The compositional character of visual correspondence

Given two images of a scene, the problem of finding a map relating the points in the two images is known as the correspondence problem. Stereo correspondence is a special case in which corresponding points lie on the same row in the two images; optical flow is the general case. In this thesis, we argue that correspondence is inextricably linked to other problems such as depth segmentation, occlusion detection and shape estimation, and cannot be solved in isolation without solving each of these problems concurrently within a compositional framework. We first demonstrate the relationship between correspondence and segmentation in a world devoid of shape, and propose an algorithm based on connected components which solves these two problems simultaneously by matching image pixels. Occlusions are found by using the uniqueness constraint, which forces one pixel in the first image to match exactly one pixel in the second image. Shape is then introduced into the picture, and it is revealed that a horizontally slanted surface is sampled differently by the two cameras of a stereo pair, creating images of different width. In this scenario, we show that pixel matching must be replaced by interval matching, to allow intervals of different width in the two images to correspond. A new interval uniqueness constraint is proposed to detect occlusions. Vertical slant is shown to have a qualitatively different character than horizontal slant, requiring the role of vertical consistency constraints based on non-horizontal edges. Complexities which arise in optical flow estimation in the presence of slant are also examined. For greater robustness and flexibility, the algorithm based on connected components is generalized into a diffusion-like process, which allows the use of new local matching metrics which we have developed in order to create contrast invariant and noise resistant correspondence algorithms. Ultimately, it is shown that temporal information can be used to assign correspondences to occluded areas, which also yields ordinal depth information about the scene, even in the presence of independently moving objects. This information can be used for motion segmentation to detect new types of independently moving objects, which are missed by state-of-the-art methods

G-C3N4 (2D)/CdS (1D)/rGO (2D) dual-interface nano-composite for excellent and stable visible light photocatalytic hydrogen generation

A 2D/1D/2D dual-interface nano-composite configuration in the form of CdS nanorods sandwiched between g-C3N4 and rGO sheets with intimate interfacial contact is synthesized by a facile wet-chemical method and is shown to exhibit excellent photocatalytic H2 generation under visible-light irradiation. In particular, the optimal g-C3N4/CdS/rGO dual-interface nano-composite shows H2 production rate of ∼4800 μmol h-1 g-1, which is almost 44, 11 and 2.5 times higher than that shown by pure g-C3N4 nanosheets, and the g-C3N4/rGO and g-C3N4/CdS single interface heterostructures, respectively. It is shown that the synergic effects involving the band structure match and close interfacial contact, which can accelerate the separation and transfer of photoinduced charge carriers, and the enhanced visible-light absorption together contribute to the impressive photocatalytic performance and photostability of the g-C3N4/CdS/rGO ternary nano-composite system. Specific advantages of a dual-interface triple-composite system over a single interface case(s) are also brought out

g-C3N4/ NiAl-LDH 2D/2D Hybrid Heterojunction for High-Performance Photocatalytic Reduction of CO2 into Renewable Fuels

2D/2D interface heterostructures of g-C3N4 and NiAl-LDH are synthesized utilizing strong electrostatic interactions between positively charged 2D NiAl-LDH sheets and negatively charged 2D g-C3N4 nanosheets. This new 2D/2D interface heterojunction showed remarkable performance for photocatalytic CO2 reduction to produce renewable fuels such as CO and H2 under visible-light irradiation, far superior to that of either single phase g-C3N4 or NiAl-LDH nanosheets. The enhancement of photocatalytic activity could be attributed mainly to the excellent interfacial contact at the heterojunction of g-C3N4/NiAl-LDH, which subsequently results in suppressed recombination, and improved transfer and separation of photogenerated charge carriers. In addition, the optimal g-C3N4/NiAl-LDH nanocomposite possessed high photostability after successive experimental runs with no obvious change in the production of CO from CO2 reduction. Our findings regarding the design, fabrication and photophysical properties of 2D/2D heterostructure systems may find use in other photocatalytic applications including H2 production and water purification