Catálogo de publicaciones - libros

The notion of topographic features like ridges, trenches, hills, etc. is formed by visualising the 2D image function as a surface in 3D space. Hence, properties of such a surface can be used to detect features from images. One such property, the curvature of the image surface, can be used to detect features characterised by a sharp bend in the surface. Curvature based feature detection requires an efficient technique to estimate/calculate the surface curvature. In this paper, we present an alternative measure for curvature and provide an analysis of the same to determine its scope. Feature detection algorithms using this measure are formulated and two applications are chosen to demonstrate their performance. The results show good potential of the proposed measure in terms of efficiency and scope.

This paper presents a novel image enhancement algorithm using a multilevel windowed inverse sigmoid (MWIS) function for rendering images captured under extremely non uniform lighting conditions. MWIS based image enhancement is a combination of three processes viz. adaptive intensity enhancement, contrast enhancement and color restoration. Adaptive intensity enhancement uses the non linear transfer function to pull up the intensity of underexposed pixels and to pull down the intensity of overexposed pixels of the input image. Contrast enhancement tunes the intensity of each pixel’s magnitude with respect to its surrounding pixels. A color restoration process based on relationship between spectral bands and the luminance of the original image is applied to convert the enhanced intensity image back to a color image.

Visibility culling of a scene is a crucial stage for interactive graphics applications, particularly for scenes with thousands of objects. The culling time must be small for it to be effective. A hierarchical representation of the scene is used for efficient culling tests. However, when there are multiple view frustums (as in a tiled display wall), visibility culling time becomes substantial and cannot be hidden by pipelining it with other stages of rendering. In this paper, we address the problem of culling an object to a hierarchically organized set of frustums, such as those found in tiled displays and shadow volume computation. We present an adaptive algorithm to unfold the twin hierarchies at every stage in the culling procedure. Our algorithm computes from-point visibility and is conservative. The precomputation required is minimal, allowing our approach to be applied for dynamic scenes as well. We show performance of our technique over different variants of culling a scene to multiple frustums. We also show results for dynamic scenes.

Recent advances in three dimensional Electron Microscopy (3D EM) have given an opportunity to look at the structural building blocks of proteins (and nucleic acids) at varying resolutions. In this paper, we provide algorithms to detect the secondary structural motifs (-helices and -sheets) from proteins for which the volumetric maps are reconstructed at 5−10 Å resolution. Additionally, we show that when the resolution is coarser than 10 Å, some of the tertiary structural motifs can be detected from 3D EM. For both these algorithms, we employ the tools from computational geometry and differential topology, specifically the computation of stable/unstable manifolds of certain critical points of the distance function induced by the molecular surface. With the results in this paper, we thus draw a connection between the mathematically well-defined concepts with the bio-chemical structural folds of proteins.

Terrains and other geometric models have been traditionally stored locally. Their remote access presents the characteristics that are a combination of file serving and realtime streaming like audio-visual media. This paper presents a terrain streaming system based upon a client server architecture to handle heterogeneous clients over low-bandwidth networks. We present an efficient representation for handling terrains streaming. We design a client-server system that utilizes this representation to stream virtual environments containing terrains and overlayed geometry efficiently. We handle dynamic entities in environment and the synchronization of the same between multiple clients. We also present a method of sharing and storing terrain annotations for collaboration between multiple users. We conclude by presenting preliminary performance data for the streaming system.

High-resolution portable projectors have become commodity items now to own – but not to use. It is not always possible to find a display area where the camera can be properly aligned so that an undistorted image be seen. We present a method to project an undistorted image using a digital projector on a piecewise-planar display area.

We use uncalibrated structured light ranging to segment the unknown projection area and further compute the homographies that map the projector space to the camera space through each of the planes. The edge detection and point-correspondences are subpixel precise. Finally, we use these computed homographies to pre-warp the display image so that a distortion-free image is visible. Our results show a seamless and correct rectification with accurate segmentation of the planes.

A novel algorithm to derive an approximate cellular envelope of an arbitrarily thick digital curve on a 2D grid is proposed in this paper. The concept of “cellular envelope” is newly introduced in this paper, which is defined as the smallest set of cells containing the given curve, and hence bounded by two tightest (inner and outer) isothetic polygons on the grid. Contrary to the existing algorithms that use thinning as preprocessing for a digital curve with changing thickness, in our work, an optimal cellular envelope (smallest in the number of constituent cells) that entirely contains the given curve is constructed based on a combinatorial technique. The envelope, in turn, is further analyzed to determine polygonal approximation of the curve as a sequence of cells using certain attributes of digital straightness. Since a real-world curve/curve-shaped object with varying thickness and unexpected disconnectedness is unsuitable for the existing algorithms on polygonal approximation, the curve is encapsulated by the cellular envelope to enable the polygonal approximation. Owing to the implicit Euclidean-free metrics and combinatorial properties prevailing in the cellular plane, implementation of the proposed algorithm involves primitive integer operations only, leading to fast execution of the algorithm. Experimental results including CPU time reinforce the elegance and efficacy of the proposed algorithm.

We present a real-time painterly rendering technique for geometric models. The painterly appearance and the impression of geometric detail is created by effectively rendering several brush strokes. Unlike existing techniques, we use the textures of the models to come up with the features and the positions of strokes in 3D object space. The strokes have fixed locations on the surfaces of the models during animation, this enables frame to frame coherence. We use vertex and fragment shaders to render strokes for real-time performance. The strokes are rendered as sprites in two-dimensions, analogous to the way artists paint on canvas. While animating, strokes may get cluttered since they are closely located on screen. Existing techniques ignore this issue; we address it by developing a level of detail scheme that maintains a uniform stroke density in screen space. We achieve painterly rendering in real-time with a combination of object space positioning and image space rendering of strokes. We also maintain consistency of rendering between frames . We illustrate our method with images and performance results.

An interesting alternative to traditional geometry based rendering is Light Field Rendering [1,2]. A camera gantry is used to acquire authentic imagery and detailed novel views are synthetically generated from unknown viewpoints. The drawback is the significant data on disk.

Moving from static images, a walkthrough or a through the implied virtual world is often desirable but the repeated access of the large data makes the task increasingly difficult. We note that although potentially infinite walkthroughs are possible, for any given path, only a subset of the previously stored light field is required. Our prior work [3] exploited this and reduced the main memory requirement. However, considerable computational burden is encountered in processing even this reduced subset. This negatively impacts real-time rendering.

In this paper, we subdivide the image projection plane into “cells,” each of which gets all its radiance information from the cached portions of the light field at select “nodal points.” Once these cells are defined, the cache is visited to find the radiance efficiently. The net result is camera walks.

This article reports the result of an experiment which integrates GPU-accelerated skinning, sprite animation, and character behavior control. The experiment shows that the existing techniques can be neatly integrated: thousands of characters are animated at real-time and the overall motion is natural like fluid. The result is attractive for games, especially where a huge number of non-player characters such as animals or monsters should be animated.