Author Morten Engell-Nørregård
This PhD thesis concerns itself with modelling and simulation of human motion. The research subjects in this thesis have at least two things in common.
First, they are concerned with Human Motion. Even though the models may be used for other things as well, the main focus is on modelling the human body.
Second, they are all concerned with simulation as a tool to synthesize motion and thus, get animations. This is an important point since it means we are not only creating tools for animators to make fun and interesting animations, but also models for simulation of realistic motion. As the project progressed, the focus has shifted from purely graphics oriented to something which may be at least as interesting for the biomechanics community:
The main scientific contributions of this work are:
• An efficient method for solving interactive constrained inverse kinematics problems, using an optimization based approach. The method is usable for motion planning of complex articulated mechanisms, with a large degree of interdependency between different parts such as a human body.
• A general and fast joint constraint model. The joint constraint model is well suited for modelling joints with highly non-convex limits and multiple degrees of freedom. Even though this constraint model may have many other uses we believe it is very well suited for the modelling of human joints which exhibit both non-convexity and multiple degrees of freedom.
• A general and versatile model for activating soft bodies. The model may be used as an animation tool but would be equally well suited for simulation of human muscles since it adheres to basic physical principles. Further, it can be used with any softbody simulation method such as finite elements or mass spring systems.
• A control method for deformable bodies based on the space time optimization. The method may be used to control the contraction of muscles in a muscle simulation.
Projected Linesearch Video
Distance cones Video
Spline activation video
Thesis (pdf) Download
Authors Morten Engell-Nørregård and Kenny Erleben
We present a method for simulating the active contraction of deformable models, usable for interactive animation of soft deformable objects.
We present a novel physical principle as the governing equation for the coupling between the low dimensional 1D activation force model and the higher dimensional 2D/3D deformable model.
Our activation splines are easy to set up and can be used for physics based animation of deformable models such as snake motion and locomotion of characters. Our approach generalises easily to both 2D and 3D simulations and is applicable in physics based games or animations due to its simplicity and very low computational cost.
Videos Video available Here
Poster (pdf) Download
Marek. K. Misztal, Kenny Erleben, Adam Bargteil, Jens Fursund, Brian Bunch Christensen, J. Andreas. Bærentzen and Robert Bridson
In this paper, we present a method for animating multiphase flow of immiscible fluids using unstructured moving meshes. Our underlying discretization is an unstructured tetrahedral mesh, the deformable simplicial complex (DSC), that moves with the flow in a Lagrangian manner. Mesh optimization operations improve element quality and avoid element inversion. In the context of multiphase flow, we guarantee that every element is occupied by a single fluid and, consequently, the interface between fluids is represented by a set of faces in the simplicial complex. This approach ensures that the underlying discretization matches the physics and avoids the additional book-keeping required in grid-based methods where multiple fluids may occupy the same cell. Our Lagrangian approach naturally leads us to adopt a finite element approach to simulation, in contrast to the finite volume approaches adopted by a majority of fluid simulation techniques that use tetrahedral meshes. We characterize fluid simulation as an optimization problem allowing for full coupling of the pressure and velocity fields and the incorporation of a second-order surface energy. We introduce a preconditioner based on the diagonal Schur complement and solve our optimization on the GPU. We provide the results of parameter studies as well as a performance analysis of our method.
Videos Available from here
SCA best paper award
Authors Jan Bender, Kenny Erleben, Jeff Trinkle, and Erwin Coumans
Interactive rigid body simulation is an important part of many modern computer tools. No authoring tool nor a game engine can do without. The high performance computer tools open up new possibilities for changing how designers, engineers, modelers and animators work with their design problems. This paper is a self contained state-of-the-art report on the physics, the models, the numerical methods and the algorithms used in interactive rigid body simulation all of which has evolved and matured over the past 20 years. The paper covers applications and the usage of interactive rigid body simulation.
Besides the mathematical and theoretical details that this paper communicates in a pedagogical manner the paper surveys common practice and reflects on applications of interactive rigid body simulation. The grand merger of interactive and off-line simulation methods is imminent, multi-core is everyman’s property. These observations pose future challenges for research which we reflect on. In perspective several avenues for possible future work is touched upon such as more descriptive models and contact point generation problems. This paper is not only a stake in the sand on what has been done, it also seeks to give newcomers practical hands on advices and reflections that can give experienced researchers afterthought for the future.
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Paper download This is the author’s version of the work.
Authors Morten Engell-Nørregård, Sarah Niebe and Kenny Erleben
Abstract We present a local joint-constraint model for a single joint which is based on distance fields. Our model is fast, general, and well suited for modeling human joints. In this work, we take a geometric approach and model the geometry of the boundary of the feasible region, i.e., the boundary of all allowed poses. A region of feasible poses can be built by embedding motion captured data points in a signed distance field. The only assumption is that the feasible poses form a single connected set of angular values. We show how signed distance fields can be used to generate fast and general joint-constraint models for kinematic figures. Our model is compared to existing joint-constraint models, both in terms of generality and computational cost.
The presented method supports joint-constraints of up to three degrees of freedom and works well with sampled motion data. Our model can be extended to handle inter-joint dependencies, or joints with more than three degrees of freedom. The resolution of the joint-constraints can be tweaked individually for each degree of freedom, which can be used to optimize memory usage. We perform a comparative study of the key-properties of various joint-constraint models, as well as a performance study of our model compared to the fastest alternative, the box limit model. The study is performed on the shoulder joint, using a motion captured jumping motion as reference.
Authors Kenny Erleben, Marek Krzysztof Misztal, and J. Andreas Bærentzen
We present the mathematical foundation of a fluid animation method for unstructured meshes. Key contributions not previously treated are the extension to include diffusion forces and higher order terms of non-linear force approximations. In our discretization we apply a fractional step method to be able to handle advection in a numerically simple Lagrangian approach. Following this a finite element method is used for the remaining terms of the fractional step method. The key to deriving a discretization for the diffusion forces lies in restating the momentum equations in terms of a Newtonian stress tensor. Rather than applying a straightforward temporal finite difference method followed by a projection method to enforce incompressibility as done in the stable fluids method, the last step of the fractional step method is rewritten as an optimization problem to make it easy to incorporate non-linear force terms such as surface tension.
Videos N/A (yet)
Paper download ACM, (2011) This is the author’s version of the work. It is posted here by permission of ACM for your personal use. Not for redistribution. The definite version will be published in Proceedings of the 2011 ACM SIGGRAPH/Eurographics Symposium on Computer Animation (2011).