Interactive Comp Graph
Interactive Comp Graph CS 5610
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Marian Kertzmann DVM
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This 2 page Class Notes was uploaded by Marian Kertzmann DVM on Monday October 26, 2015. The Class Notes belongs to CS 5610 at University of Utah taught by Charles Hansen in Fall. Since its upload, it has received 37 views. For similar materials see /class/229977/cs-5610-university-of-utah in ComputerScienence at University of Utah.
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Date Created: 10/26/15
MIT Laboratory for Computer Science March 2003 417 Billboard Clouds for Extreme Model Simpli cation Xavier Decoret Fredo Durand Francois X Silliort Julie Dorsey Introduction We introduce billboard clouds a new approach for extreme simpli cation in the context of real time rendering 3D models are simpli ed onto a set of planes with texture and transparency maps We present an optimization approach to build a billboard cloud for a textured polygonal model and a given geometric error threshold We demonstrate our technique on a large class of models including smooth manifolds and composite objects a b C d Figure 1 Example of a billboard cloud a Original model 5138 polygons b falsecolor rendering using one color per billboard to show the faces that were grouped on it c View of the automatically generated 32 textured billboards d the billboards side by side ApproachWe iteratively replace large sets of faces by a textured plane The texture is generated by projecting the associated set of faces on the plane The faces and the plane are chosen so that the distance between vertices and their projection is bounded error threshold and that the projected area of faces is maximized To approximate the whole model with a minimum number of planes we uses a greedy approach that iteratively chooses planes that approximate great quantities of faces which we call dense planes Density is evaluated in plane space using a discretization and adaptative re nement for ef ciency A good surface approximation is ensured by favoring planes that are nearly tangent to the model This method does not require connectivity information but still avoids cracks by projecting primitives onto multiple planes when needed To avoid large textures with a lot of empty space which can occur when distant parts of the model are simpli ed on the same plane our algorithm takes into account the compacity of the generated textures when selecting set of faces to be simpli ed For extreme simpli cation our approach combines the strengths of mesh decimation and image based impostors Billboard clouds is a new representation that can be used not only for rendering but also quick shadow display or fast ray object intersection as shown on Figure 2 d Figure 2 Billboard clouds encode an object shape and appearance through a few quads and transparent textures This representation can be used for fast rendering of complex objects a for shadow computations b or ray object intersection c Storing additional information such as normal map in the textures also allow real time relighting for improved realism d ProgresszThe simpli cation of complex object with billboard clouds gives good results The shape and details can be captured with a surprisingly low number of billboards But this low number of billboards can not be straightforwardly compared to the number of faces in a mesh simpli cation The rendering of a billboard cloud is penalized mainly by the ll rate when 100 interleaved billboards are rendered lots of pixels are drawn many times the worst case being parallel billboards viewed from front Therefore a billboard clouds is useful when its projected screen size is 418 MIT Laboratory for Computer Science March 2003 not too large When an object is far away using a billboard cloud simpli cation gives higher quality results than a geometrically simpli ed version of the objectl of same rendering cost FuturezFor billboard clouds to be interesting when viewed from closer range we have to diminish the number of billboards Unfortunately if we want the billboard cloud to be viewable from any angle this number of billboards cannot be lowered too much The extrem case of a single billboard would obviously look at from many viewpoints Therefore we plan to investigate object based view dependent billboards clouds The space around an object is divided into regions as indicated on Figure 3 A billboard cloud is constructed for each of these regions or viewcells By limit ing the view angle within a region we expect to get a low billboard count for each cloud Moreover the distance range delimited by each viewcell also allows to optimize the resolution of billboards textures Therefore the extra storage caused by multiple view dependent clouds versus a single view independent one should not be too high compared to the bene ts Preliminary results seems promising but there is still a lot to investigate and notably the transitions object based viewcells close range middle range far range Figure 3 Examples of viewdependent billboards A billboard cloud is build for each viewcell Depending of the distance to the object the algorithm detects that more and more billboards are required to represent the shape and appearance of a tree model 20 000 polygons Note also that the further the billboard cloud is the lower the resolution of the texture is Research Support This research is supported by funding from the NTT Corporation References l M Garland and P Heckbert Surface simpli cation using quadric error metrics Proceedings of SIGGRAPH 97 1997 2 X D coret Preprocessing of large databases for interactive visualisation PhD thesis Universit Joseph Four rier Grenoble october 2002 3 X D coret F Sillion F Durand and J Dorsey Billboard clouds Tech Rep RR 4485 INRIA Grenoble June 2002
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