Research On The Two-Layer Microfacet Model With Diffraction

Rendering is an important branch of computer graphic(CG),and the main content of the research is how to calculate and generate a 2D picture from a 3D scene.Rendering is widely used in many industries:animation,games,education,astronomy,aerospace,3D printing,and so on.The metrics that measure the pros and cons of rendering algorithms include accuracy and speed.Optics belongs to physics and physically based rendering follows the law of light propagation and reflection,like energy conservation,energy non-negative,light transmission and scattering;thus,the images rendered based on physically based rendering are more realistic and higher quality than the traditional rendering models.One research component of the physically based rendering is the bidirectional scattering distribution function(BSDF)including bidirectional reflection distribution function(BRDF)and bidirectional transmittance distribution function(BTDF).BRDF describes how light reflected at a surface and BTDF describes how light transmitted into the lower layer and back to the top layer.The materials are mainly divided into conductors and dielectrics,whereas the dielectrics are divided into plastics and subsurface scattering materials.The conductors are not transparent;thus,they do not have transmittance component.The subsurface scattering materials have subsurface scattering component and the plastics have transmittance component.We mainly discuss BSDF because our model is based on the plastic materials.The rendering experiments of this paper are all based on the physically based rendering system(PBRT)with C/C+H-programming language.The bidirectional scattering distribution function describes the energy distribution when light scatters from surfaces.The main parameters are incident direction and outgoing direction vectors.The BSDF value mainly represents the ratio of outgoing radiance to incident irradiance.The main coefficients of the function contain the transmittance ratio and the coefficients of the normal distribution function(NDF).Coefficient oriented models are widely used in rendering.The most widely used microfacet model is the Cook-Torrance(CT)model,which is relatively good to fit the measured materials,but it ignores the diffraction effect.The most important function is normal distribution function(NDF).The Cook-Torrance diffraction(CTD)model computes the specular diffraction effects but fails to fit the diffuse layer diffraction effects.There is a two-layer structure on the plastic materials.The upper layer is the Cook-Torrance microfacet model and the lower layer is diffuse layer.Our two-layer diffraction model focuses on the implementation of diffuse layer diffraction effects.We take the Oren-Nayar(ON)model as the lower-layer BSDF of the plastic materials.Based on the CTD model,we make a further research for making up the CTD model on plastic materials.We name the proposed model as the Oren-Nayar diffraction(OND)model.We take 100 typical materials in MERL as ground truth to validate the OND model.One hundred materials in MERL are with high angular resolution,including 9 conductors,6 subsurface scattering materials and 85 plastic materials.Furthermore,we make a comparnson between the CTD and proposed OND models.We divide the plastic materials into three categories.Among the third-class plastic materials,the proposed OND model is obviously better than the CTD model.Ultimately,we make a conclusion and outlook.The main contrnbutions are as follows:1.We present a two-layer model integrating specular and diffuse microfacet models.Based on the CTD model,we add two coefficients:a and a sd to represent the a m-level and nm-level roughness values respectively.The proposed OND model provides a better approximation to the plastic materials.2.Three techniques are applied in our experiments:convolution computation,Gauss-Newton method,and precomputation.The three techniques make the proposed model more accurate and higher rendering efficiency.