Integration of Voxel Colouring Technique in the Volumetric Textures Representation Based on Image Layers

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1 Journal of Computer Scence 2 (7) : 6-66, 26 ISSN Scence Publcatons Integraton of Voxel Colourng Technque n the Volumetrc Textures Representaton Based on Image Layers 1 Babahenn Mohamed Chaouk, 2 Hemd Djallel, 2 Djed Noureddne 1 Insttut de Recherche en Informatque de Toulouse. Equpe SIRV. Unversté Paul Sabater. Toulouse France 2 Department of Computer Scence, Mohamed Khder Unversty, Bskra, Algera Abstract: Ths paper presents a method for ntegratng a technque of reconstructon scene (voxel colourng) startng from mages of the reference element of a volumetrc texture, ths one wll be converted n a second phase nto a whole of layers (2D mages consdered as transparent textures), whch wll be projected and composed successvely on surface defned as volumetrc grd usng the Z- buffer algorthm. The model suggested allows prmarly made realstc of repettve complex scenes lower cost of calculaton due to the effectve explotaton of the capactes of the graphcs boards and to the fact that t takes account of the level of detal accordng to the dstance of the observer and the vson angle, n the representaton of the reference element. Key words: Real tme, Image layers, Level of detals, Textures volumetrc, Voxel colorng INTRODUCTION Volumetrc textures offer realstc renderng of the repettve complex scenes, n acceptable tmes (from 5 to 2 mnutes) by usng the ray castng. However, for applcatons requrng the real tme, obtanng an addtonal proft appears not easly possble by preservng ths technque n ts ntal form (representaton of the reference volume by an octal tree). Among the approaches of mage based renderng, dealng wth the problems of geometrcal and temporal complexty, exst the renderng based on mages layers. Ths technque allows realstc renderng of scenes and objects n lower calculaton cost than wth a tradtonal method. Ths beneft of calculaton tme s due to the explotaton of capactes of the graphcs boards treatng, effectvely, the polygons. So each layer of mage must be represented by a textured polygon. For renderng, t s enough to project and successvely compose the layers on the mage plan (Z- Buffer) and to obtan, thus, the fnal mage. Each projected layer s combned wth the precedng result by takng account of the densty of each pxel. The projecton of a layer s faster than calculatons of projecton for each voxel along a ray, whch allows an apprecable beneft of tme. Ths paper present a method for ntegratng the technque of colourng voxel n volumetrc textures contanng layers of mages [1], whch wll make t possble to generalze the reference volume whch wll be ntally defnte from mages key (real photographs or syntheszed mages), transform t nto a whole of layers and to adopt the method of Meyer [2] to carry out one made real tme by explotng the graphcs boards capactes. Prevous work: Intal representaton: In 1989 Kajya and Kay [3] presented the volumetrc textures technque appled to fur renderng, whch consttutes an effcent way of representng and renderng complex repettve data. The prncple conssts n makng a sample of volumetrc texture; whch s stored n a volume of reference, whose deformed copes, called texels, wll be mapped on a surface made up of blnear patches. The reference volume represents area of space sampled onto voxels. Each voxel contans nformaton space of presence (densty) and functon ndcatng the local photometrc behavour, whch conssts of a local orentaton and an analytcal functon of reflectance. The volume of reference s then plated n a repettve way on all surface as for a tradtonal surface texture, t s then deformed n order to ensure contnuty between the close texels. The renderng s done by volumetrc ray tracng when a texel s crossed. A stochastc samplng of the ray s carred out n order to reduce calculatons; the local llumnaton s then carred out usng the model of reflectance whch smulates the presence of a cylnder. A ray s sent towards the source of lght n order to test the shade of the current voxel. In [3] the authors wanted to represent a partcular texture, the fur on a bear. The texel and the reflectance functon whch they used are very specfc. In order to Correspondng Author : Babahenn Mohamed Chaouk, IRIT, équpe SIRV, Unversté Paul Sabater, 118 Route de Narbonne, F-3162 Toulouse Cedex 9, France 6

