| With the rapid development of technology,3D models are widely used in the fields of film and television animation,3D games,computer-aided design,geographic information systems,etc,bringing people more realistic effects and friendly interaction.But,the complexity of 3D models continues to increase,which not only takes up more storage space,but also consumes more computing resources.Especially in real-time rendering,in order to ensure user-friendly interaction,rendering must be completed within a certain period of time,which requires that in practical applications not only can maintain the visual effect of the 3D model,but also can improve the 3D model rendering efficiency as possible.Therefore,the research on model simplification algorithm has extremely high academic and application value.In this thesis,the existing model simplification algorithm is comprehensively studied.Based on the billboard cloud algorithm,the original algorithm is improved from three aspects:plane optimization,texture optimization and gap optimization in order to not only simplify the 3D model at the same time,but also maintain the geometric characteristics of the original 3D model to the greatest extent.(1)plane optimizationThe core of billboard cloud algorithm is to select the appropriate billboard plane.There is still room for improvement in the quality of plane selection and rendering efficiency in the original algorithm.Therefore,in order to avoid long and narrow areas or broken points in the simplified plane,the normal vector offset is introduced.In order to improve the visual effect of the simplified model,this thesis introduces a horizontal plane to avoid creating a horizontal billboard.In order to make the billboard plane in the original algorithm more representative,this thesis introduces distance weight in the selection of planes to improve the simplified quality of the billboard plane.In the final fitting plane,SVD fitting plane method is introduced to ensure the minimum distance between the effective point set selected by the algorithm and the fitting plane.At the same time,considering the efficiency of algorithm,plane polygon processing is introduced to improve the efficiency of billboard cloud rendering.(2)texture optimizationWhen simplifying the 3D model with the billboard cloud algorithm,the billboard plane loses a lot of detail,unless the selected texture resolution is quite large.So the super-sampling technique is used to improve the quality of texture map in this thesis.For a model with transparency,this thesis will use transparency order independent rendering to produce a properly mixed texture map.(3)gap optimizationThe idea of billboard cloud algorithm is to project the set of surfaces in the model that meet the error condition onto the billboard plane.If a vertex sharing multiple faces is projected to a plane,there will inevitably be gaps.When the surface is simplified,the gaps become obvious and the geometric features of the original model are severely lost.Therefore,correlation projection is introduced to reduce the number of gaps.For the easily separated surface part of the model,the method of surface simplification based on LOD will be adopted in order to preserve the geometric features of the original model to the greatest extent.The algorithm simplifies the separated 3D model by using a fixed axial billboard algorithm,imposter algorithm and an improved QEM simplification algorithm from far to near according to the distance principle.Experimental results show that the improvement scheme proposed by this thesis for the deficiency of the billboard cloud algorithm is effective.The rendering efficiency of improved algorithm is increased by 5%compared with the original billboard cloud algorithm.The simplification effect is also better than the original algorithm. |
