Ricardo Oliveira

Optimal point correspondence through the use of rank constraints

We propose a solution to the n-frame correspondence problem under the factorization framework. During the matching process, our algorithm takes explicitly into account the geometrical constraints associated to the reconstruction process. To this end, a rank constraint is imposed on the measurement matrix. Since our method relies solely on geometric constraints, it is not dependent on corners as image features and can consequently match generic points (e.g. contours). Outlier rejection is integrated as a part of the actual matching process. In general the problem is formulated as combinatorial, but we develop a method, which can provide a solution with a low computational complexity. Because of this, our algorithm is able to handle high-dimensional matching problems that are common in real-life applications.

Contour point tracking by enforcement of rigidity constraints

The aperture problem is one of the omnipresent issues in computer vision. Its local character constrains point matching to high textured areas, so that points in gradient-oriented regions (such as straight lines) can not be reliably matched. We propose a new method to overcome this problem by devising a global matching strategy under the factorization framework. We solve the n-frame correspondence problem under this context by assuming the rigidity of the scene. To this end, a geometric constraint is used that selects the matching solution resulting in a rank-4 observation matrix. The rank of the observation matrix is a function of the matching solutions associated to each image and as such a simultaneous solution for all frames has to be found. An optimization procedure is used in this text in order to find the solution.

Multi-view correspondence by enforcement of rigidity constraints

Establishing the correct correspondence between features in an image set remains a challenging problem amongst computer vision researchers. In fact, the combinatorial nature of feature matching effectively hinders the solution of large scale problems, which have direct applications in important areas such as 3D reconstruction and tracking. The solution is obtained by imposing a geometric constraint - rigidity - that selects the matching solution resulting in a rank-4 observation matrix. Since this is a global criterion, issues usually associated to local matching algorithms (such as the aperture problem) do not present an obstacle in this case. The use of a geometric constraint of this type assumes that all feature points are visible in every image, so as to obtain a complete observation matrix. The rank of the observation matrix is a function of the matching solutions associated to each image and as such a simultaneous solution for all frames has to be found. For each frame, correspondence is modeled through a permutation matrix, which also allows for the rejection of wrong candidates. Although each image is matched individually, an iterative algorithm is used to integrate correspondence information associated to all remaining images. Each individual matching process results in a linear problem: the reduced computational complexity allows the solution of large problems in an acceptable time interval. Although the algorithm has intrinsically been designed for calibrated systems, some instances of the uncalibrated case can also be solved provided a convenient bootstrap is available.

Enhanced Efficiency of PTB7 : PC61BM Organic Solar Cells by Adding a Low Efficient Polymer Donor

Ternary blend polymer solar cells combining two electron-donor polymers, poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7) and poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), as electron-acceptor, were fabricated. The power conversion efficiency of the ternary cells was enhanced by 18%, with respect to the reference binary cells, for a blend composition with 25% (wt%) of pBTTT in the polymers content. The optimized device performance was related to the blend morphology, nonrevealing pBTTT aggregates, and improved charge extraction within the device.

Improved stability of organic solar cells by cross-linking of the electron-donor polymer

We fabricated organic photovoltaic cells with enhanced performance stability measured after temperature annealing at 100 °C and over a period of 24 days at room temperature. This improvement was achieved upon chemical cross-linking of the photo-active layer of the devices, combining a cross-linkable polymer and a fullerene, PC61BM. The cells efficiency decayed by up to 14 % upon heating at 100 °C and by 37 % after the 24 days period, while the efficiency of control devices based on noncross-linked layers was decreased by 58 % and 55 %, respectively.