Full-range in-plane rotation measurement for image recognition with hybrid digital-optical correlator

A hybrid digital-optical correlator (HDOC) based on volume holographic memory is able to compute the correlation of images at a high speed. HDOC is suitable for real-time image processing and has potential usage in big data processing areas. A 7500-channel HDOC system is experimentally set up, and the target image is correlated with all the channels. The large number of parallel correlation channels could contribute to the precise rotation measurement as well as the translation measurement. In the image recognition applications, the target image involves rotation distortion with respect to the template images. A method with two coarse-fine steps is proposed to measure the rotation at a full range of 360 deg. In the coarse step, the target image is rotated 36 times at an increment of 10 deg. The 36 new images are sent into the HDOC to compute with the template images. Each new image corresponds to a correlation matrix. By searching the smallest value throughout the 36 minimums of the 36 correlation matrixes, the rotation of the target image is narrowed into perpendicular to 5 deg. In the fine step, the new image is rotated another 10 times at an increment of 1 deg. The rotation measurement error is <0.3 deg. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)