John F. Hughes

As-rigid-as-possible shape manipulation

We present an interactive system that lets a user move and deform a two-dimensional shape without manually establishing a skeleton or freeform deformation (FFD) domain beforehand. The shape is represented by a triangle mesh and the user moves several vertices of the mesh as constrained handles. The system then computes the positions of the remaining free vertices by minimizing the distortion of each triangle. While physically based simulation or iterative refinement can also be used for this purpose, they tend to be slow. We present a two-step closed-form algorithm that achieves real-time interaction. The first step finds an appropriate rotation for each triangle and the second step adjusts its scale. The key idea is to use quadratic error metrics so that each minimization problem becomes a system of linear equations. After solving the simultaneous equations at the beginning of interaction, we can quickly find the positions of free vertices during interactive manipulation. Our approach successfully conveys a sense of rigidity of the shape, which is difficult in space-warp approaches. With a multiple-point input device, even beginners can easily move, rotate, and deform shapes at will.

Direct manipulation of free-form deformations

Free-form deformation (FFD) is a powerful modeling tool, but controlling the shape of an object under complex deformations is often difficult. The interface to FFD in most conventional systems simply represents the underlying mathematics directly; users describe deformations by manipulating control points. The difficulty in controlling shape precisely is largely due to the control points being extraneous to the object; the deformed object does not follow the control points exactly. In addition, the number of degrees of freedom presented to the user can be overwhelming. We present a method that allows a user to control a free-form deformation of an object by manipulating the object directly, leading to better control of the deformation and a more intuitive interface. CR Categories: I.3.5 [Computer Graphics]: Computational Geometry and Object Modeling Curve, Surface, Solid, and Object Representations; I.3.6 [Computer Graphics]: Methodology and Techniques Interaction Techniques. Additional

Real-time nonphotorealistic rendering

Nonphotorealistic rendering (NPR) can help make comprehensible but simple pictures of complicated objects by employing an economy of line. But current nonphotorealistic rendering is primarily a batch process. This paper presents a real-time nonphotorealistic renderer that deliberately trades accuracy and detail for speed. Our renderer uses a method for determining visible lines and surfaces which is a modification of Appel’s hidden-line algorithm, with improvements which are based on the topology of singular maps of a surface into the plane. The method we describe for determining visibility has the potential to be used in any NPR system that requires a description of visible lines or surfaces in the scene. The major contribution of this paper is thus to describe a tool which can significantly improve the performance of these systems. We demonstrate the system with several nonphotorealistic rendering styles, all of which operate on complex models at interactive frame rates. CR

Orientable textures for image-based pen-and-ink illustration

We present an interactive system for creating pen-and-ink-style line drawings from greyscale images in which the strokes of the rendered illustration follow the features of the original image. The user, via new interaction techniques for editing a direction field, specifies an orientation for each region of the image; the computer draws oriented strokes, based on a user-specified set of example strokes, that achieve the same tone as the image via a new algorithm that compares an adaptively-blurred version of the current illustration to the target tone image. By aligning the direction field with surface orientations of the objects in the image, the user can create textures that appear attached to those objects instead of merely conveying their darkness. The result is a more compelling pen-and-ink illustration than was previously possible from 2D reference imagery. CR

Multiperspective panoramas for cel animation

We describe a new approach for simulating apparent camera motion through a 3D environment. The approach is motivated by a traditional technique used in 2D cel animation, in which a single background image, which we call a multiperspective panorama, is used to incorporate multiple views of a 3D environment as seen from along a given camera path. When viewed through a small moving window, the panorama produces the illusion of 3D motion. In this paper, we explore how such panoramas can be designed by computer, and we examine their application to cel animation in particular. Multiperspective panoramas should also be useful for any application in which predefined camera moves are applied to 3D scenes, including virtual reality fly-throughs, computer games, and architectural walk-throughs. CR Categories: I.3.3 [Computer Graphics]: Picture/Image Generation. Additional

Scheduled Fourier volume morphing

We describe an easily implemented and computationally feasible method for smoothly transitioning from one sampled volumetric model to another. This induces a transition between isosurfaces of the two models. The technique is based on interpolating smoothly between the Fourier transforms of the two volumetric models and then transforming the results back. A linear interpolation between the transformed datasets yields unsatisfactory results in some cases. We use a schedule for the interpolation in which the high frequencies of the first model are gradually removed, the low frequencies are interpolated to those of the second, and the high frequencies of the second model are gradually added in. Such scheduling yields more satisfactory results. We give several examples and comment briefly on preprocessing models to make the morphing smoother.

Modeling surfaces of arbitrary topology using manifolds

Manifolds describe complicated objects that are locally

Defocus video matting

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Art-based rendering of fur, grass, and trees

by defining a set of overlapping maps from the object to

Smooth interpolation of orientations with angular velocity constraints using quaternions

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