Research On The Fast Calculation Of Hologram For Natural Light Field

Holographic display can reconstruct the whole optical wave field of a three-dimensional(3D)scene and has the potential to provide all the depth cues that human eyes can perceive.Hence holographic display can be considered as a promising candidate for future 3D display technologies.With the development of computer technology and liquid crystal display(LCD)technology,we can get rid of complicated interference recording procedure,namely eliminate the necessity of a coherent optic system and recording films,but use computer-generated holograms(CGHs)to reconstruct 3D images for both existing and synthetic objects as long as the mathematical descriptions of the 3D scenes are provided.There were three category methods developed for calculating CGH numerically,which are point source algorithm,polygon-based method and stereoscopic hologram algorithm respectively.For point source algorithm,the 3D object was divided into multiple point sources,then the optical transmission process from the 3D scene to the hologram plane was numerically simulated.Finally the whole light field of the 3D scene was calculated by superimposing the light field of each point source.This method can optically reconstruct continuous motion parallax and accurate depth information.As we know,the more number of point sources,the more accurate details of 3D scenes can be described.However,for complex 3D object,the huge number of point sources significantly increased the amount of computation,so that the calculation efficiency was very low.In addition,it is hard to express some view-dependent properties of the 3D scenes,such as shading and occlusion.To deal with the occlusion problem,several modified point source algorithms has been developed.Hidden surface removal method was designed to solve the absence of the hidden surface's data.However,it brings in the problem of distortions.An inverse orthographic projection technique was proposed to avoid the geometric distortions,but it limits the depth ranges of the reconstructed scenes.Similar with the point source algorithm,3D objects can also be composed of multiple planar segments.We can calculate a rotary transform for each planar segment,then the diffraction waves from inclined planes were superimposed to form the light field distribution on the holographic plane.But complex 3D object would increase the computational complexity,thus the real-time calculation is difficult to achieve.Moreover,the shading and occlusion effect also remains a problem for this method.Recently,ray casting and ray tracing techniques were introduced to provide occlusion,refractive and reflective effects.However,complex ray-object processes would slow down the calculation speed.This thesis focuses on the stereoscopic hologram algorithm.Firstly,based on the theory of light field,2D projection views that record the intensity and direction information of the object wave are generated by the computer graphics rendering techniques such as 3Dmax.There are two projection geometry,generating perspective and orthographic projection views respectively.Then recording to the reconstructed principle of stereoscopic holography,complex amplitude hologram is calculated by using FFT on each projection view.As the limitation of Spatial Light Modulator(SLM),the amplitude and phase of hologram can't be modulated simultaneously.Due to the high diffraction efficiency of phase-only SLM,which is 100%in theory,it is more widely used for holographic display.We proposed two method to encoding the distribution of complex amplitude to phase-only information.One is to add a random phase to each projection view before the implementation of FFT,then take the phase component from the complex distribution.The other is to apply the Gerchberg-Saxton(GS)iterative algorithm on each projection view instead of FFT.Finally phase-only hogels are generated.The formula to calculate the sampling rate of holographic plane is also deduced.Simulated and optical experiments of each projection geometry and phase-only encoding method are conducted respectively,and the results are compared to each other.The run time of calculating the stereoscopic holography is also given,demonstrating the feasibility of real-time application.Eventually the fast calculation for the hologram of natural light field is completed.According to the process of calculating stereoscopic hologram,it is easy to find that,the computational complexity is depended on the resolution of the projection views and the number of the viewpoints.Therefore the calculation speed could be promoted by optimizing these parameters.And the computer graphics rendering techniques would add multiple shading effects and eventually make the 3D scene more realistic,so the shading and occlusion problem can be automatically solved during the rendering processes.