Journals and Conferences

Copyright Notice The electronic material below is protected by copyright. The author’s version of this material is provided here by permission of ACM or IEEE for your personal and non-commercial use only. Not for redistribution. The definitive versions of the papers and accompanying videos can be downloaded from either the ACM or the IEEE digital library.

LinkEdit: Interactive Linkage Editing using Symbolic Kinematics

M. Bächer, S. Coros, B. Thomaszewski

Proceedings of ACM SIGGRAPH (Los Angeles, USA, August 9-13, 2015), ACM Transactions on Graphics, vol. 34, no. 4.

Abstract We present a method for interactive editing of planar linkages. Given a working linkage as input, the user can make targeted edits to the shape or motion of selected parts while preserving other, e.g., functionally-important aspects. In order to make this process intuitive and efficient, we provide a number of editing tools at different levels of abstraction. For instance, the user can directly change the structure of a linkage by displacing joints, edit the motion of selected points on the linkage, or impose limits on the size of its enclosure. Our method safeguards against degenerate configurations during these edits, thus ensuring the correct functioning of the mechanism at all times. Linkage editing poses strict requirements on performance that standard approaches fail to provide. In order to enable interactive and robust editing, we build on a symbolic kinematics approach that uses closed-form expressions instead of numerical methods to compute the motion of a linkage and its derivatives. We demonstrate our system on a diverse set of examples, illustrating the potential to adapt and personalize the structure and motion of existing linkages. To validate the feasibility of our edited designs, we fabricated two physical prototypes.

Spin-It: Optimizing Moment of Inertia for Spinnable Objects

M. Bächer, E. Whiting, B. Bickel, O. Sorkine-Hornung

Proceedings of ACM SIGGRAPH (Vancouver, CAN, August 10-14, 2014), ACM Transactions on Graphics, vol. 33, no. 4.

Abstract Spinning tops and yo-yos have long fascinated cultures around the world with their unexpected, graceful motions that seemingly elude gravity. We present an algorithm to generate designs for spinning objects by optimizing rotational dynamics properties. As input, the user provides a solid 3D model and a desired axis of rotation. Our approach then modifies the mass distribution such that the principal directions of the moment of inertia align with the target rotation frame. We augment the model by creating voids inside its volume, with interior fill represented by an adaptive multi-resolution voxelization. The discrete voxel fill values are optimized using a continuous, nonlinear formulation. Further, we optimize for rotational stability by maximizing the dominant principal moment. We extend our technique to incorporate deformation and multiple materials for cases where internal voids alone are insufficient. Our method is well-suited for a variety of 3D printed models, ranging from characters to abstract shapes. We demonstrate tops and yo-yos that spin surprisingly stably despite their asymmetric appearance.

Fabricating Articulated Characters from Skinned Meshes

M. Bächer, B. Bickel, D. L. James, H. Pfister

Proceedings of ACM SIGGRAPH (Los Angeles, USA, August 5-9, 2012), ACM Transactions on Graphics, vol. 31, no. 4.

Abstract Articulated deformable characters are widespread in computer animation. Unfortunately, we lack methods for their automatic fabrication using modern additive manufacturing (AM) technologies. We propose a method that takes a skinned mesh as input, then estimates a fabricatable single-material model that approximates the 3D kinematics of the corresponding virtual articulated character in a piecewise linear manner. We first extract a set of potential joint locations. From this set, together with optional, user-specified range constraints, we then estimate mechanical friction joints that satisfy inter-joint non-penetration and other fabrication constraints. To avoid brittle joint designs, we place joint centers on an approximate medial axis representation of the input geometry, and maximize each joint’s minimal cross-sectional area. We provide several demonstrations, manufactured as single, assembled pieces using 3D printers.

Design and Fabrication of Materials with Desired Deformation Behavior

B. Bickel, M. Bächer, M. A. Otaduy, H. R. Lee, H. Pfister, M. Gross, W. Matusik

Proceedings of ACM SIGGRAPH (Los Angeles, USA, July 25-29, 2010), ACM Transactions on Graphics, vol. 29, no. 4.

Abstract This paper introduces a data-driven process for designing and fabricating materials with desired deformation behavior. Our process starts with measuring deformation properties of base materials. For each base material we acquire a set of example deformations, and we represent the material as a non-linear stress-strain relationship in a finite-element model. We have validated our material measurement process by comparing simulations of arbitrary stacks of base materials with measured deformations of fabricated material stacks. After material measurement, our process continues with designing stacked layers of base materials. We introduce an optimization process that finds the best combination of stacked layers that meets a user’s criteria specified by example deformations. Our algorithm employs a number of strategies to prune poor solutions from the combinatorial search space. We demonstrate the complete process by designing and fabricating objects with complex heterogeneous materials using modern multi-material 3D printers.