2 carry out an effectve representaton of the complex geometres by generalzng the precedng method and by ntroducng the prncple of level of detal nto the representaton of the texel. F. Neyret [4] proposed the use of the octree to store reference volume. Any voxel n the octree smulates the photometrc local behavour of the object represented n the texel, and contans the local densty and a mcro-prmtve modellng the local reflectance. Volumetrc texture renderng s then modfed accordng to the multscale data structure and to the reflectance. So t s encodng n two passes one made total whch s done on the level of each texel by usng a tradtonal algorthm of cone tracer. Ths algorthm course respectvely space scene, space texel then space object and uses a trlnear nterpolaton and one made local whch s done on the level of each voxel by ntegratng the model of local llumnaton of Phong. J. Computer Sc., 2 (7) : 6-66, 26 the user specfed amount. Ths mpostor s composed of multple layers of textured meshes, whch replace the dstant geometry and s much faster to render. It captures n the model the complexty of sutable depth wthout resortng to a complete of the scene. The layers can be updated dynamcally durng vsualzaton. A very nterestng adaptaton of volumetrc textures contanng mage layers was developed by Lengyel [8], for real tme renderng of fur. As a pre-process, he smulates vrtual har wth a partcle system, and smplfes t nto a volume texture. Next, he parameterzes the texture over a surface of arbtrary topology usng lapped textures whch s an approach for applyng a texture sample to a surface by repeatedly pastng patches of the texture untl the surface s covered. At runtme, the patches of volume textures are rendered as a seres of concentrc shells of semtransparent medum. The method generates convncng magery of fur at nteractve rates for models of moderate complexty. Models contanng bllboard [9] allow a representaton of the objects contanng sem- Representaton based on mage layers: To mnmze the tme of renderng, Lacroute and Levoy [5] ntroduced volumetrc renderng by mage layers where the volumetrc data are nterpreted as layers and the voxels transparent textures, the renderng engne always drects are factorzed n texture and then treated n parallel. The towards the observer. Jakuln [1] proposes a hybrd algorthms rely on a factorzaton of the vewng model between the texels. He uses ordnary mesh-based transformaton that smplfes projecton from the renderng for the sold parts of a tree, ts trunk and volume to the mage, whch allow constructng an lmbs, the sparse parts of a tree, ts twgs and leaves, are object-order algorthm wth the same advantages as an nstead represented wth a set of slces, an mage based mage-order algorthm. They call the factorzaton the representaton. For renderng, t blend between the shear-warp factorzaton. The prncpal advantage s the nearest two slcng accordng to the angle between the benefts of tme because the projecton of a layer s vewpont and the normal wth the slce. faster than the calculaton of projecton for each voxel Wth the advent of rsng generaton of the graphcs along a ray. Westermann and Ertl n 1998 [6] presented a hardware, Sénégas [11] could develop a new verson of model ncludng dffuse lghtng n the medcal mages; volumetrc textures n real tme by addng a calculaton they used a sngle texture of NxXxY resoluton. For of llumnaton to the texels. These graphcs hardware renderng they poston and turn correctly the cube n make possble to evaluate llumnaton n each pxel of the scene and then determne the polygons formng the the polygon and ether at the tops of the grds. Then ntersecton of parallel plans to the mage plan wth the these values wll be nterpolated durng the process of cube n the scene. To smulate the dffuse shadng, they rastersaton whch conssts n fllng the 2D polygon ntroduced another 3D texture whose color nformaton pxel by pxel. A layer of a texel s then made up of two corresponds to the normal n each pont of volumetrc secton: texture. 