Zeljka Mihajlovic

Visualization Service Based on Web Services

Scientific visualization evolved up to the stage where collaborative work in virtual coexistence of various experts becomes unavoidable to identify and solve specific visualization tasks. These problems encouraged us to research and define key aspects of visualization service which will enable collaborative work over the Internet. In this paper we define architecture of the whole system and unified interface for all requirements in such environment. Unified requests are sent to visualization processing unit to execute required visualization routine. Collaborative work on the volumetric medical data is especially demanding task, so we present development of VMI (Visualization of Medical data over Internet) system, where segmentation for visualization is based on joint histogram.

Frequency domain analysis of B-spline interpolation

This paper describes B-spline interpolation and compares it with other reconstruction methods, especially in three-dimensional space. We first consider the B-spline bases in the terms of convolution in signal processing. The presented analysis requires careful usage of continuous and discrete representation of B-spline. Emphasis is given to the important difference between B-spline interpolation and approximation. The difference is shown through frequency domain analysis, so we derive frequency responses of the B-spline interpolation and approximation. We conclude by demonstrating the use of several reconstruction filters and appropriate gradient estimators in volume rendering. Exact reconstruction in volume visualization is very important in many industrial applications, such as material cavity control.

Camera space shadow maps for large virtual environments

This paper presents a new single-pass shadow mapping technique that achieves better quality than the approaches based on perspective warping, such as perspective, light-space, and trapezoidal shadow maps. The proposed technique is appropriate for real-time rendering of large virtual environments that include dynamic objects. By performing operations in camera space, this solution successfully handles the general and the dueling frustum cases and produces high-quality shadows even for extremely large scenes. This paper also presents a fast nonlinear projection technique for shadow map stretching that enables complete utilization of the shadow map by eliminating wastage. The application of stretching results in a significant reduction in unwanted perspective aliasing, commonly found in all shadow mapping techniques. Technique is compared with other shadow mapping techniques, and the benefits of the proposed method are presented. The proposed shadow mapping technique is simple and flexible enough to handle most of the special scenarios. An API for a generic shadow mapping solution is presented. This API simplifies the generation of fast and high-quality shadows.

Gradient of B-splines in volume rendering

This paper deals with surface reconstruction and the gradient reconstruction in the volume rendering. In the volume rendering procedure, reconstruction according to the discrete set of samples is required. Due to the reconstruction procedure alias artifacts in the final image, it could not be neglected. In this paper, we focus our attention on the gradient reconstruction based on the surface reconstruction. For the surface reconstruction we use the cubic B-splines and for the gradient reconstruction corresponding derivative. If the noise is present in the input data, the approximation of B-spline is used, and the interpolation B-spline is used for the input signal without the noise. The shading procedure requires normal estimation. Two approaches are used for normal estimation. The classic approach for normal estimation is central difference calculation, and we propose derivative calculation of the reconstruction B-spline function. We show that the calculation of the normal vector has important influence on the alias artifacts in the result.

Automatic recognition of handwritten corrections for multiple-choice exam answer sheets

Automated grading of multiple-choice exams is of great interest in university courses with a large number of students. We consider an existing system in which exams are automatically graded using simple answer sheets that are annotated by the student. A sheet consists of a series of circles representing possible answers. As annotation errors are possible, a student is permitted to alter the annotated answer by annotating the“error” circle and handwriting the letter of the correct answer next to the appropriate row. During the scanning process, if an annotated“error” circle is detected, the system raises an alarm and requires intervention from a human operator to determine which answer to consider valid. We propose rather simple and effecive computer vision algorithm which enables automated reading of a limited set of handwritten answers and minimizes the need for a human intervention in the scanning process. We test our algorithm on a large dataset of real scanned answer sheets, and report encouraging performance rates.

Adaptive tool path planning applied in manufacturing optimization

Paper describes research done on algorithms for the generation of gouge-free nonisoparametric tool paths across surface plane segments obtained through triangulation partitioning. Along with contiguous trajectory sequences, adaptive tool path planning introduces partially discontinuous tool trajectories. Algorithm achieves the necessary trajectory continuity by the insertion of the auxiliary tool-orientation trajectories. The tool passes over the path gaps are accomplished through rapid tool traverses. The evaluation of surface machining optimization is provided by simulation of computed tool motion trajectories applied to triangulation models. Tool tracks and tool trajectories obtained with different surface models are used for presentation and examination of the optimized tool path modeling and algorithm computing performance issues.

Reconstruction of gradient in volume rendering

This paper deals with reconstruction of the gradient in the volume element space that is required for shading in the volume rendering. The accuracy of the surface reconstruction in the volume rendering is very important task as well as reconstruction accuracy of the normal vector. The central difference operator is often used as approximation of the gradient. The gradient reconstruction is an essential operation for determination of illumination and it has a great impact on the appearance of the reconstructed surface of an object. Our reconstruction of the gradient is based on the cubic approximation or interpolation B-splines. Volume rendering is visualization technique with widespread applicability in many fields, from the medical visualization to the engine visualization. Artefacts introduced by the surface reconstruction or the gradient reconstruction are of the crucial importance for perception of the projected object.

Volume rendering with least squares spline reconstruction

This paper presents a volume rendering procedure with embedded least square spline reconstruction. The first part of this paper describes a technique for the continuous representation of discrete signals by using B-splines. Volume element space is an example of the discrete signal which can be continuously represented in terms of the B-spline. Volume rendering is a very time-consuming procedure and therefore we down-sample the initial size of the volume. Before the down-sampling, some low-pass prefiltering is required. Our prefilter design is based on considering the combined operation of the prefiltering and the reconstruction postfiltering. The second part of this paper deals with embedding of the least square spline filter into the volume rendering procedure. The position of the reconstruction point is arbitrary in the volume element space. Reconstruction of the normal vector required for shading could be easily derived from the continuous reconstruction function.

A system for head-neck rehabilitation exercises based on serious gaming and virtual reality

Acute and chronic neck pain are common medical conditions, and the treatment typically includes physical therapy involving daily exercises. Insufficient motivation of people afflicted with neck pain to adhere to the prescribed exercise regimen may delay their recovery. Accordingly, in this work, we propose a system that motivates the users to perform neck exercises by engaging them in a serious exergame within virtual reality (VR) environment. The system measures the users’ neck movements via a few static and dynamic kinematic tests and a novel VR serious game, tailored to the neck range of motion of each individual user. The game is designed to make the users perform rehabilitative neck movements according to the prescribed exercise regimen while playing. The analysis of acquired data from VR hardware provides insight into flexibility of the neck during head movements and overall neck kinematics, which is valuable for assessment of pain-related stiffness, as well as for progress monitoring. In a user study performed with the proposed system and the Oculus Rift DK2 VR headset, we show that the users find exercising more interesting and engaging when using the proposed system, and that introducing visually rich VR environments makes the users more motivated to continue exercising.

A platform independent tool for programming, visualization and simulation of simplified FPGAs

During the last few decades complex programmable circuits have seen a widespread usage in various digital circuit applications. One prominent example are Field Programmable Gate Arrays (FPGAs). Teaching FPGA technology has become an integral part of introductory digital logic courses. However, implementing Boolean functions in this technology requires understanding of several steps that are not trivial, including Boolean function decomposition, mapping the design into physical programmable units and routing. We have developed a portable Java-based tool which allows students to experiment with the required steps for arbitrary Boolean functions and to simulate the obtained implementation. In this paper, we give an overview of the developed tool and discuss its usage in class.