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.

Spin-It: Optimizing Moment of Inertia for Spinnable Objects

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

Communications of the ACM, vol. 60, no. 8, 2017.


Abstract Spinning tops and yo-yos have long fascinated cultures around the world with their unexpected, graceful motions that seemingly elude gravity. Yet, due to the exceeding difficulty of creating stably spinning objects of asymmetric shape in a manual trial-and-error process, there has been little departure from rotationally symmetric designs. With modern 3D printing technologies, however, we can manufacture shapes of almost unbounded complexity at the press of a button, shifting this design complexity toward computation.
In this article, we describe an algorithm to generate designs for spinning objects by optimizing their mass distribution: as input, the user provides a solid 3D model and a desired axis of rotation. Our approach then modifies the interior mass distribution such that the principal directions of the moment of inertia align with the target rotation frame. To create voids inside the model, we represent its volume with an adaptive multiresolution voxelization and optimize the discrete voxel fill values using a continuous, nonlinear formulation. We further optimize for rotational stability by maximizing the dominant principal moment. 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.

A Computational Design Tool for Compliant Mechanisms

V. Megaro, J. Zehnder, M. Bächer, S. Coros, M. Gross, B. Thomaszewski

Proceedings of ACM SIGGRAPH (Los Angeles, USA, July 30-August 3, 2017), ACM Transactions on Graphics, vol. 36, no. 4.

Abstract We present a computational tool for designing compliant mechanisms. Our method takes as input a conventional, rigidly-articulated mechanism defining the topology of the compliant design. This input can be both planar or spatial, and we support a number of common joint types which, whenever possible, are automatically replaced with parameterized flexures. As the technical core of our approach, we describe a number of objectives that shape the design space in a meaningful way, including trajectory matching, collision avoidance, lateral stability, resilience to failure, and minimizing motor torque. Optimal designs in this space are obtained as solutions to an equilibrium-constrained minimization problem that we solve using a variant of sensitivity analysis. We demonstrate our method on a set of examples that range from simple four-bar linkages to full-fledged animatronics, and verify the feasibility of our designs by manufacturing physical prototypes.

Designing Cable-Driven Actuation Networks for Kinematic Chains and Trees

V. Megaro, E. Knoop, A. Spielberg, D.I.W. Levin, W. Matusik, M. Gross, B. Thomaszewski, M. Bächer

Proceedings of the ACM SIGGRAPH / Eurographics Symposium on Computer Animation (SCA, Los Angele, USA, July 28-30, 2017).

Abstract In this paper, we present an optimization-based approach for the design of cable-driven kinematic chains and trees. Our system takes as input a hierarchical assembly consisting of rigid links jointed together with hinges. The user also specifies a set of target poses or keyframes using inverse kinematics. Our approach places torsional springs at the joints and computes a cable network that allows us to reproduce the specified target poses. We start with a large set of cables that have randomly chosen routing points and we gradually remove the redundancy. Then we refine the routing points taking into account the path between poses or keyframes in order to further reduce the number of cables and minimize required control forces. We propose a reduced coordinate formulation that links control forces to joint angles and routing points, enabling the co-optimization of a cable network together with the required actuation forces. We demonstrate the efficacy of our technique by designing and fabricating a cable-driven, animated character, an animatronic hand, and a specialized gripper.

Enriching Facial Blendshape Rigs with Physical Simulation

Y. Kozlov, D. Bradley, M. Bächer, B. Thomaszewski, T. Beeler, M. Gross

Proceedings of Eurographics (Lyon, France, April 24-28, 2017), Computer Graphics Forum, vol. 36, no. 2.


