Holograms are a promising technology for realizing a natural three-dimensional display. While the holographic pattern records both the phase and intensity information of an object, the spatial light modulator, which is a device for reproducing it, is capable of modulating either phase or amplitude. Therefore, in order to realize a perfect holographic image, a complex hologram must be converted into a phase-only hologram. Currently, SLM resolution is 4k or less, but resolution needs to be increased to achieve better quality holograms. This will increase the computational complexity of the Fourier transform, which is needed for each iterate. Therefore, a way to create a phase-only-hologram with a minimum number of iterations is needed. It is known that in the iterative method, the initial predicted value greatly affects the result. In this study, we propose a method to make initial guess to obtain more accurate phase only hologram with fewer iteration times. To obtain the initial guess, the desired image was divided into several parts, and the phase-only hologram corresponding to each was obtained and added again. We used hybrid-input-output iteration to maximize the effect of pre-iterated guess. When iterates the same number of times, the peak signal-to-noise ratio (PSNR) value obtained by using proposed pre-iterated guess was higher than using the random initial guess. In other words, the desired PSNR value can be reached with a smaller number of iterations by using proposed method. After 15 iterations, proposed initial guess have ~0.3dB higher PSNR value, which means one or two times of reduced iterations.
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