Rendering of Complex Materials for Driving Simulators

Transcription

1 Renderng of Complex Materals for Drvng Smulators Therry Lefebvre 1,2 +33 (0) Andras Kemeny (0) andras.kemeny@renault.com Dder Arquès (0) dder.arques@unv-mlv.fr Loïc Joly (0) loc.joly@renault.com 1 Techncal Centre for Smulaton Technocentre Renault, TCR AVA O13 1 avenue du golf, Guyancourt Cedex. Renault, France. Sylvan Mcheln (0) mcheln@unv-mlv.fr 2 Laboratory of Image synthess Batîment Charles Cros 6 Cours du Danube Serrs. Unversty of Marne-la-Vallée, France. Abstract The vsual realsm of a vrtual scene s very mportant for drvng smulators. In real lfe, a scene s composed of complex materals (asphalt, vegetaton, road mark...) nteractng wth lght sources (sun, headlamps...). Our goal s to smulate such envronments n real tme under varous weather condtons (dry or wet road). The lghtng nteracton cannot be smulated drectly usng standard lghtng model mplemented n graphc board (Phong shadng). In ths paper, we present a method for renderng complex materals wth BTF (Bdrectonal Texture Functon). Ths method, whch allows to vew varous and complex lghtng phenomenon (such as nterreflectons, self-shadowng and maskng effects), requres a long computaton chan and uses a large data set (one or more ggabytes). The man steps of ths method are exposed : BTF acquston (both vrtual and through measurement), data compresson (a dedcated algorthm can acheve compressons rates above 2000:1), and real tme mplementaton technques usng vertex and pxel shaders. We present our frst results, applcatons and dscuss about lmtatons and future works of ths renderng technque. 313

2 Introducton Objectves The realstc smulaton of vrtual scenes s one of the man challenges n today s computer graphcs. It depends mostly on the nteractons between lght sources (sun, headlamp ) and materals (asphalt, road markng, vegetaton ). These nteractons are much more complex than the lghtng model mplemented n graphc board (Phong lghtng model). Ths paper presents a renderng technque of complex materals based on BTF (Bdrectonal Texture Functon). In the frst part, we present man concepts of materal representaton. In the second part, we descrbe an algorthm dedcated to real tme renderng of materals usng BTF. Fnally, we dscuss about advantages and lmtatons of the method before presentng results and future works. Our fnal goal s to smulate realstc envronments under varous weather condtons (dry or wet road) for both headlght smulator and daytme drvng smulator. Today s lghtng smulaton The Techncal Centre for Smulaton of Renault (CTS) has developed a drvng smulaton software (SCANeR II). It reles on several ndependant modules usng a network protocol. The software enables programmers to buld complete drvng smulator wth mmersve stereoscopc vsualsaton, dynamc models, trafc and scenaro generaton, sound smulaton, and comprses many tools for data analyss and road network modelng. It can be used for many dfferent studes such as human factors, vehcle systems ergonomcs, vehcle behavour, percepton (navgaton), basc and advanced drver tranng, headlght assessment or even road nfrastructure. Fgure 1 : headlght smulator (left), lghtng smulaton wthn fog (center), glare effects (rght) The real-tme headlght smulaton module s used for the assessment of new projectors snce 1998 [LK99]. Its headlght man features comprses lghtng smulaton wthn fog and smulaton of glare effects comng from trafc vehcles (fgure 1). 314