1. a secton codng color (RGB) and the densty To carry out volumetrc textures n real tme, (channel alpha). Meyer and Neyret [2] tred to beneft from the capacty of 2. a secton codng the normals. the graphcs hardware to treat textured polygons quckly. In ths context, a cubc volume s represented Representaton based ponts: M.Levoy and T.Whtted by a seres of slces covered wth a transparent texture. ntroduce n 1985, a new hybrd representaton located A 3D object s thus translated nto sectons. As the between the mage representaton and polygonal slces are not front the observer and do not have a representatons [12] ; t used ponts lke sngle prmtve thckness, one s lkely to see between them, thus they of renderng. In 1998, J.P. Grossman and Dally [13] took have defned three drectons of slces and used one up ths dea, the objects are represented as a dense set of among t accordng to the pont of vew of the user. The surface pont samples whch contan colour, depth and generaton of the texels can be done n two manners, by normal nformaton. These pont samples are obtaned convertng a polygonal representaton or startng from a by samplng orthographc vews on an equlateral Perln texture. trangle lattce. They are rendered drectly and Schaufler [7], ntroduced a new type of mpostor, ndependently wthout any knowledge of surface whch has the property to lmt the errors of occluson to topology. Pfster and al [14] defned the surfel as an n- 61

3 J. Computer Sc., 2 (7) : 6-66, 26 tuple of dmenson wth attrbutes of form and of texture color whch locally approxmates the surface of an object. The surfels can be consdered as an object wth out explct connectvty wth ther neghbours, whch s n fact a very flexble representaton to handle. As a pre-process, an octree-based surfel representaton of a geometrc object s computed. Durng samplng, surfel postons and normals are optonally perturbed and dfferent levels of texture colours are prefltered and stored n every surfel. Durng renderng, a herarchcal forward warpng algorthm projects surfels to a z-buffer. The dsplay cost of an object s proportonal to ts sze wth the screen and not to ts ntrnsc complexty. The reconstructon of surface can be made ether n space mage [15], by takng account of transparent sem surfaces (use of an A-buffer), or n space object [16] by usng an acceleraton by the graphc materal. To deal wth the problem of the alasng for the objects havng a complex texture, one uses a flter by ellptc average weghtng (EWA Flter, '' Ellptcal Weghted Average '') whch gves very good vsual results but have heavy cost n computng tmes [17]. Varous accelerated methods of ths flter were presented [18, 19], but ther performances are not suffcent. In 22 [2], Guannabeau, adopted a technque for vsualzaton of volumetrc textures contanng surfels where the reference element s represented usng a LDC tree. Motvatons: The motvaton of our representaton s: - Encodng of any knd of shape ntally represented by a seres of real photographs or syntheszed mages usng voxel colourng technque n volume pattern. - Provdng mult-scale representaton by convertng obtaned volumes to mage layers wtch wll be projected and decomposed successvely on surface support defned as volumetrc grd. - Usng capabltes of graphcs hardware, wtch makes volume renderng nteractve. Integraton of voxel colorng technque n the representaton of volumetrc textures based on mages layers: Modellng of the reference element by the voxel colourng technque: Voxel colourng technque was ntroduced by Setz [21], t can consdered as a voxelbased 3D reconstructon technque to compute photo realstc volume models from multple color mages V,, V n. It conssts n rebuldng a scene 3D by assgnng colors (radances) to voxels (ponts) n a 3D volume and by guaranteeng consstent wth a whole of basc mages (Fg. 1). [21]. Fg. 1: The voxel colourng; The ntal stage of the algorthm conssts n breakng up the scene nto a set of voxels. For that, we defne a subdvson of space 3D n layers of voxels of unform dstance at the camera: { V V d} v d = = ν = r =1 v d Where d 1,, d r s a sequence of ncreasng numbers and. s an occluson compatble functon. Voxel colourng algorthm s presented thus [21] : S φ For =1,, r do //Iterate through the layers For each V ν do d //Iterate through the voxels n the layer For j=,, n do //Project the voxel on each mage Compute footprnt ρ of V n I j π j { p ρ p s not marked} Compute λ V //Evaluate the voxel consstency If m> et λ V <thresh then S S { V } //color the voxel π π π //Memorze mage pxel to mark =1 End If Mark the pxel n π The threshold thresh corresponds to the maxmum allowable correlaton error. A low value of the threshold makes t possble to have a precse reconstructon but ncomplete, on the other hand, wth a larger threshold we obtan a complete reconstructon, havng some false voxels. 62