Abstract Oftentimes facial animation is created separately from overall body motion. Since convincing facial animation is challenging enough in itself, artists tend to create and edit the face motion in isolation. Or if the face animation is derived from motion capture, this is typically performed in a mo-cap booth while sitting relatively still. In either case, recombining the isolated face animation with body and head motion is non-trivial and often results in an uncanny result if the body dynamics are not properly reflected on the face (e.g. the bouncing of facial tissue when running).
We tackle this problem by introducing a simple and intuitive system that allows to add physics to facial blendshape animation. Unlike previous methods that try to add physics to face rigs, our method preserves the original facial animation as closely as possible. To this end, we present a novel simulation framework that uses the original animation as per-frame rest-poses without adding spurious forces. As a result, in the absence of any external forces or rigid head motion, the facial performance will exactly match the artist-created blendshape animation. In addition, we propose the concept of blendmaterials to give artists an intuitive means to account for changing material properties due to muscle activation. This system allows to automatically combine facial animation and head motion such that they are consistent while preserving the original animation as closely as possible. The system is easy to use and readily integrates with existing animation pipelines.

DefSense: Computational Design of Customized Deformable Input Devices

M. Bächer, B. Hepp, F. Pece, P. G. Kry, B. Bickel, B. Thomaszewski, O. Hilliges

Proceedings of ACM CHI (San Jose, USA, May 7-12, 2016), ACM Transactions on Computer-Human Interaction.

Abstract We propose to embed piezoresistive sensing elements into flexible 3D printed objects. These sensing elements are then utilized to recover rich and natural user interactions at runtime. Designing such objects manually is a challenging and hard problem for all but the simplest geometries and deformations. Our method simultaneously optimizes the internal routing of the sensing elements and computes a mapping from low-level sensor readings to user-specified outputs in order to minimize reconstruction error. We demonstrate the power and flexibility of the approach by designing and fabricating a set of flexible input devices. Our results indicate that the optimization based design greatly outperforms manual routings in terms of reconstruction accuracy and thus interaction fidelity.

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

Contact Pressure Distribution as an Evaluation Metric for Human-Robot Hand Interactions

E. Knoop, M. Bächer, P. Beardsley

Proceedings of International Workshop on Reproducible HRI Experiments (ReHRI), 2017.


Abstract Soft robotic technologies are paving way for physical human-robot hand interactions, creating a need for structured evaluation metrics for robot hands. We propose that the contact pressure distribution of the grasp should be used as a hand benchmark both for naturalness and comfort, and present our initial work in this direction. We describe an experimental setup for measuring the contact pressure distribution and present a case study comparing the pressure distributions from a robotic hand and a human hand. The grasping force of the human hand is ten times greater than the robot, but the robot hand produces higher peak contact pressures and smaller contact areas.

Balancing 3D Models with Movable Masses

R. Prévost, M. Bächer, W. Jarosz, O. Sorkine-Hornung

Vision, Modeling, and Visualization (VMV), 2016.


Abstract We present an algorithm to balance 3D printed models using movable embedded masses. As input, the user provides a 3D model together with the desired suspension, standing, and immersion objectives. Our technique then determines the placement and suitable sizing of a set of hollow capsules with embedded metallic spheres, leveraging the resulting multiple centers of mass to simultaneously satisfy the combination of these objectives. To navigate the non-convex design space in a scalable manner, we propose a heuristic that leads to near-optimal solutions when compared to an exhaustive search. Our method enables the design of models with complex and surprising balancing behavior, as we demonstrate with several manufactured examples.

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.

A Lattice Boltzmann Simulation of Hemodynamics in a Patient-Specific Aortic Coarctation Model

A. Peters Randles, M. Bächer, H. Pfister, E. Kaxiras

Statistical Atlases and Computational Models of the Heart (STACOM), 2012.

Abstract In this paper, we propose a system to determine the pressure gradient at rest in the aorta. We developed a technique to efficiently initialize a regular simulation grid from a patient-specific aortic triangulated model. On this grid we employ the lattice Boltzmann method to resolve the characteristic fluid flow through the vessel. The inflow rates, as measured physiologically, are imposed providing accurate pulsatile flow. The simulation required a resolution of at least 20 microns to ensure a convergence of the pressure calculation. HARVEY, a large-scale parallel code, was run on the IBM Blue Gene/Q supercomputer to model the flow at this high resolution. We analyze and evaluate the strengths and weaknesses of our system.

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

Articulated Character Fabrication

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

U.S. Patent. Pub. No.: US 2015/0187134, Pub. Date: July, 2, 2015.


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.