3 To compute lghtng n real tme, the tool calls upon classcal renderng technques : projected textures (for headlamps) and hardware lghtng model (Phong [Pho75]) for surfaces reflecton. Headlamps are descrbed by ther photometrc data (lumnance and color nformaton). Surfaces are purely dffuse, the same amount of lght s reflected n all drectons (fgure 2). Fgure 2 : dffuse reflecton on road surface. Materal representatons Overvew Tradtonally the geometry of a surface s modeled explctly (e.g. wth mesh) only up to a certan scale (fgure 3). Fne and planar detals on surfaces are represented by textures. Geometrc detals are generally modeled usng trcks, such as bump mappng or dsplacement mappng. The mcro-structure responsble for the reflectance behavour of a materal s smulated usng lghtng models (Phong or reflectance functon descrbed n the followng secton). BRDF (Bdrectonal Reflectance Dstrbuton Functon) Fgure 3 : modelng technques. Reflectance propertes of a materal are commonly represented by a functon called BRDF (fgure 4). Ths functon descrbes, for an ncomng llumnance, the amount of reflected lumnance n a specfc drecton (albedo) and can be descrbed as follow: r L ( x, ω, λ ) r ω Lght vector dω r r n r L o ( x, ω o, λ ) r = ωo = x r ω θ d x S r r r Lo ( x, ωo, λ) fr ( ω, ωo, x, λ) = r L ( x, ω, λ)cosθ dω r ω o Vew Vector dω = Sold angle around lght vector θ = Angle between lght vector and surface normal λ = Wavelength L = Receved Lumnance L = Reflected lumnance o 315

4 Fgure 5 : BRDF representaton BRDF measurements of wet and dry asphalt are represented n false colors on fgure 5. You can see a large retro dffuson around the ncomng ray (rght) for dry asphalt whereas a fne specular peak around the reflected ray for wet asphalt. Some nose s also vsble (especally on wet asphalt) at grazng angles due to sold angle (more the sgnal s weak, more the nose s relevant). Thus, t could be nterestng to apply a nose flter or fnd a numercal model that Fgure 4 : measures of dry (left) and wet (rght) asphalt. match the measured data. Usng a BRDF functons, materals wth coarse varatons can be well represented, only for far vewers (lghtng varatons are smaller than a pxel sze). The dffculty arses from the complex mesostructure and reflectance behavour defnng the unque look-andfeel of a materal. Fgure 6 : renderng based on measured BRDF of wet asphalt combned wth normal mappng. Snce the BRDF already contans lght nteractons wthn the asphalt, t s not physcally correct to apply bump-mappng technques to render asphalt roughness. Thus, we need a spatally varant functon to descrbe complex and heterogeneous materals (asphalt, vegetaton ). We descrbe n the next part a sutable functon recently ntroduced by Dana and al. n 1999 [DvGNK99]. 316

5 BTF (Bdrectonal Texture Functon) Overvew The BTF can be seen as a set of 2D-textures (fgure 8), where each one corresponds to a gven lghtng and vewng drecton. A BTF s very smlar to BRDF but provdes a texture nstead of an albedo. Thus, t can model the fne-scale self-shadows, self-occlusons, and speculartes caused by surface mesostructure. A wde class of materals (asphalt, marble, wool ) can be descrbed by a such functon. (vew) ω r o (lght) ω r Fgure 7 : example of ABRDF A BTF can also be represented as a set of Apparent BRDF (fgure 7). An ABRDF s not exactly defned as a BRDF snce t does not respect fundamental physc prncples (Helmotz recprocty and energy conservaton). Effectvely, there are strong non-lnear effects dues to self-shadowng (and maskng) of the materal. Ths functon has at least sx dmensons : 4D as non-spectral BRDF and 2D for spatalty nformaton. Texture resoluton 256x256 x Eye postons 81 x Lght postons 81 x Component RGB 3 = 1,3 Gb Fgure 8 : samples of measured BTF from Bonn database [BonnDB] for few angles of lghtng and vewng drecton. Due to ts huge sze (1,3 Gb n our example), a BTF cannot be mplemented drectly n real tme wth today s graphc boards. The second dffculty arses from the dmenson of ths functon: 6D textures are not avalable. In order to acheve real-tme frame rates and acceptable memory consumpton, some sort of data-compresson has to be acheved. Method should of course preserve as much as possble, the relevant features of the BTF. It should also explot the data redundancy n an effcent way and provde a real-tme decompresson stage for our applcaton. BTF: from generaton to real-tme mplementaton Synthetc BTF Generaton The acquston of BTF can be acheved by several ways. It can be measured usng an nstallaton based on CCD camera [HP03], [KMBK03]. Few measured BTF databases are avalable onlne : CUReT database [CURetDB] and Bonn Unversty database [BonnDB].The samples from CUReT database are not spatally regstered, thus ts use n our experments s tresome. 317