4 J. Computer Sc., 2 (7) : 6-66, 26 To be able to reconstruct the 3D scene, we used a whole of syntheszed mages, generated by usng a modfed ray castng. After havng computed a whole of mages of depth around the objects of the scene, we can rebuld ths scene by usng these mages accompaned by other nformaton such as: the poston of the camera, focal angles of vson (vertcal and horzontal) and the dmenson of each mage. The process of reconstructon s an opposte projecton of the whole of pxels of each mage towards the space scene dscretzed n voxels. Ths operaton makes t possble to color the transparent voxels and to have thus the same scene, represented ths tme by a seres of voxels. Ths process (Fg. 2) s carred out n the followng way: Intally, we construct the grd 3D of the transparent voxels (the value of alpha s ). We use nformaton accompanyng the camera to compute the matrx M of change scale between the object space and the camera space. M = RX RY T (where: RX and RY are the two matrces of rotaton to brng back the vector of vson on OZ axs, and T s the translaton brngng back the camera to the centre of the scene. Once the drecton of the ray obtaned, we compute the poston of the pxel projected n space scene by usng the nformaton of depth accompanyng ths pxel: P = O + λ D. Where: Fg. 3: The passage between a voxel representaton and mage layers representaton. Renderng by usng the new model of volumetrc texture: The surface can be represented by a whole of boxes bult by usng a surface grd. Each box s thus defned by 2N ponts [2]: N ponts for the base and N ponts for the top (Fg. 4). To ndcate how the box n the texel s located, t s necessary to add to the base ponts the textures coordnates U, V. For the creaton of a surface grd, we used snusodal functons (basc ponts), whch one wll be able to be perturbed by textures of Perln (Perln nose) n order to create grounds. O s the ray orgn, D s the normalzed vector of the ray drecton and λ corresponds to the depth of the pont. P. Then we compute the poston of the correspondng voxel n the grd and we assgn the color of the pxel to the found voxel, and the value alpha of the voxel s 255. Fg. 4: Mappng texel on a surface [2]. The volume of reference s represented by a whole of N 2D mages of sze X Y to format RGBA (Red, Green, Bleu and the Alpha transparency). One can also see t lke a volume of N X Y voxels havng each one four components (three of colors and transparency) [2]. Ths volume of reference does not contan any more nformaton on reflectance snce the precalculated color s stored n the place, whch mples that the render of shades and lghtng s fxed durng ts constructon. Renderng of a scene passes by renderng of all the boxes comprsng a whole of textured transparent polygons (the mages layers). From the nformaton contaned n a box, we obtan the coordnates (n space scene X, Y, Z and n space textures U, V) of textured faces whch we dsplay wth OpenGL [2]. A box s defned by ts ponts of the base and the top. The calculaton of the coordnates of the varous sectons s done by a lnear nterpolaton between a pont of the base and a pont of the top. The face coordnates are: Fg. 2: Reconstructon from the mages (the drecton of the arrow ndcates the drecton of projecton) (a) Creaton of the mages (b) Opposte projecton of the pxels. Once the creaton of the reference volume by the technque of voxels colourng made, we can use the whole of voxels coloured for the generaton of our volume of reference contanng layers. The process of passage between these two representatons s llustrated n Fg. 3..A A n n 1 B = B +. B B n n 1 C = C +.C C n n 1 A = A 63 +

5 J. Computer Sc., 2 (7) : 6-66, 26 Where A, B, C are coordnates of bottom layer, A n-1, B n-1, C n-1 are coordnates of top layer, and n s the number of layers. The coordnates of the ponts of these faces are recomputed only when the box becomes deformed. The technque of the Z-buffer mposes that the dsplay of transparent faces s done by order of depth. When OpenGL dsplays a new polygon, t treats all the pxels of ths polygon lke ths [2] : If (the depth of the pont s further away from the eye than that of the Z-buffer) and (the pont of the Z-buffer belongs to a completely opaque polygon) Then The pont s rejected, else we dsplay t by applyng the followng formula: C screen = Alpha pont C pont +(1 - Alpha pont ) C screen If the pont belonged to an opaque sem polygon, t s necessary to make a composton of the colors [2] by dsplayng the faces of the back forwards by respectng the order of depth accordng to the pont of vew. Fg. 5: Order of the faces dsplay: Case A of wth n-1. Case B of n-1 wth. The method of renderng all textured surfaces conssts n takng each face of surface and dsplays the box beng wth