Tomoyuki Nishita

Ray tracing trimmed rational surface patches

This paper presents a new algorithm for computing the points at which a ray intersects a rational Bézier surface patch, and also an algorithm for determining if an intersection point lies within a region trimmed by piecewise Bézier curves. Both algorithms are based on a recent innovation known as Bézier clipping, described herein. The intersection algorithm is faster than previous methods for which published performance data allow reliable comparison. It robustly finds all intersections without requiring special preprocessing.

A simple, efficient method for realistic animation of clouds

This paper proposes a simple and computationally inexpensive method for animation of clouds. The cloud evolution is simulated using cellular automaton that simplifies the dynamics of cloud formation. The dynamics are expressed by several simple transition rules and their complex motion can be simulated with a small amount of computation. Realistic images are then created using one of the standard graphics APIs, OpenGL. This makes it possible to utilize graphics hardware, resulting in fast image generation. The proposed method can realize the realistic motion of clouds, shadows cast on the ground, and shafts of light through clouds.

Continuous tone representation of three-dimensional objects taking account of shadows and interreflection

The effect of shadows and interreflection created by room obstructions is an important factor in the continuous tone representation of interiors. For indirect illumination, in most cases a uniform ambient light has been considered, even though the interreflection gives very complex effects with the shaded images.The proposed method for indirect lighting with shadows results in the following advanced points:1) The indirect illuminance caused by the surfaces of objects such as ceilings, floors, walls, desks, bookcases etc. gives added realism to images.2) The proposed method is suitable for every type of light source such as point sources, linear sources, and area sources.

Display of clouds taking into account multiple anisotropic scattering and sky light

Methods to display realistic clouds are proposed. To display realistic images, a precise shading model is required: two components should be considered. One is multiple scattering due to particles in clouds, and the other factor to be considered is sky light. For the former, the calculation of cloud intensities has been assumed to be complex due to strong forward scattering. However, this paper proposes an efficient calculation method using these scattering characteristics in a positive way. The latter is a very significant factor when sky light is rather stronger than direct sunlight, such as at sunset/sunrise, even though sky light has been ignored in previous methods. This paper describes an efficient calculation method for light scattering due to clouds taking into account both multiple scattering and sky light, and the modeling of clouds. CR Categories and Subject Descriptors:

A shading model for atmospheric scattering considering luminous intensity distribution of light sources

Studio spotlights produce dazzling shafts of light, while light scattered from fog illuminated by automobile headlights renders driving difficult. This is because the particles in the illuminated volume become visible by scattering light. A shading model for scattering and absorption of light caused by particles in the atmosphere is proposed in this paper. The method takes into account luminous intensity distribution of light sources, shadows due to obstacles, and density of particles. The intensity at a viewpoint is calculated by integration of light scattered by particles between the viewpoint and a given point on an object. The regions to be treated in this manner are localized by considering illumination volumes and shadow volumes caused by obstacles in the illumination volumes.

A feature-driven approach to locating optimal viewpoints for volume visualization

Optimal viewpoint selection is an important task because it considerably influences the amount of information contained in the 2D projected images of 3D objects, and thus dominates their first impressions from a psychological point of view. Although several methods have been proposed that calculate the optimal positions of viewpoints especially for 3D surface meshes, none has been done for solid objects such as volumes. This paper presents a new method of locating such optimal viewpoints when visualizing volumes using direct volume rendering. The major idea behind our method is to decompose an entire volume into a set of feature components, and then find a globally optimal viewpoint by finding a compromise between locally optimal viewpoints for the components. As the feature components, the method employs interval volumes and their combinations that characterize the topological transitions of isosurfaces according to the scalar field. Furthermore, opacity transfer functions are also utilized to assign different weights to the decomposed components so that users can emphasize features of specific interest in the volumes. Several examples of volume datasets together with their optimal positions of viewpoints are exhibited in order to demonstrate that the method can effectively guide naive users to find optimal projections of volumes.

Curve intersection using Be´zier clipping

Abstract A technique referred to as Bezier clipping is presented. This technique forms the basis of algorithm for computing the points at which two curves intersect, and also an algorithm for robustly and quickly computing points of tangency between two curves. Bezier clipping behaves like an intelligent interval Newton method, in which geometric insight is used to identify regions of the parameter domain which exclude the solution set. Implementation tests suggest that the curve intersection algorithm is marginally slower than an algorithm based on implicitization (though faster than other algorithms) for curves of degree four and less, and is faster than the implicitization algorithm for higher degrees.

A montage method: the overlaying of the computer generated images onto a background photograph

A system of computer programs has been established to generate high quality montage image of considerable usefulness in architectural simulation which combine computer-generated images and photographed background pictures.Traditionally, there are two methods of creating architectural montages: (1) an artist paints new buildings onto a background scene usually generated photographically, and (2) a three-dimensional scale model is created to simulate the whole landscape, and this model is then photographed. The montage method described here combines aspects of both traditional montage methods with significant improvement in accuracy and reduction of time and cost of preparation. Specifically, a digitized photograph is used as a background scene onto which is superimposed a 3D computer-generated image of a new building. The outstanding points of the new method are:(i) The shading and shadows of each computer generated image are calculated with higher accuracy, (ii) the fog effect is taken into account, and (iii) a new anti-aliasing technique improves the quality of the final montage image.

Extracting depth and matte using a color-filtered aperture

This paper presents a method for automatically extracting a scene depth map and the alpha matte of a foreground object by capturing a scene through RGB color filters placed in the camera lens aperture. By dividing the aperture into three regions through which only light in one of the RGB color bands can pass, we can acquir three shifted views of a scene in the RGB planes of an image in a single exposure. In other words, a captured image has depth-dependent color misalignment. We develop a color alignment measure to estimate disparities between the RGB planes for depth reconstruction. We also exploit color misalignment cues in our matting algorithm in order to disambiguate between the foreground and background regions even where their colors are similar. Based on the extracted depth and matte, the color misalignment in the captured image can be canceled, and various image editing operations can be applied to the reconstructed image, including novel view synthesis, postexposure refocusing, and composition over different backgrounds.

A lighting model aiming at drive simulators

Many techniques for rendering natural objects such as the sea, terrains, and trees have been developed; they are indispensable for flight simulators. In this paper, techniques for rendering road surfaces under various conditions are discussed. Rendering road surfaces is quite useful for the evaluation of driving safety, and it will play an important part in the development of drive simulators. Light sources with high intensity often disturb drivers especially under wet road surface conditions.This paper proposes two models, a reflection model for road surfaces taking into account weather conditions, and a model on streaks of light taking into account both refraction and diffraction of light. Some examples demonstrate the possibility of applications for drive simulators in the future.