6 Measurement of BTF requres an expansve system and a controlled sophstcated process. In order to experment BTF, we thought of data processng sequences for generatng synthetc BTF samples. We generate good qualty mages (fgure 9), usng a ray-tracer (Povray) because t s easly customzable (anmaton scrpts are used for both camera and lght dsplacements). We have developed a tool that computes homographc transformaton n order to replace the BTF samples n the camera space (spatally regstered samples) Compresson algorthm Fgure 9 : synthetc BTF sample We have chosen to mplement an algorthm ntroduced by Suykens and al. [SvBLD03]. They provde a BTF renderng method wth hgh compresson rates at nteractve frame rates. Ths algorthm s based on the Sngular Value Decomposton (SVD) and k-mean clusterng. The SVD s used n order to represent a 4D functon (ABRDF) by a product of 2D functon (sngular vectors). Ths dea has been ntroduced n [KM99] and [MAA01]. The compresson s acheved by reducng the sze n of the matrx D (for n = 1, sze decreases from n. m to 2 n + 1). M = U. DV. T M = matrx (n x m) U = orthogonal matrx (m x n) D = dagonal matrx (n x n) V = orthogonal matrx (n x m) Results of SVD compresson are much better usng the GSHD (Gramm-Schmdt Halfway-Dfference) parameterzaton, snce t correspond to the optmal space for representng ABRDF. Although the sngular value decomposton of all ABRDFs, the BTF remans too bg. A sort of k-mean clusterng s appled to reduce datasze by gatherng smlar sngular vectors (We keep only 256 dfferent ABRDFs). An overvew of the complete process can be descrbed as follow: Apply GSHD parameterzaton on nput ARBDFo. Compute SVD, keep only one sngular vector for halfway component (H). Reconstruct the compressed ABRDFh and dvde the orgnal one (ARBDFo) by ABRDFh, you obtan the resultant ABRDFr Compute SVD on ABRDFr, keep two sngular vectors (U and V) Apply k-mean clusterng on sngular vectors, keep 256 vectors represented as parabolc maps (fgure 10) for each group U, V and H. Save an ndex map to retreve the correspondng clustered map to each texel. 318

7 Interactve renderng The Fnal representaton of materal requres only 640 Kb, correspondng to a compresson rate of 2030:1, ndependently on the materal complexty. Fgure 10 : BTF sample (mpalla from Bonn database) after compresson process (U, V, H and an ndex map). The ndexmap s a classcal 2D texture mapped on geometry as a standard texture. Each group U, V, H s stored n a 3D texture. We can rebuld the BTF very easly by multplyng components of sngular vectors (U*V*H). Therefore, ths algorthm s very sutable for real tme renderng usng a shadng language (GLSL or other). Results We have tested two natural BTF samples provded by Bonn Unversty (fgure11 and 12). We have also tested the algorthm on a synthetc BTF (fgure 13). As expected, results are good for very repettve materals (Corduroy sample). Shadows are also well vsble on squares borders on mpalla sample but the overall mage qualty s not very satsfyng. Fgure 11 : Impalla sample (from Bonn database) Fgure 12 : Corduroy sample (from Bonn database) 319

8 Fgure 13 : synthetc pavement BTF The synthetc BTF of pavement (Fgure 13) has been generated usng a ray-tracer. Specular reflectons can be seen between pavng stones that comng from shallow water. Image qualty s qute damaged compared to the orgnal sample (see Fgure 9) whch s very detaled. The renderng performances for the gven examples s n the range of about 20 to 60 frames per second, at a resoluton of All performance measurements were obtaned on a BXeon 2.6GHz usng an Nvda Quadro FX2000. Concluson and future work In ths paper, we have presented a real tme mplementaton of measured BRDF combned wth bump mappng technques (geometry nformaton). We have also presented an mage based renderng method (BTF) for realstc materal smulaton, wthout assumpton on the underlyng geometry. The presented method provdes hgh compresson rate, easy and fast reconstructon (only three texture products) whch allows real tme renderng of BTF. We have developed a method to generate synthetc BTF samples (mage computaton and homographc transformaton) and we have tested ths algorthm on both synthetc and natural BTF samples. However, ths method has a few dsadvantages. Image qualty can be poor dependng on the materal complexty. A vertex local space (tangent, bnormal, normal) must be correctly defned to compute angles for accessng the BTF, therefore, hgh qualty mesh are requred. 320

