Leonid I. Dimitrov

Enhanced Voxelization and Representation of Objects with Sharp Details in Truncated Distance Fields

This paper presents a new method for voxelization of solid objects containing sharp details. Voxelization is a sampling process that transforms a continuously defined object into a discrete one represented as a voxel field. The voxel field can be used for rendering or other purposes, which often involve a reconstruction of a continuous approximation of the original object. Objects to be voxelized need to fulfill certain representability conditions; otherwise, disturbing artifacts appear during reconstruction. The method proposed here extends the traditional distance-based voxelization by an a-priori detection of sharp object details and their subsequent modification in such a way that the resulting object to be voxelized fulfills the representability conditions. The resulting discrete objects are represented by means of truncated (i.e., narrow-band) distance fields, which provide reduction of memory requirements and further processing by level set techniques. This approach is exemplified by two classes of solid objects that normally contain such sharp details: implicit solids and solids resulting from CSG operations. In both cases, the sharp details are rounded to a specific curvature dictated by the sampling distance.

The Reflection of Cognitive Tasks in EEG and MRI and a Method of Its Visualization

SummaryUp until recently, neurology was dominated by localisatory thinking. Language and other so-called“centers” were considered to be centers of command controlling the respective functions. Today, there is general agreement that, instead, for every brain function numerous brain regions must act together. For the exploration of these manifold topographic cooperations produced by cognitive tasks, coherence of long-term EEG periods proved to be a proficient parameter for the representation of functionally essential connections. Because of the unequivocal meaningfulness of absolute coherence values, instead, only the signs of significant differences between coherence values during cognitive tasks and periods of EEG at rest before and after the task were considered for all possible electrode pairings and charted on schematic maps of the brain. In addition, the signs of significant changes of amplitude were entered. This procedure was performed for each of 6 frequency bands and for the 19 electrodes of the 10/20 system, thus yielding 171 possible plus or minus values for coherence and 19 for amplitude, respectively. The positions of the electrodes were marked by an MRI contrast medium. After the EEG, MRI examination was performed. The MRI data were segmented and the cortex was mapped onto a plane using a method similar to cartography. The exact electrode positions are registered from a similarly obtained map of the scalp and the electrode position pattern is used as basis for the coherence graphs. A detailed map of the cortex based on the segmented MRI data with the electrode positions marked is provided as a reference enabling allocation of the electrodes to the cortical structures. The usefulness of this procedure is demonstrated with a single subject by means of different cognitive tasks including musical thinking.

f3d - a file format an tools for storage an manipulation of volumetric data sets

Different file formats for storage and manipulation of volumetric data exist today, but none of them has been accepted as a standard by the volume visualization and volume graphics communities until now. This paper tries to change this situation by proposing a new, freely available file format f3d, which, as we believe, fulfills the demands of research and educational environments. We hope that this new format will be accepted by the above mentioned communities and thus will provide a basis for improved communication between different groups and applications.

Correction of voxelization artifacts by revoxelization

Earlier proposed antialiasing techniques for voxelization of geometric objects in some cases do not result in completely alias-free data and image renditions. This is often the case for some implicit solids and CSG trees. In this paper we propose a set of operations, which can correct such corrupted data sets and subsequently lead to alias-free image renditions.

Processing of volumetric data by slice- and process-based streaming

Although the main memory capacity of modern computers is constantly growing, the developers and users of data manipulation and visualization tools fight all over again with the problem of its shortage. In this paper, we advocate slice-based streaming as a possible solution for the memory shortage problem in the case of preprocessing and analysis of volumetric data defined over Cartesian, regular and other types of structured grids. In our version of streaming, data flows through independent processing units---filters---represented by individual system processes, which store each just a minimal fraction of the whole data set, with a slice as a basic data entity. Such filters can be easily interconnected in complex networks by means of standard interprocess communication using named pipes and are executed concurrently on a parallel system without a requirement of specific modification or explicit parallelization. In our technique, the amount of stored data by a filter is defined by the algorithm implemented therein, and is in most cases as small as one data slice or only several slices. Thus, the upper bound on the processed data volume is not any more defined by the main memory size but is shifted to the disc capacity, which is usually orders of magnitude larger. We propose implementations of this technique for various point, local and even global data processing operations, which may require multiple runs over the input data or eventually temporary data buffering. Further, we give a detailed performance analysis and show how well this approach fits to the current trend of employing cheap multicore processors and multiprocessor computers.

Using 3D-Bresenham for resampling structured grids

Structured grids and some of their applications in natural sciences are discussed. The problem of their visualization and quantitative evaluation is considered and possible ways for its solution sketched. Resampling a structured grid onto a regular one is such a possible solution offering the additional benefit of enabling quantitative evaluations, too. This resampling is achieved by a preliminary tetrahedronization of the structured grid(s) and a subsequent digitalization of the constituent tetrahedrons using an adaptation of the 3D-Bresenham algorithm. Advantages and disadvantages of this approach as compared to other possible schemes are discussed. An implementation based on our open source f3d-file format for storage and transmission of volumetric data is presented, and results from applying it to real-world data shown.

CSG operations with voxelized solids

We present a new technique for CSG operations with voxelized geometric objects, which are represented by truncated discrete distance fields supplemented by additional information about the surface normal. The technique removes artifacts of straightforward volumetric CSG operations by taking into account conditions for object representability, according to which sharp details are not correctly representable in discrete distance fields. The proposed technique solves this shortcoming by rounding edges and other sharp details. It works at the voxel level without the necessity for reconstruction of continuous object models

Collagen dysplasia in idiopathic carpal tunnel syndrome.

Surgical biopsies of dissected transverse carpal ligaments of patients with idiopathic carpal tunnel syndrome were examined with an electron microscope revealing collagen fibrils with extremely varying diameters. Morphometric analysis was performed on electron micrographs exhibiting fibrils with a small diameter comparable to that in control tissue as well as fibrils with a far larger diameter than could be observed in control tissue. Morphometric parameters were evaluated in order to analyse the relation between the number of and the area covered by collagen fibrils in the electron micrographs. In control tissue the numerical density per image area was twice the numerical density in carpal tunnel syndrome. However, the area fraction of the electron micrographs occupied by collagen fibrils in carpal tunnel syndrome and controls were equal.

Morphometric Analysis of Collagen Fibrils in Idiopathic Carpal Tunnel Syndrome

Surgical biopsies of dissected transverse carpal ligaments of patients with idiopathic carpal tunnel syndrome were examined with an electron microscope revealing collagen fibrils with varying diameters. Morphometric analysis of transversely cut collagen fibrils was performed on photomicrographs exhibiting fibrils with a small diameter comparable to that in normal tissue as well as fibrils with a large diameter that could not be observed in normal tissue.

Computer-generated display macro holograms

Two computer-generated display macro holograms (CGDMH) have been synthesized to demonstrate the possibility of holographic display of 3D objects given by their mathematical descriptions only. Three dimensional models of the objects and shaded 2D projections in varying viewing directions were generated using the methods of computer graphics. For each projection, a Fourier hologram was synthesized and encoded by the kinoform method. The recording of the obtained digital kinoforms on a commercially available photographic film was done by a computer controlled laser device. This process produces, after film development and bleaching, a facet CGDMH. The complete CGDMHs have a size of 672 X 672 mm2 and consist of 900 elementary holograms of 256 X 256 samples each, calculated for different directions within the solid angle of +/- 90 degree(s). They allow the visual representation of 3D objects with good quality.