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Detecting and Rendering Silhouette in a Point Data Set
Abstract
A point data set is a set of independent point samples that are typically acquired from a D scanning process. We are interested in detecting and rendering silhouettes in such data set, for the purpose of Non-Photorealistic Rendering (NPR). Traditionally, work on silhouette extraction and rendering tends to assume a polygonal mesh input, where connectivity information is available and the formation of silhouettes involves tracking of edges. A point data set, however, does not contain connectivity information. In a previous paper (38), we presented the first hybrid image/object-space method to directly detect and render silhouettes for point data set. In this paper, we describe the essence of the method in more detail, emphasizing on the mathematics applied. We describe as well a simple variant of the method to detect silhouette points using ray casting instead of point splatting. This variant is useful in the absence of dedicated graphics hardware.
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Silhouette is a drawing feature that is popular in illustrations and line-drawing artworks and is an important focus of research in the field of Non-Photorealistic Rendering (NPR). Current work on silhouette extraction and rendering tends to assume a polygonal mesh input, where connectivity information is available and the formation of silhouettes involves tracking of edges. On the other hand, the availability of D scanning devices nowadays leads to the proliferation of dense point data set for which no connectivity information is initially available. We present in this paper a hybrid image/object-space method to directly detect and render stylized silhouettes for such data set. Our method is simple to implement and runs at an interactive frame rate. Its basic form involves rendering silhouette points, each with a unique color value, to a color buffer. We show how lines can then be quickly fitted to the points projected. We further show how stylized silhouette can be used effectively with two other NPR techniques that happen to work very naturally with point-sampled geometry: stippling, and direct drawing onto surfaces.
We present an algorithm suitable for real-time, high quality rendering of complex objects. Objects are represented as a dense set of surface point samples which contain colour, depth and normal information. These point samples are obtained by sampling orthographic views on an equilateral triangle lattice. They are rendered directly and independently without any knowledge of surface topology. We intro- duce a novel solution to the problem of surface reconstruction using a hierarchy of Z-buffers to detect tears. Our algorithm is fast and requires only modest resources.
Finding and displaying silhouette edges is important in applications ranging from computer vision to nonphotorealistic rendering. To render visible silhouette edges of a polygonal object in a scene from a given viewpoint, we must first find all silhouette edges, i.e. boundaries between adjacent front facing and back-facing surfaces. This is followed by solving the partial visibility problem so that only those parts of silhouette edges, which are not occluded by interior of any front facing surface, are rendered. The scene may optionally be rendered with a lighting model. This paper describes a simple general-purpose method to combine all three operations for any scene composed of objects that can be scan-converted. Using a depth buffer, the rendering process computes the intersection of adjacent front facing and back-facing surfaces in image space at interactive rates. All operations are performed in image-precision and hence special care is taken for the limited numerical precision of the depth buffer. A solution is suggested using view-dependent modification of polygonal objects. The method does not require any preprocessing or adjacency information and hence is applicable for dynamic scenes.
We present a sketching interface for quickly and easily designing freeform models such as stuffed animals and other rotund objects. The user draws several 2D freeform strokes interactively on the screen and the system automatically constructs plausible 3D polygonal surfaces. Our system supports several modeling operations, including the operation to construct a 3D polygonal surface from a 2D silhouette drawn by the user: it inflates the region surrounded by the silhouette making wide areas fat, and narrow areas thin. Teddy, our prototype system, is implemented as a Java™ program, and the mesh construction is done in real-time on a standard PC. Our informal user study showed that a first-time user typically masters the operations within 10 minutes, and can construct interesting 3D models within minutes.
Most commercial 3-D software packages used for animated films and still picture production offer merely rudi- mentary support for non-photorealistic rendering. However, nearly all of these packages possess interfaces which allow the user to add custom shader programs. We present a solution which uses a custom shader combined with post-processing software in order to draw outlines and creases of 3-D scenes in a way which simulates artistic tools like pencils and ink brushes.
We present a new technique for extracting line-type features on point-sampled geometry. Given an unstructuredpoint cloud as input, our method first applies principal component analysis on local neighborhoods toclassify points according to the likelihood that they belong to a feature. Using hysteresis thresholding, we thencompute a minimum spanning graph as an initial approximation of the feature lines. To smooth out the featureswhile maintaining a close connection to the underlying surface, we use an adaptation of active contour models.Central to our method is a multi-scale classification operator that allows feature analysis at multiplescales, using the size of the local neighborhoods as a discrete scale parameter. This significantly improves thereliability of the detection phase and makes our method more robust in the presence of noise. To illustrate theusefulness of our method, we have implemented a non-photorealistic point renderer to visualize point-sampledsurfaces as line drawings of their extracted feature curves.
closely matches the one she is used to - namely pencil and paper. The ability to specify nonplanar 3D curves is of fundamen- tal importance in 3D modeling and animation systems. Ef- fective techniques for specifying such curves using 2D in- put devices are desirable, but existing methods typically re- quire the user to edit the curve from several viewpoints. We present a novel method for specifying 3D curves with 2D in- put from a single viewpoint. The user first draws the curve as it appears from the current viewpoint, and then draws its shadow on the floor plane. The system correlates the curve with its shadow to compute the curve's 3D shape. This method is more "natural" than existing methods in that it leverages skills that many artists and designers have devel- oped from work with pencil and paper.
We present a system that lets a designer directly annotate a D model with strokes, imparting a personal aesthetic to the non-photorealistic rendering of the object. The artist chooses a "brush" style, then draws strokes over the model from one or more viewpoints. When the system renders the scene from any new viewpoint, it adapts the number and placement of the strokes appropriately to maintain the original look.
