Ing. Petr Felkel, Ph.D.

A procedure is described for unbiased identification of all π-electron chromophore pair geometry choices that locally maximize the rate of conversion of a singlet exciton into singlet biexciton (triplet pair), using a simplified version of the diabatic frontier orbital model of singlet fission (SF). The resulting approximate optimal geometries provide insight and are expected to represent useful starting points for searches by more advanced methods. The general procedure is illustrated on a pair of ethylenes as the simplest model of a π-electron system, but it is applicable to pairs of much larger molecules, with dozens of non-hydrogen atoms, and not necessarily planar. We first examine the value of |TA|², the square of the electronic matrix element for SF with initial excitation fully localized on partner A, on a grid of several billion geometries within the six-dimensional space of physically realizable possibilities. The optimized pair geometries are found to follow the qualitative guidance proposed earlier. In the neighborhood of each local maximum of |TA|², consideration of mixing with charge-transfer configurations and of excitonic interaction between partners A and B determines the SF energy balance and yields squared matrix elements |T*|² and |T**|² for the lower and upper excitonic states S* and S**, respectively. Assuming Boltzmann populations of these states, the geometry is further optimized to maximize k, the sum of the SF rates obtained from Marcus theory, and this reorders the suitable geometries substantially. At 87 pair geometries, the |T*|² and |T**|² values are compared with those obtained from high-level ab initio non-orthogonal configuration interaction calculations and found to follow the same trend. Finally, the biexciton binding energy at the optimized geometries is calculated. ...

Geometric transformations play an important role in a vast variety of disciplines. Although they belong to the fundamental concepts, they are also difficult to comprehend. Thousands of students take courses of algebra every year and although they may conceptually understand the transformations and mechanically solve the presented problems, they often struggle in visualizing the effect of the transformation on 3D objects represented as matrices. We explored the hypothesis that using interactive 3D computer graphics to visualize the transformations has its learning benefit.We have developed a novel framework for interactive 3D transformations called Interactive 3D Transformations (I3T) that allows for exploring and visualizing immediate effect of 3D transformations on rigid objects. We tested nine graduate students with I3T and compared them with the control group of another nine participants that used traditional passive methods. Moreover, we have tested the students spatial abilities by using a standardized test and we have evaluated how this affects their ability to comprehend the 3D transformations. Overall results showed that students increased their understanding of transformations between the pretest and posttest in both groups. When comparing the two groups, although the mean score in the posttest was two times higher for the I3T group, it did not show that this was statistically significantly higher than for the Traditional Group. The written responses showed higher enthusiasm of the students who used the interactive tool as opposed to using the passive learning method.

In this study, a new method for visualization of a virtually machined surface based on construction of a field of straight lines perpendicular to the machined surface is presented. The method is introduced in connection with the machine tool virtual model. The qualitative verification of the virtual machining results is performed and the precision of the model with the real machined surface is compared.

Computer programs devoted to visually impaired children are rare in the Czech Republic. Therefore, we started a project supporting the developers of such programs by a 2D game development environment. This environment allows drag-and-drop scene modeling, physics simulation, and script programming. We started with reviews of scripting languages and physics libraries. Based on the evaluation of speed and clarity of syntax, we selected the "Boo" scripting language. Based on computational stability and usability in the .NET Framework, C#, and Silverlight environments, we have chosen "Farseer Physics". We have demonstrated our tool by development of the educational program "Discovering with Louisa"; an educational program for visually impaired children, created in the frame of a joint project with the Charles University in Prague.

Pro intaktní děti existuje celá řada výukových počítačových programů různých témat. Některé z nich jsou využitelné i pro děti s postižením zraku s pomocí reedukačních pomůcek. Specializovaných programů pro předškolní a mladší školní děti se zrakovým postižením je na českém trhu minimum. Proto jsme se rozhodli využít prostředí pro tvorbu her, vyvinuté na fakultě elektrotechnické ČVUT, a vytvořit v něm čtyři ukázkové scénáře. V rámci tématu "poznávání zvířat" se v nich děti učí, jak různá zvířata vypadají, jaké vydávají zvuky, jaké zanechávají stopy a v jakém prostředí žijí. Zároveň program slouží nenásilnou formou k reedukaci zraku. Program jsme podrobili uživatelským testům s předškolními dětmi s poškozením zraku. Děti si po počátečním váhání ovládání rychle osvojily. Proto plánujeme pokračovat ve vývoji dalších scénářů.

In this paper, a survey on the most typical mesh errors is given. Each error is described in detail, it is illustrated on an example and surface based techniques for its detection and correction are presented. Covered errors include cracks, holes, T-joints, overlaps, zero volume parts, duplicated geometry, self intersections, inconsistent normal orientation, invisible polygons, degenerate faces and concavities. We consider the separation of the detection and the correction phases advantageous as it gives the user a better control over the mesh correction process, allowing better corrected meshes without introducing new errors, simplifications, or deformations.

Neoficiální VŠ učebnice počítačové grafiky v Česku a na Slovensku. Obsahuje široký a ucelený průřez moderními metodami rovinné a prostorové počítačové grafiky.

Visualization and quantitative analysis of vessel data is an important preprocessing step in diagnosis of vascular diseases, monitoring, surgery planning, blood flow simulation, education and training of surgeons. This paper surveys several geometric methods to solve basic visualization and quantification problems like centerline computation, boundary detection, projection techniques, and geometric model generation.

