In this paper, a method based on point source and view-window is proposed that covers the features as full parallax, depth and accurate occlusion cue, shading and lighting, and ensures the computational efficiency in the meantime, to calculate the on-the-fly computer-generated holograms. With the acceleration of graphics processing unit (GPU), a bunch of point data of reconstructed image go through the pipeline of OpenGL and finish with coordinates transformation, fragment interpolation, lighting calculation, occlusion test, calculation and superposition of complex amplitude, and finally hologram are generated and loaded into the spatial light modulator (SLM). The experimental result shows that the lifelike complex full-parallax objects can be reconstructed at a high speed with varying gloss and accurate occlusion when viewed from different perspectives in the view-window.
Holographic exposure mosaic technology is a feasible solution to fabricate large-area pulse compression gratings, where the mosaic grating method of developing region by region is one of the mosaic approaches. In this method, an exposed area of the substrate is firstly developed, and then the developed photoresist grating mask is put back into the previous exposure system. The next area grating mask is fabricated by aligning the interference fringes formed by the exposure beam and the developed real grating. However, since the unequal exposure and inconsistent development, the groove shapes of grating masks in two areas, including groove depth and duty cycle, will be different. When detecting the mosaic grating error, the differences of the groove shapes will cause the dislocation in the -1-order reflected diffraction wavefront at gap of the mosaic grating. It will be superimposed on the phase change caused by the lateral displacement error, so that the judgement of lateral displacement error will be seriously interfered. To solve this problem, the measurement method of the 0-order diffraction wavefront under multiple incident angles is proposed to precisely judge the lateral displacement error in the mosaic grating. In this paper, the grating diffraction analysis program based on the rigorous coupled-wave analysis is firstly written, and then the initial phase of 0-order reflected diffraction wavefront of mosaic grating mask is calculated. Subsequently, the sample is tested by atomic force microscopy and interferometer. By importing the measurement data into the searching program, the groove parameters of grating masks are obtained by the library matching method. Then, the lateral displacement error of the mosaic grating is further deduced. Finally, the feasibility of the proposed judgment method is confirmed by the experiment.
KEYWORDS: Holograms, Digital holography, Fresnel lenses, Computer generated holography, Spatial light modulators, Reconstruction algorithms, 3D modeling, 3D image processing, 3D displays, 3D image reconstruction
In this paper, a voxel-based algorithm for calculating phase-only three-dimensional (3D) computer generated holograms (CGHs) with occlusion effect is proposed. The 3D object can be firstly decomposed into a number of self-luminous voxels, which is the minimum imaging unit of the object. According to occlusion relationship, the 3D position and propagation direction of each voxel can be determined, where the axial position, lateral position and the propagation direction are precisely controlled by digital Fresnel lens, digital grating and hologram segmentation, respectively. Then, in order to reconstruct the 3D object composed of multiple voxels, the pre-calculated holograms for all of the voxels are synthesized with the interweaving arrangement method. Moreover, the look-up-table (LUT) method is used in the hologram generation process to improve the calculating speed. An experimental verification system for the proposed algorithm is constructed using a single SLM. The optical reconstruction results demonstrate that the CGHs generated by the proposed algorithm can successfully provide 3D sensation with occlusion effect.
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