Design and analysis of coded aperture for 3D scene sensing
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In this thesis, the application of coded aperture in depth estimation is studied. More specifically, depth from defocus (DfD) is considered. DfD is a popular computer vision technique, which utilises the defocus blur cue for depth estimation. A general review of studies about the defocus blur, both its properties as a depth cue and its relation with the disparity cue, is presented. DfD methods are comprehensively investigated under two types of solving strategies. One is image restoration-based, whose success depends on the quality of image restoration; while the other strategy directly focuses on the depth estimation without requiring image restoration, and thus is referred to as the restoration-free strategy. The defocus blur is actually characterised by the point spread function (PSF) of the camera imaging system. The PSF of the camera can be modified by inserting a physical mask in the camera aperture position. A recent technique called coded aperture, which refers to the insertion of a coded mask in the aperture position, utilises this fact to improve the performance of DfD. Optimisation of the mask pattern for depth estimation is discussed in detail. A camera with a coded mask is built. The existing coded aperture methods for depth estimation are implemented and tested in both simulations and real experiments. Wave-optics based PSF calculation is proposed to have an accurate imaging model and avoid capturing PSFs in real experiments. Finally, several stereo cameras equipped with different sets of masks are analysed to explore the possible improvements in depth estimation by jointly utilising disparity and defocus blur cues. Results show that DfD can give valuable complementary depth information to stereo vision where stereo matching suffers from the correspondence problem. On the other hand, a stereo camera arrangement is shown to be useful for getting a single shot coded aperture system which employs a pair of complementary masks. A modified DfD algorithm is developed for that system.