Kenshi Takayama

Lapped solid textures: filling a model with anisotropic textures

We present a method for representing solid objects with spatially-varying oriented textures by repeatedly pasting solid texture exemplars. The underlying concept is to extend the 2D texture patch-pasting approach of lapped textures to 3D solids using a tetrahedral mesh and 3D texture patches. The system places texture patches according to the user-defined volumetric tensor fields over the mesh to represent oriented textures. We have also extended the original technique to handle nonhomogeneous textures for creating solid models whose textural patterns change gradually along the depth fields. We identify several texture types considering the amount of anisotropy and spatial variation and provide a tailored user interface for each. With our simple framework, large-scale realistic solid models can be created easily with little memory and computational cost. We demonstrate the effectiveness of our approach with several examples including trees, fruits, and vegetables.

GeoBrush: Interactive Mesh Geometry Cloning

We propose a method for interactive cloning of 3D surface geometry using a paintbrush interface, similar to the continuous cloning brush popular in image editing. Existing interactive mesh composition tools focus on atomic copy‐and‐paste of preselected feature areas, and are either limited to copying surface displacements, or require the solution of variational optimization problems, which is too expensive for an interactive brush interface. In contrast, our GeoBrush method supports real‐time continuous copying of arbitrary high‐resolution surface features between irregular meshes, including topological handles. We achieve this by first establishing a correspondence between the source and target geometries using a novel generalized discrete exponential map parameterization. Next we roughly align the source geometry with the target shape using Green Coordinates with automatically‐constructed cages. Finally, we compute an offset membrane to smoothly blend the pasted patch with C continuity before stitching it into the target. The offset membrane is a solution of a bi‐harmonic PDE, which is computed on the GPU in real time by exploiting the regular parametric domain. We demonstrate the effectiveness of GeoBrush with various editing scenarios, including detail enrichment and completion of scanned surfaces.

Volumetric modeling with diffusion surfaces

The modeling of volumetric objects is still a difficult problem. Solid texture synthesis methods enable the design of volumes with homogeneous textures, but global features such as smoothly varying colors seen in vegetables and fruits are difficult to model. In this paper, we propose a representation called diffusion surfaces (DSs) to enable modeling such objects. DSs consist of 3D surfaces with colors defined on both sides, such that the interior colors in the volume are obtained by diffusing colors from nearby surfaces. A straightforward way to compute color diffusion is to solve a volumetric Poisson equation with the colors of the DSs as boundary conditions, but it requires expensive volumetric meshing which is not appropriate for interactive modeling. We therefore propose to interpolate colors only locally at user-defined cross-sections using a modified version of the positive mean value coordinates algorithm to avoid volumetric meshing. DSs are generally applicable to model many different kinds of objects with internal structures. As a case study, we present a simple sketch-based interface for modeling objects with rotational symmetries that can also generate random variations of models. We demonstrate the effectiveness of our approach through various DSs models with simple non-photorealistic rendering techniques enabled by DSs.

Sketch-based generation and editing of quad meshes

Coarse quad meshes are the preferred representation for animating characters in movies and video games. In these scenarios, artists want explicit control over the edge flows and the singularities of the quad mesh. Despite the significant advances in recent years, existing automatic quad remeshing algorithms are not yet able to achieve the quality of manually created remeshings. We present an interactive system for manual quad remeshing that provides the user with a high degree of control while avoiding the tediousness involved in existing manual tools. With our sketch-based interface the user constructs a quad mesh by defining patches consisting of individual quads. The desired edge flow is intuitively specified by the sketched patch boundaries, and the mesh topology can be adjusted by varying the number of edge subdivisions at patch boundaries. Our system automatically inserts singularities inside patches if necessary, while providing the user with direct control of their topological and geometrical locations. We developed a set of novel user interfaces that assist the user in constructing a curve network representing such patch boundaries. The effectiveness of our system is demonstrated through a user evaluation with professional artists. Our system is also useful for editing automatically generated quad meshes.

A procedural method for modeling the purkinje fibers of the heart.

The Purkinje fibers are located in the ventricular walls of the heart, just beneath the endocardium and conduct excitation from the right and left bundle branches to the ventricular myocardium. Recently, anatomists succeeded in photographing the Purkinje fibers of a sheep, which clearly showed the mesh structure of the Purkinje fibers. In this study, we present a technique for modeling the mesh structure of Purkinje fibers semiautomatically using an extended L-system. The L-system is a formal grammar that defines the growth of a fractal structure by generating rules (or rewriting rules) and an initial structure. It was originally formulated to describe the growth of plant cells, and has subsequently been applied for various purposes in computer graphics such as modeling plants, buildings, streets, and ornaments. For our purpose, we extended the growth process of the L-system as follows: 1) each growing branch keeps away from existing branches as much as possible to create a uniform distribution, and 2) when branches collide, we connect the colliding branches to construct a closed mesh structure. We designed a generating rule based on observations of the photograph of Purkinje fibers and manually specified three terminal positions on a three-dimensional (3D) heart model: those of the right bundle branch, the anterior fascicle, and the left posterior fascicle of the left branch. Then, we grew fibers starting from each of the three positions based on the specified generating rule. We achieved to generate 3D models of Purkinje fibers of which physical appearances closely resembled the real photograph. The generation takes a few seconds. Variations of the Purkinje fibers could be constructed easily by modifying the generating rules and parameters.