A new method for generating surfaces of branching tubular structures with given centerlines and radii. As the centerlines are not straight lines, the cross-sections are not parallel and well-known algorithms for surface tiling from parallel cross-sections cannot be used. The proposed method tiles non-parallel circular cross-sections and constructs a topologically-correct surface mesh. The method is not artifact-free, but it is fast and simple. The surface mesh serves as a data representation of a vessel tree suitable for real-time Virtual Reality operation planning and operation support within a medical application. Proposed method extracts a "classical" polygonal representation, which can be used in common surface-oriented graphic accelerators.

Image stitching is a prerequisite for obtaining a high resolution image if scanned parts of an image are too large to fit on a scanner. The proposed stitching is based on combination of two approaches used in image registration: on matching sets of external markers and on similarity evaluation of overlaps of neighboring

We report on using computed tomography (CT) as a model acquisition tool for complex objects in computer graphics. Unlike other modeling and scanning techniques the complexity of the object is irrelevant in CT, which naturally enables to model objects with, for example, concavities, holes, twists or fine surface details. Once the data is scanned, one can apply post-processing techniques for data enhancement, modification or presentation. For demonstration purposes we chose to scan a Christmas tree which exhibits high complexity which is difficult or even impossible to handle with other techniques. However, care has to be taken to achieve good scanning results with CT. Further, we illustrate post-processing by means of data segmentation and photorealistic as well as non-photorealistic surface and volume rendering techniques.

Visualization of tubular structures such as blood vessels is an important topic in medical imaging. One way to display tubular structures for diagnostic purposes is to generate longitudinal, cross-sections in order to show their lumen, wall, and surrounding tissue in a curved plane. This process is called Curved Planar Reformation (CPR). We present three different methods to generate CPR images. A tube-phantom was scanned with Computed Tomography (CT) to illustrate the properties of the different CPR methods. Furthermore we introduce enhancements to these methods: thick-CPR, rotating-CPR and multi-path-CPR.

This paper describes our work on the reconstruction of the complex liver vessel tree. We demonstrate our method for constructing an arbitrary smooth polygonal representation based on subdivision surfaces using vessel centerlines and radii as input.

The main goal of this paper is an implementation of the IFT (Imge Foresting Transform) algorithm with a priority queue with buckets and careful timing of this implementation to reach as minimal memory consumption as possible. The paper presents five possible modifications and methods of implementation of the IFT algorithm. All presented implementations keep the time complexity of the standard priority queue with buckets but the best one minimizes the costly memory allocation and needs only 19-45% of memory for typical 3D medical imaging datasets. Memory saving was reached by an IFT algorithm simplification, which stores more elements in temporary structures but these elements are simpler and thus need less memory.

není

Vylepseny algoritmus oplastovani povrchu

Oliva proposed an interesting algorithm for a 3D surface reconstruction from contours in parallel crosssections. As stated in the article [9], the algorithm constructs the surface for any non-self-intersecting contour shape also with holes [9] by means of adding appropriate number of intermediate cross-sections between complicated contours and triangulation of every pair of contours in different slices separately. For this task a Straight skeleton (Angular Bisector Network) [1, 9] is exploited. We have implemented Oliva's algorithm and we have found cases, which are not handled properly. The resulting surface can contain overhangs and selfintersected triangles can occur. We propose a modification of a triangulation step in Oliva's algorithm which handles differently the cases when overhangs (artifacts looking like folds) can appear and generates an overhang-free surface. By exclusion of overhangs we also prevent the creation of degenerated surface with mutually-intersected triangles.

Rekonstrukce prostorových objektů oplášťováním kontur

Straight skeleton (Angular Bisector Network, ABN) of a planar polygon, which can be grasped as a modification of a planar Voronoi diagram without parabolic arcs, has been successfully used by Oliva et al. as a part of a system for three dimensional reconstruction of objects from a given set of 2D contours in parallel cross sections. The algorithm itself is used for the construction of intermediate contour layers during the reconstruction process, in order not to create self intersected surface triangles or a surface with holes. But Oliva's algorithm is not publicly available and we have not found any other useful code on the net. We have followed our older ideas and implemented our version of straight skeleton. Our algorithm runs in O(nm + n log n) time, where n denotes the total number of polygon vertices and m the number of reflex ones.

Straight skeleton (Angular Bisector Network, ABN) of a planar polygon, which can be grasped as a modification of a planar Voronoi diagram without parabolic arcs, has been successfully used by Oliva et al. as a part of a system for three dimensional reconstruction of objects from a given set of 2D contours in parallel cross sections. The algorithm itself is used for the construction of intermediate contour layers during the reconstruction process, in order not to create self intersected surface triangles or a surface with holes. But Oliva's algorithm is not publicly available and we have not found any other useful code on the net. We have followed our older ideas and implemented our version of straight skeleton. Our algorithm runs in O(nm + n log n) time, where n denotes the total number of polygon vertices and m the number of reflex ones.

Segmentation forms the crucial step of Magnetic Resonance (MR) data visualization. The aim of our article is to present two fundamental methods for segmentation of medical data acquired by MR scanners. After a short overview of rendering techniques the appropriate segmentation step for each of them is discussed. Firstly, we describe a segmentation algorithm suitable for surface rendering. We present a model of object shape and appearance, and how how to match the model to the actual image data. Genetic algorithms perform the initial, rough estimation and the position is then refined by a gradient method. We demonstrate the functionality of a simple, one-level model, as well as that of a multiple resolution extended model. Secondly, an algorithm suitable for volume rendering is discussed. It is based on combined model which stores not only grey level and gradient information, but takes into account also adjacency of organs and their hierarchical partitioning.