A Material Measuring System Based On Hemispherical Lighting

Nowadays,with the development of industrial applications such as AR / VR,the photorealistic visual effects in computer graphics are increasingly demanded,Photorealistic Rendering develops rapidly and has great potential.Photorealistic Rendering is to generate graphics that simulate the material surface of real objects by computer.It's an important issue in computer graphics research currently,and it includes two critical elements: the texture of the material surface and the illumination model based on physical analysis.Material surface data is the basis of Photorealistic Rendering,normally it can be described by Bidirectional Reflection Distribution Function(BRDF)and its derivative forms,Spatially Varying Bidirectional Reflection Distribution Function(SVBRDF)and Bidirectional Texture Function(BTF).In order to acquire and measure material data,following research work has been carried out in this thesis:1.In order to acquire the appearance images of materials,a system for material acquisition and measurement based on hemispherical omnidirectional lighting was developed.The main body of the system is a hemispherical structure with latitude and longitude grids.The light sources distributes uniformly in all directions.The camera array acquires material appearance images in parallel.The camera,light source and material sample provide a total of 4 degrees of freedom.The system considers three important aspects about cost,speed and accuracy,and improves the deficiencies of traditional methods.2.In order to measure and calculate the BRDF and SVBRDF / BTF data of material accurately,the adjustment and calibration methods of the system was provided,including physical adjustment of light source and camera position,and optical calibration of camera parameters and lighting condition.And the image acquisition method,image processing algorithm and material data generation algorithm corresponding to the acquisition type was proposed.According to above process,a common material database was initially established.3.In order to verify the measurement data of the system,a test method including fit and rendering for BRDF data was given.Based on subjective data evaluation,the Levenberg-Marquardt based optimization method is used to perform fitting on Cook-Torrance model.And the fitted parameter is used for rendering.The result is basically consistent with the visual effects of real materials,which proves that the data measured by the system has certain authenticity and reliability.