Real-time example-based elastic deformation

We present an example-based elastic deformation method that runs in real time. Example-based elastic deformation was originally presented by Martin et al. [MTGG11], where an artist can intuitively control elastic material behaviors by simply giving example poses. Their FEM-based approach is, however, computationally expensive requiring nonlinear optimization, which hinders its use in real-time applications such as games. Our contribution is to formulate an analogous concept using the shape matching framework, which is fast, robust, and easy to implement. The key observation is that each overlapping local regions right stretch tensor obtained by polar decomposition is a natural choice for a deformation descriptor. This descriptor allows us to represent the pose space as a linear blending of examples. At each time step, the current deformation descriptor is linearly projected onto the example manifold, and then used to modify the rest shape of each local region when computing goal positions. Our approach is two orders of magnitude faster than Martin et al.s approach while producing comparable example-based elastic deformations.

Pattern-Based Quadrangulation for N-Sided Patches

We propose an algorithm to quadrangulate an N‐sided patch (2 ≤ N ≤ 6) with prescribed numbers of edge subdivisions at its boundary. Our algorithm is guaranteed to succeed for arbitrary valid input, which is proved using a canonical simplification of the input and a small set of topological patterns that are sufficient for supporting all possible cases. Our algorithm produces solutions with minimal number of irregular vertices by default, but it also allows the user to choose other feasible solutions by solving a set of small integer linear programs. We demonstrate the effectiveness of our algorithm by integrating it into a sketch‐based quad remeshing system. A reference C++ implementation of our algorithm is provided as a supplementary material.

A Sketch-Based Interface for Modeling Myocardial Fiber Orientation that Considers the Layered Structure of the Ventricles

We propose a sketch-based interface for modeling the myocardial fiber orientation required in the electrophysiological simulation of the heart, especially the ventricles. The user can create a volumetric vector field that represents the myocardial fiber orientation in two steps. First, a depth field over the three-dimensional (3D) ventricular model is defined to create layers of myocardium. The user can then peel these layers and draw strokes on them to specify the myocardial fiber orientation in each layer. We represent the 3D ventricular model as a tetrahedral mesh and perform Laplacian smoothing over the mesh vertices to interpolate the vector field defined by the user-drawn strokes. Our method also allows the user to perform deformations on volumetric models of myocardial fiber orientation, which is very important for studying heart disease associated with morphological abnormalities. We created several examples of myocardial fiber orientation and applied them to a simplified simulator to demonstrate the effectiveness of our method.

ProcDef: Local‐to‐global Deformation for Skeleton‐free Character Animation

Animations of characters with flexible bodies such as jellyfish, snails, and, hearts are difficult to design using traditional skeleton‐based approaches. A standard approach is keyframing, but adjusting the shape of the flexible body for each key frame is tedious. In addition, the character cannot dynamically adjust its motion to respond to the environment or user input. This paper introduces a new procedural deformation framework (ProcDef) for designing and driving animations of such flexible objects. Our approach is to synthesize global motions procedurally by integrating local deformations. ProcDef provides an efficient design scheme for local deformation patterns; the user can control the orientation and magnitude of local deformations as well as the propagation of deformation signals by specifying line charts and volumetric fields. We also present a fast and robust deformation algorithm based on shape‐matching dynamics and show some example animations to illustrate the feasibility of our framework.

Light shower: a poor man's light stage built with an off-the-shelf umbrella and projector

Compositing an actor or real-world object into a virtual background is a powerful and widely used tool in movic and TV production. To create a natural composite, it is necessary to maintain photometric consistency between the foreground object and the background environment. Various light stage systems have been developed to achieve this goal. Debevec et al. [Debevec et al. 2002] illuminated the actor by an array of inward-pointing RGB light-emitting diodes, and Mitsumine et al. [Mitsumine et al. 2005] surrounded the actor with back-projection screens and projected virtual images onto these screens. One problem with both of these strategies is that the systems involved are very expensive and time-consuming to build. We propose an inexpensive light stage system, Light Shower, which consists of an off-the-shelf projector and a white umbrella. The projector projects an image onto the white umbrella, which creates environment light for a human face or a real object inside the umbrella (see Figure 1).