the top. The algorthm used s as follows: For = all boxes do For n = all sectons box do load texture n dsplay secton n of box The loadng of a texture (layer) n the assocated memory corresponds to change a context (the use of the functon glbndtexture to allot an dentfer to texture and glteximage 2D to load texture [22] ). Fg. 6: A texel wth ts 3 drectons of faces. To know whch drecton s the best, one carres out the three scalar products between the axs of sght V and the three drectons of the texel NR, T, B (Normal, Tangent, B-normal), and one chooses the drecton whose scalar product s largest n absolute value. Introducton of the mult-resoluton aspect: The ntroducton of the mult-resoluton aspect conssts n reducng the number of dsplay layers whle keepng a constant vsual qualty, the objectve beng to fnd rght balance between the realsm and the computng speed. Thus, we seek to descrbe the mportance of an object n a scene n order to choose the smoothness of hs representaton. - When the texel s seen perfectly n the axs or when the pont of vew devates a lttle from the drecton of vson, the effect of relef s not mportant. We could thus dsplay only one lmted number of textured faces whch would be the precalculated result of the stackng of all the textured faces [2]. The dea s to dsplay a varable number of layers whch depends on the angle of vew, and more precsely of the dstance d (see Fg. 7) whch corresponds to the vsble depth nsde the object. The qualty crteron C1 s defned by rasng the dstance d by d, whch gves: H tan( a) < d, t s n deduced that H tan( a) < n. Where n s the mnmum d number of sectons whch should be used to keep a qualty connect constant. Fg. 7: The frst crteron. Fg. 8: The second crteron. When the angle of sght of the texel s very acute t s seen clearly that surface s a superposton of faces. To solve ths problem, we chose to ntroduce two other drectons of alternate sectons, to use when the frst gves a too degenerated qualty. We choose the most adapted drecton to the poston from the pont of vew (see an example of swng n case A of Fg. 5 on the rght) [2] The C 2 crteron conssts n defnng the spacng between two layers of a texel (.e. we wants that the dstance H projected n the screen s lmted by d 1 (n pxels)). The dstance h projected n the screen s l (see fg. 8). We want that l<d 1, what gves: F H sn( a) < n Z d1 Ths crteron takes account of the perspectve; therefore a texel beng far from the pont of vew wll

6 J. Computer Sc., 2 (7) : 6-66, 26 have a mnmum number of layers very low. Ths crteron also takes account of the orentaton of the texel compared to the axs of slce. The mnmal number of layers to be dsplay when the two crtera are used s the maxmum of the two found values (.e.: max (C 1, C 2 )). To dsplay the texel result wth the level of detal calculated accordng to the precedng crteron, we precalculates ntermedate mages where each one s the combnaton of the two precedng ones n the followng way: From the N = 2 k mages (N beng power of 2 mmedately hgher to n), we buld N 1 = 2 k by 2 combnng them two by two accordng to the prncple of the compostng [23], and so on untl obtanng only one last mage, whch wll be dsplayed accordng to the poston and the orentaton from the pont of vew. (a) (b) Fg. 1: Reference volumes obtaned by usng the voxels colorng technque. Fg. 11: Reference volumes (grass and tree) obtaned from trangular meshes N =2 k mages 2 k mages last mage Fg. 9: layer mages compostng RESULTS AND DISCUSSION All the computatons are done on a P4 at 3. GHz wth a GeForce FX 52 graphc card wth 128 Mb of memory. Fg. 1 represents two reference volumes generated by voxels colourng technque whch uses the nformaton of depth accompanyng each pxel by an mage source, once volumes obtaned they are translated nto a whole of layers. The mage (fg. 1a) s regenerated by usng sx mages of depth whch resoluton s 256x256 pxels. In ths case, there are coloured voxels between voxels (64 3 ) for a computng tme 3,52 seconds. The mages (fg. 1b) are obtaned n the same way wth 12 coloured key mages and voxels, and n fgure 11 we have two other volumes of reference constructed by usng a voxels colourng startng from a trangular grd (or cloud of ponts). Fg. 12 shows a whole of vews taken from dfferent ponts of vson. It represents complex terran made up of 1. patches bult from a Perln texture equpped by trees. The frame rate s 2 to 3 fps dependng on the vew and the tunng of parameters. The frame rate s nversely proportonal to resoluton, and transtons between LODs of the same type of slces are qute unnotceable. Renderng s carred out n an nteractve tme when the dsplayng frequency vares between 9 and 21 F.P.S and the reference volume resoluton s Fg. 12 : Dfferent vew poston of a terran of 1. patches equpped wth trees 65

