Computers&Graphics29(2005)203–208 Apolar-plane-basedmethodfornatural illuminationofplantsandtrees Marı´aJ.Vicent,VicenteRosell,RobertoVivo´ Ã Dpto. SIC, Universidad Polite´cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain Abstract We introduce an illumination method for outdoor scenes containing plants and trees. Our method supports multiresolutionplantandtreemodelsbasedonrandomL-systems.Themethodincludesmodelingandrenderingofthe skyatdifferenttimesfordifferentlocationsontheearth.Weproposeanefficientalgorithmtoilluminatetheleavesofa treeusingformfactorspre-computedwithapolar-plane-basedmethod.Wealsopresenttwosolutionstothevisibility problem,aheuristicsolutionandasolutionintegratedwiththepolar-planemethod. r 2005ElsevierLtd.Allrightsreserved. Keywords: Naturallighting;Formfactors;Plantandtreerendering 1. Introduction Simulatinglightinteractionwithplantssupportsboth generatingtheirappearanceattributes,likecolor,bright- nessandtransparency,andsimulatingtheirgrowthand their interaction with the environment. Rendering realistic outdoor images with plants and trees requires using lighting models that take into account natural light sources, like the sky and the sun. Also, the complexity of plant and tree models prevents the use oftraditionalglobalilluminationalgorithms. Most plant and tree rendering algorithms are based ontexturemapping.Jakulin [1] proposesaninteractive tree rendering algorithm that defines boxes or slicings thatcontainthetree’sbranchesandleaves.Slicingsare defined for certain directions parallel to the ground. They are divided into slices texture-mapped with those branches and leaves that are closest to each slice. To render the slices from an arbitrary direction, Jakulin blends the two slicings closest to the desired view. Texturemapsarerenderedusingray-tracingsoftware. ThemethodproposedbyMeyeretal. [2] generatesa hierarchy of bidirectional textures (HBTs). For each pair illumination direction-viewing direction, the meth- odpre-computesaself-orientedimpostor.Atrendering time, a new image is rendered by interpolating the impostors closest to the desired pair illumination direction-viewingdirection. Other plant and tree rendering algorithms use local illuminationmodels.Forexample,Qinetal. [3] propose an algorithm that uses a set of 2D buffers containing surfaceshadinginformation. Complex radiosity algorithms can also be used for plant and tree rendering. Sillion [4], for instance, uses hierarchical radiosity with grouping. Max et al. [5] employaradiancetransportmethod.Theirassumption is that radiance depends only on the direction of the light flow and the height with respect to the ground. This makes the method applicable to dense vege- tation, instead of single isolated trees. Hondermarck and Chelle [6] use a nested radiosity method that distinguishesbetweencloseandfarradiosity,depending ARTICLE IN PRESS www.elsevier.com/locate/cag 0097-8493/$-seefrontmatter r 2005ElsevierLtd.Allrightsreserved. doi:10.1016/j.cag.2004.12.005 Ã Correspondingauthor.Tel.:+34963877795; fax:+34963877359. E-mail address: rvivo@dsic.upv.es(R.Vivo´).