials since we explicitly model small scale details We propose


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