Capture and Modeling of Non-Linear Heterogeneous Soft Tissue

B. Bickel, M. Bächer, M. A. Otaduy, W. Matusik, H. Pfister, M. Gross

Proceedings of ACM SIGGRAPH (New Orleans, USA, August 3-7, 2009), ACM Transactions on Graphics, vol. 28, no. 3.

Abstract This paper introduces a data-driven representation and modeling technique for simulating non-linear heterogeneous soft tissue. It simplifies the construction of convincing deformable models by avoiding complex selection and tuning of physical material parameters, yet retaining the richness of non-linear heterogeneous behavior. We acquire a set of example deformations of a real object, and represent each of them as a spatially varying stress-strain relationship in a finite-element model. We then model the material by non-linear interpolation of these stress-strain relationships in strain-space. Our method relies on a simple-to-build capture system and an efficient run-time simulation algorithm based on incremental loading, making it suitable for interactive computer graphics applications. We present the results of our approach for several nonlinear materials and biological soft tissue, with accurate agreement of our model to the measured data.

Volume MLS Ray Casting

C. Ledergerber, G. Guennebaud, M. Meyer, M. Bächer, H. Pfister

IEEE Transactions on Visualization and Computer Graphics (Proceedings of Visualization 2008), 14(6):1372-1379, 2008.

Abstract The method of Moving Least Squares (MLS) is a popular framework for reconstructing continuous functions from scattered data due to its rich mathematical properties and well-understood theoretical foundations. This paper applies MLS to volume rendering, providing a unified mathematical framework for ray casting of scalar data stored over regular as well as irregular grids. We use the MLS reconstruction to render smooth isosurfaces and to compute accurate derivatives for high-quality shading effects. We also present a novel, adaptive preintegration scheme to improve the efficiency of the ray casting algorithm by reducing the overall number of function evaluations, and an efficient implementation of our framework exploiting modern graphics hardware. The resulting system enables high-quality volume integration and shaded isosurface rendering for regular and irregular volume data.

Workshops

Hallucination:
A Mixed-Initiative Approach for Efficient Document Reconstruction

H. Zhang, J. K. Lai, M. Bächer

The 4th Human Computation Workshop (HCOMP), 2012.

Abstract We introduce a mixed-initiative approach for document reconstruction that can significantly reduce the amount of time and effort required to reassemble a document from shredded pieces or an artifact from broken fragments. We focus in particular on the hardest subproblem, which is the problem of identifying a matching neighbor for any given piece. Our approach, called hallucination, combines human and machine intelligence by leveraging people’s ability to draw what a neighboring piece may look like, and then using the drawing as a template based on which the computer computes likely matches. Experiments on a puzzle from the DARPA Shredder Challenge demonstrate that the hallucination approach significantly reduces the search space for identifying a match, outperforming humans and computers working in isolation.

Theses

From Digital to Physical: Computational Aspects of 3D Manufacturing

M. Bächer

Ph.D. Thesis, Advisor: H. Pfister, Harvard University, 2013.


Inverse Modeling of (Facial) Contact

M. Bächer

Master Thesis, Advisor: B. Bickel, Supervisor: M. Gross, Swiss Federal Institute of Technology, 2008.


Abstract In this thesis, a novel representation and technique for simulating static non-linear material behavior based on Finite Elements (FE) is presented. All required simulation parameters can be acquired and fitted from a set of example deformations of a real-world object or subject. The simulation is therefore closely related to the person or object specific deformation behavior. We first acquire a single static surface scan and several measurements of static surface displacements by probing an object at many positions and orientations using a force sensor. A trinocular stereo system measures the surface displacements at colored marker locations on the object. The volume of the object is discretized into tetrahedral elements, and for each element and every measurement material parameters are fitted. Our material model consists of material parameters and the corresponding material strain. During run time, we blend these parameters by using a novel strain-based interpolation scheme in material strain space, modeling therefore intuitively the non-linear material stress-strain relationship. Furthermore, since the model is based on a linear deformation FEM, simulations of new interactions are stable and also computationally efficient.

Patents

Method and system for determining poses of semi-specular objects

P. A. Beardsley, M. Bächer

U.S. Patent. Pub. No.: US 2009/0297020 A1, Pub. Date: Dec. 3, 2009.



Unpublished

Copyright Notice The electronic material below is protected by copyright. This material is provided here for your personal and non-commercial use only. Not for redistribution.

A Regression Framework for Image Processing

M. Bächer

Harvard University, 2009.


Abstract In this project, I describe an image processing framework that uses locally weighted least squares regression to denoise, reconstruct and upsample images. Classic, bilateral and robust kernel regression is derived and discussed.