Compared with the size of a calibration board, the inspected area within the camera field of view is much bigger in many applications such as visual surveillance systems. In this paper, we present a global image-to-ground homography calibration method to obtain the mapping between the image and the planar scene lying in the whole camera field of view. The calibration approach is proposed by fusing multiple local homography matrices, with each of them only reflecting the relationship between a small calibration board and its corresponding small image part. By further calibrating a height-related homography, visual measurement can be extended for arbitrary planar surfaces with known heights. The proposed method presents such merits as no requirement for camera internal parameters, high calibration accuracy, and ease of implementation. Experimental results show that (1) the proposed global homography method achieves more accurate measurement results than the local homography method; (2) the heightrelated homography is calibrated with high accuracy; (3) the proposed approach can be used for mobile robot localization with accuracy close to the performance limit of a monocular camera.

Material innovation plays an important role in improving the output performance of a triboelectric nanogenerator (TENG). In this work, a flexible ZnO@ZnSnO 3 /ZnSnO 3 /polydimethylsiloxane (PDMS) composite film is designed and prepared as the negative friction layer of TENG by a facile hydrothermal method based on in situ growth of the semiconductor ZnO on ferroelectric nanocubic ZnSnO 3 , which is leadfree and has a high dielectric constant, and cooperative uniform dispersion of nanoparticles. The effects of surface charge density, effective contact area, and interfacial polarization of the negative friction layer on the output performance of TENG are discussed experimentally and theoretically. Compared with the TENG involving pure PDMS, the overall performance of TENG based on the composite film is greatly enhanced, including an open-circuit voltage of 218 V, a short-circuit current of 14.2 μA, a transferred charge of 60 nC, and a power density of 24.625 μW/cm 2 . The achieved output voltage, current, and transferred charge are 6.8, 9.7, and 10 times higher than those of TENG comprising of pure PDMS, respectively. The TENG designed also successfully lights up 212 light-emitting diodes and charges the electronic devices without any charging equipment. Besides, it can be used as a favorable motion monitoring device. This work provides a facile and effective approach to enhance the overall performance of TENG by utilizing an elaborately designed flexible negative friction layer. The resulting TENG will have a respectable application prospect in self-powered sensing detection and flexible wearable devices.

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