9 Ths method does not support mp-map flterng whch s very mportant for scene observed at a drver poston (grazng angles wth long egocentrc dstances). In the future, we wll make more precse compresson error assessments and comparsons wth further algorthms based on prncpal component analyss (PCA) [Jol86], or Local PCA [MMK03], [Sch04]. We wll also study new algorthms more sutable for drvng smulator (mpmapng support and random mappng to avod vsble repettve patches [TZL02]). We are also nterested by consderng human percepton of speed. Snce our dsplay system has lmtatons (spatal resoluton, temporal alasng, lumnance ), t could be very mportant to consder specfc algorthms (moton blur, tone mappng ) to generate a vrtual scene n accordance wth the most relevant human percepton propertes. References [LK99] Lecocq P., Kelada J-M., Kemeny A., Interactve Headlght Smulaton, n Proceedngs of DSC 00 Drvng Smulaton Conference, 1999, pp [DvGNK99] DANA K. J., VAN GINNEKEN B., NAYAR S. K., KOENDERINK J. J.: Reflectance and texture of real world surfaces. ACM Transactons on Graphcs 18, 1 (1999), [HP03] HAN J. Y., PERLIN K.: Measurng bdrectonal texture reflectance wth a kaledoscope. ACM Trans. Graph. 22, 3 (2003), [Jol86] JOLLIFFE I.: Prncpal Component Analyss. Sprnger-Verlag, [KM99] KAUTZ J., MCCOOL M.: Interactve Renderng wth Arbtrary BRDFs usng Separable Approxmatons. In Tenth Eurographcs Workshop on Renderng (1999), pp [KMBK03] KOUDELKA M. L., MAGDA S., BELHUMEUR P. N., KRIEGMAN D. J.: Acquston, compresson and synthess of bdrectonal texture functons. In 3rd Internatonal Workshop on Texture Analyss and Synthess (2003). [LHZ04] LIU X., HU Y., ZHANG J., TONG X., GUO B.,SHUM H.-Y.: Synthess and Renderng of Bdrectonal Texture Functons on Arbtrary Surfaces. IEEE Transactons on Vsualzaton and Computer Graphcs 3 (2004), [MAA01] MCCOOL M. D., ANG J., AHMAD A.: Homomorphc Factorzaton of BRDFs for Hgh- Performance Renderng. In Proceedngs of the 28th annual conference on Computer graphcs and nteractve technques (2001), ACM Press, pp [MMK03] MÜLLER G., MESETH J., KLEIN R.: Compresson and real-tme Renderng of Measured BTFs usng local PCA. In Vson, Modelng and Vsualsaton 2003 (November 2003), pp [MMSK04] J., MÜLLER G., SATTLER M., KLEIN R : Acquston, Synthess and Renderng of Bdrectonal Texture Functons. State of The Art Report (STAR) n Eurographcs 2004, pp [MMSK03] MESETH J., MÜLLER G., SATTLER M., KLEIN R.: Btf renderng for vrtual envronments. In Vrtual Concepts 2003 (November 2003), pp

10 [Pho75] PHONG B. T.: Illumnaton for computer generated pctures. Communcatons of the ACM 18, 6 (1975), [Sch04] SCHNEIDER M.: Real-Tme BTF Renderng. In Proceedng of CESCG 2004 (2004). [SvBLD03] SUYKENS F., VOM BERGE K., LAGAE A., DUTRÉ P.: Interactve Renderng of Bdrectonal Texture Functons. In Eurographcs 2003 (September 2003), pp [TZL02] TONG X., ZHANG J., LIU L., WANG X., GUO B., SHUM H.-Y.: Synthess of bdrectonal texture functons on arbtrary surfaces. In Proceedngs of the 29 th annual conference on Computer graphcs and nteractve technques (2002), ACM Press, pp [BonnDB] BTF DATABASE BONN: [ColDB] COLUMBIA-UTRECHT REFLECTANCE AND TEXTURE DATABASE: 322