We present a method for rendering 3D models in the traditional line-drawing style used in artistic and scientific illustrations. The goal is to suggest the 3D shape of the object using a small number of lines drawn with carefully chosen line qualities. The system combines several known techniques into a simple yet effective non-photorealistic line renderer. Feature edges related to the outline and interior of a given 3D mesh are extracted, segmented, and smoothed, yielding chains of lines with varying path, length, thickness, gaps and enclosures. The paper includes sample renderings obtained for a variety of models.
In recent years, several techniques have been proposed for automatically producing line-art illustrations. In this paper a new non photo-realistic rendering scheme for triangulated surfaces is presented. In contrast to prior approaches with parametric surfaces, there is no global parameterization for triangle meshes. So a new approach is made to automatically generate a direction field for the strokes. Discrete curvature analysis on such meshes allows to estimate differential parameters. Lines of curvature are then constructed to be used as strokes. Using triangulated surfaces allows to render aesthetically pleasing line drawings from a huge class of models. Besides, experiments show that even real time visualization is possible.
One of the best choices for fast, high quality shadows is the shadow volume algorithm. However, for real timeapplications the extraction of silhouette edges can significantly burden the CPU, especially with highly tessellatedinput geometry or when complex geometry shaders are applied.
In this paper we show how this last, expensive part of the shadow volume method can be implemented on programmablegraphics hardware. This way, the originally hybrid shadow volumes algorithm can now be reformulatedas a purely hardware-accelerated approach.
The benefits of this implementation is not only the increase in speed. Firstly, all computations now run on thesame hardware resulting in consistent precision within all steps of the algorithm. Secondly, programmable vertextransformations are no longer problematic when applied to shadow casting objects.
Categories and Subject Descriptors (according to ACM CCS): I.3.1 [Computer Graphics]: Hardware Architecture;I.3.3 [Computer Graphics]: Picture/Image Generation; I.3.7 [Computer Graphics]: Three-Dimensional Graphicsand Realism
Stippling is a efficient technique for representing surfaces in pen-and-ink. We present new methods for stipple drawings by computer. In contrast to already existing techniques arbitrary shapes can be used for the dots. An extension of Lloyd's method enables us to position small objects on the plane in a visually pleasing form. This allows us to generate new illustration styles, similar methods can also be used for positioning objects in other applications.
An algorithm to allow real time interactive extraction and orthographic display of silhouettes of complex two-manifold polygonal object(s) is presented. An off-line pre-processing of all the edges of all polygons enables the efficient extraction of the silhouette edges in real time, once a viewing direction is prescribed. During the interactive session, the time complexity of extracting the silhouette edges is linear in the number of edges in the silhouette, and is typically in the order of O(√n), where n is the number of polygons in the scene. The time complexity of the pre-processing stage is linear in n
A technique is presented for line art rendering of scenes composed
of freeform surfaces. The line art that is created for parametric
surfaces is practically intrinsic and is globally invariant to changes
in the surface parameterization. This method is equally applicable for
line art rendering of implicit forms, creating a unified line art
rendering method for both parametric and implicit forms. This added
flexibility exposes a new horizon of special, parameterization
independent, line art effects. Moreover, the production of the line art
illustrations can be combined with traditional rendering techniques such
as transparency and texture mapping. Examples that demonstrate the
capabilities of the proposed approach are presented for both the
parametric and implicit forms
A way to create effective stylized line drawings is to draw strokes that start and stop at visible portions along the silhouette of an object to be portrayed. In computer graphics to date, algorithms to extract silhouette edges are many, although putting these edges into a form such that stylized strokes may be applied to them has not been greatly covered, so that existing methods are either time-consuming or presented vaguely. In this paper, we introduce an algorithm that takes a set of silhouette edges originating from polygonal meshes and efficiently computes the visible parts of the edges before connecting them to form long smooth silhouette strokes to which stylization algorithms may be effectively applied. Features of our algorithm that gain efficiency and accuracy over existing methods is that we directly exploit the analytic connectivity information of the mesh in combination with the availablez-bufferinformation during rendering, and filter artifacts in connected edges during the process to improve the visual quality of strokes after stylization.
Categories and Subject Descriptors (according to ACM CCS):
1.3.3 [Computer Graphics]: Picture/Image Generation—Line and curve generation
1.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism—Hidden line/surface removal
We present a technique for rendering animations in a painterly style. The difficulty in using existing still frame methods for animation is getting the paint to "stick" to surfaces rather than randomly change with each frame, while still retaining a hand-crafted look. We extend the still frame method to animation by solving two major specific problems of previous techniques. First our method eliminates the "shower door" effect in which an animation appears as if it were being viewed through textured glass because brush strokes stick to the viewplane not to the animating surfaces. Second, our technique provides for frame-to-frame coherence in animations so that the resulting frames do not randomly change every frame. To maintain coherence, we model surfaces as 3d particle sets which are rendered as 2d paint brush strokes in screen space much like an artist lays down brush strokes on a canvas. We use geometric and lighting properties of the surfaces to control the appearanceof brush stro...
We present a method for computer generated pen-and-ink illustrations by the simulation of stippling. In a stipple drawing, dots are used to represent tone and also material of surfaces. We create such drawings by generating an initial dot set which is then processed by a relaxation method based on Voronoi diagrams. The point patterns generated are approximations of Poisson disc distributions and can also be used for integrating functions or the positioning of objects. We provide an editor similar to paint systems for interactively creating stipple drawings. This makes it possible to create such drawings within a matter of hours, instead of days or even weeks when the drawing is done manually. 1. Introduction Creating drawings by using mostly dots is a powerful and widely used illustration method. Typically, several tens of thousands of dots are manually arranged to generate a single drawing. A variety of methods for placing dots and controling their size are used to represent d...
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