7 CONCLUSION AND FUTURE WORK We have descrbed a multscale scheme for realtme volumetrc textures by ntegratng new way to represent reference element: the voxel colourng technque, whch permts to generalze the use of the texel to objects defned only by key mages (real photographs or mages obtaned by a synthess process). To map texel over surface support, we use technque based on mage layers [2] that was partcularly effectve and s well adapted to vsualzaton of volumetrc textures usng graphcs board. We have shown that our results are convncng: we can move nteractvely above a scene whch shows a contnuous range reference element. However, t stll remans of certan ponts to mprove: - The use of three drectons of layers extends three tmes the memory capacty used to store the reference volume, what results n an ncrease n the space occuped n the memory textures. - Generalzaton of the scene to be equpped wth other types of surface and ntroducton of 3D textures. [24] - Generatng local llumnaton on texel by ntegratng mage based renderng technques usng graphcs capabltes. - Anmaton of volumetrc textures. REFERENCES 1. Decaudn, P. and Neyret, F., 24. Renderng Forest Scenes n Real-Tme. In Eurographcs Symposum on Renderng. 2. Meyer, A. and Neyret, F., Interactve volumetrc textures. In Eurographcs Renderng Workshop, pp: Kajya, J. and Kay, T., Renderng fur wth three dmensonal textures. Proceedngs of SIGGRAPH 89, vol. 23, N 3, pp: Neyret, F., 1998: Modelng anmatng and renderng complex scenes usng volumetrc textures. IEEE Transactons on Vsualzaton and Computer Graphcs, vol. 4, N Lacroute, P. and Levoy, M., Fast volume renderng usng a shear warp factorzaton of the vewng transformaton. Proceedngs of SIGGRAPH 94, pp: Westermann, R. and Ertl, T., Effcently usng graphcs hardware n volume renderng applcatons. Proceedngs of ACM SIGGRAPH, pp: Schaufler, G., Image-based object representaton by layered mpostors. ACM Symposum on Vrtual Realty Software and Technology 98, pp: Lengyel, J., Praun, E., Fnkelsten, A. and Hugues, H., 21. Real-Tme Fur over Arbtrary Surfaces. Symposum on Interactve 3D Graphcs, pp: J. Computer Sc., 2 (7) : 6-66, Decoret, X., Durand, F., Sllon, F. X., and Dorsey J., 23. Bllboard clouds for extreme model smplfcaton. Proceedngs of ACMSIGGRAPH, ACM Press, vol. 22, N 3, pp: Jakuln, A., 2. Interactve Vegetaton Renderng wth Slcng and Blendng. Proc. eurographcs (Short Presentatons) 11. Sénégas, F., 21. Rendu de forêts en temps-réel à base de représentatons alternatves. Rapport de DEA IVR, INPG. 12. Levoy, M. and Whtted, T., The use of ponts as a dsplay prmtve. TR CS Department, Unversty of North Carolna. 13. Grossman, J. P. and Dally, W. J., Pont sample renderng. EG workshop on renderng, Sprnger-Verlag. pp: Pfster, H., Zwcker, M., van Baar, J., and Gross, M., 2. Surfels: Surface elements as renderng prmtves. Proc. ACM SIGGRAPH. pp: Gross, M., Pfster, H., Zwcker, M., Pauly, M., Stammnger, M., and Alexa, M., 22. Pont based computer graphcs. In Tutoral T6, Eurographcs. 16. Rusnkewcz, S. and Levoy, M., 2. Qsplat : A multresoluton pont renderng system for large meshes. Proceedngs of conference on Computer graphcs and nteractve technques. pp: Zwcker, M., Gross, M., Pfster, H., and van Baar, J., 21. Surface splattng. Proceedngs of the 28th annual conference on Computer graphcs and nteractve technques. pp: Guennebaud, G. and Pauln, M., 23. Effcent screen space approach for Hardware Accelerated Surfel Renderng. In Vson, Modelng and Vsualzaton, Munch, pp: Ren, L., Pfster, H., and Zwcker, M. 22. Object space ewa surface splattng: A hardware accelerated approach to hgh qualty pont renderng. Eurographcs, vol 21, N 3 pp: Guennebaud, G., Pauln, M. 22. Vsualsaton par surfels des textures volumques. AFIG 22, pp: Setz, S. M. and Dyer, C. R., Photorealstc Scene Reconstructon by Voxel Colorng, Proc. Computer Vson and Pattern Recognton Conf., pp: Mller, N., 2. OpenGL Texture Mappng : An Introducton, grammng/features/ogltm/. 23. Porter, T. and Duff, T., Compostng Dgtal Images Computer Graphcs Volume 18, N 3, pp: Lefebvre, S., Hornus, S. and Neyret, F., 25. Texture Sprtes: Texture Elements Splatted on Surfaces I3D - ACM SIGGRAPH Symposum on Interactve 3D Graphcs and Games. pp: