Alla Sheffer

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Recovering 3D shape from concept and pose drawings (2016)

Modern tools to create 3D models are cumbersome and time-consuming. Sketching is a natural way to communicate ideas quickly, and human observers, given a sketch, typically imagine a unique 3D shape; thus, a tool to algorithmically interpret sketches recovering the intended 3D shape would significantly simplify 3D modeling. However, developing such tool is known to be a difficult problem in computer science due to multitude of ambiguities, inaccuracies and incompleteness in the sketches. In this thesis, we introduce three novel approaches in CAD and character modeling that successfully overcome those problems, inferring artist-intended 3D shape from sketches. First, we introduce a system to infer the artist-intended surface of a CAD object from a network of closed 3D curves. Second, we propose a new system for recovering a 3D model of a character, given a single complete drawing and a correspondingly posed 3D skeleton. Finally, we introduce a novel system to pose a 3D character using a single gesture drawing. While developing each system, we derive our key insights from perceptual and artist literature, and confirm our algorithmic choices by various evaluations and comparisons to ground truth data.

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Capturing and modeling of deformable objects (2010)

Modeling the behavior of deformable virtual objects has important applications incomputer graphics. There are two prevalent approaches for modeling deformableobjects, an active one by deforming existing virtual models and a passive one bycapturing the geometry and motion of real objects. This thesis explores the problemof modeling and acquisition of objects undergoing deformations, and proposes aset of practical deformation and capturing tools.The first contribution is a new approach to model deformation that incorporatesnon-uniform materials into the geometric deformation framework. This techniqueprovides a simple and intuitive method to control the deformation using materialproperties that can be specified by the user with an intuitive interface or can belearned from a sequence of sample deformations facilitating realistic looking results.Some deformable objects such as garments exhibit a complex behavior undermotion and thus are difficult to model or simulate, making them suitable targetfor capture methods. Methods for capturing garments usually use special markersprinted on the fabric to establish temporally coherent correspondences betweenframes. Unfortunately, this approach is tedious and prevents the capture of interesting,off-the-shelf fabrics. A marker-free approach to capturing garment motion thatavoids these problems is presented in chapter three. The method establishes temporallycoherent parameterizations between incomplete geometries that are extracted at each time step using a multiview stereo algorithm, and the missing geometry isfilled in using a template.Garment motion is characterized by dynamic high-frequency folds. However,these folds tend to be shallow, making them difficult to capture. A new method forreintroducing folds into the sequence using data-driven dynamic wrinkling is presentedin chapter four. The method first estimates the folds in the video footage andthen wrinkle the surface using space-time deformation. The validity of the methodis demonstrated on several garments captured using several recent techniques.While this markerless reconstruction method is tailored specifically for garments,this thesis also proposes a more general method for reconstructing a consistentframe sequence from a sequence of point clouds captured using multiplevideo streams. The method uses optical flow to guide a local-parameterizationbased cross-parameterization method. This reconstruction method accumulates geometricinformation from all the frames using a novel correction and completionmechanism.

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Master's Student Supervision (2010 - 2018)
FoldSketch : enriching garments with physically reproducible folds (2018)

While folds and pleats add interest to garments and cloth objects, incorporating them into an existing design manually or using existing software requires expertise and time. This thesis presents FoldSketch, a new system that supports simple and intuitive fold and pleat design. FoldSketch users specify the fold or pleat configuration they seek using a simple schematic sketching interface; the system then algorithmically generates both the fold-enhanced 3D garment geometry that conforms to user specifications, and the corresponding 2D patterns that reproduce this geometry within a simulation engine. While previous work aspired to compute the desired patterns for a given target 3D garment geometry, the main algorithmic challenge here is that the target geometry is missing. Real-life garment folds have complex profile shapes, and their exact geometry and location on a garment are intricately linked to a range of physical factors; it is therefore virtually impossible to predict the 3D shape of a fold-enhanced garment using purely geometric means. At the same time, using physical simulation to model folds requires appropriate 2D patterns and initial drape, neither of which can be easily provided by the user.FoldSketch obtains both the 3D fold-enhanced garment and its corresponding patterns and initial drape via an alternating 2D-3D algorithm. We first expand the input patterns by allocating excess material for the expected fold formation; then we use these patterns to produce an estimated fold-enhanced target drape geometry that balances designer expectations against physical reproducibility. Next, we generate an initial reproducible output using the expanded patterns and the estimated target drape as input to a garment simulation engine. Then we improve the output's alignment with designer expectations by progressively refining the patterns and the estimated target drape, converging to a final fully physically reproducible fold-enhanced garment. The experiments confirm that FoldSketch reliably converges to a desired garment geometry and corresponding patterns and drape, and works well with different physical simulators. My collaborators and I demonstrate the versatility of this approach by showcasing a collection of garments augmented with diverse fold and pleat layouts specified via the FoldSketch interface, and further validate this approach via feedback from potential users.

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A method for real-time dynamic cloth wrinkling (2015)

This work presents a novel technique for generating a plausible rest shape to cloth animations which have none, and adding dynamic folds and wrinkles to them inreal-time. The nature of real-time animations makes this task very challenging. The models used are typically very coarse, and the animations used are often nonphysical and exhibit poor temporal and spatial coherence. Since animations can move and deform in non-physical ways, the notion of a valid rest shape or reference shape is not well defined. Instead, we utilize a graph-cut framework to smoothly and consistently measure temporally local deformation in the animation, and use that to construct a per-triangle temporally adaptive pseudo-reference shape. From this shape we compute a stretch tensor field whose eigenvectors can be used to trace plausible dynamic wrinkle paths. We then harness the GPU tessellation unit to refine and deform the cloth along these paths to create wrinkle geometry. Our method runs in real-time on a variety of data sets.

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Augmentation of coarse meshes with wrinkles (2015)

Folds and wrinkles are an important visual cue in the recognition of realistically dressed characters in virtual environments. Wrinkles must, however, move dynamically within the context of an animation to retain much of this realism. Adding wrinkles to real-time cloth visualization proves challenging, as the animations used in games, pre-render visualization, and other such applications, often have no reference shape, an extremely low triangle count, and poor temporal and spatial coherence. I contribute approaches towards the persistence of wrinkles over time, and the creation and rendering of wrinkle geometry in a real-time context, towards a novel real-time method for adding believable, dynamic wrinkles to coarse cloth animations. With this method we trace spatially and temporally coherent wrinkle paths and overcomes the inaccuracies and noise in low-end cloth animation. We employ a two stage stretch tensor estimation process, first detecting regions of consistent surface behaviour, and then using these regions to construct a per-triangle, temporally adaptive reference shape and a stretch tensor based on it. We use this tensor to dynamically generate new wrinkle geometry on coarse cloth meshes through use of the GPU tessellation unit. Our algorithm produces plausible fine wrinkles on real-world data sets at real-time frame rates, and is suitable for the current generation of consoles and PC graphics cards.

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Flowshape : 3d concept modeling from single-view design sketches (2013)

This work presents an automatic creation of 3D models from single view design sketches. We observe that these sketches typically employ networks of trim curves and cross-sections to convey shape. Combined together these curves define the representative flow-lines of the model, reflecting principal curvature and feature lines across the target surface. Artists design these lines to serve as a visual proxy of the 3D object and to effectively convey all surface details. Cross-sections, in particular, are known to impose a set of geometric constraints on the imagined surface, that help position the surface in 3D. We formulate the problem of reconstructing believ- able 3D curve geometry from design sketches via an optimization framework that leverages the geometric properties of flow-line networks, the constraints imposed by cross-section curves, and observations on how people perceive and sketch such networks. This algorithm utilizes these criteria to simultaneously construct the 3D curves and correct for inevitable inaccuracies in free-hand sketches, which if retained would hinder constraint satisfaction and may lead to noticeable artifacts in the reconstructed 3D models. We validate our framework by producing believable 3D curve networks and surfaces from design sketches based on cross-section and trim curves, conducting a qualitative comparison to artist-estimated models, and visual validation by designers.

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Digital micrography (2012)

No abstract available.

Globally consistent space-time reconstruction (2012)

We present a novel algorithm for space-time reconstruction of deforming meshes. Based on partial meshes at every frame, and sparse optical flow information between frames, we reconstruct a globally consistent, crossparameterized, and hole filled sequence of meshes. Our method is based on pair-wise merging of frame sequences while correcting for changes in topology, filling in missing geometry, and repairing inconsistencies. We also introduce a robust method for filling in missing geometry in each frame of the sequence using geometry from another frame. Using this method we can propagate geometry over the full frame sequence, correcting errors and filling in holes even in regions of the object that are not observed in the input meshes for extended periods of time. Unlike other approaches, our method does not require template geometry, nor is it limited to narrow classes of objects or purely isometric deformations.

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Plausible cloth wrinkling for captured mesh sequences (2011)

Cloth modeling has become an important research topic in computer graphics, as garments are ubiquitous in virtual environments used in movies or games. While recent studies can successfully capture the general shape of cloth as well as a continuous cloth motion, they usually lose high frequency details such as wrinkles or folds. This work provides a method of recovering high frequency details for captured cloth animation sequences without any aids of additional information. The method analyzes the stretch/shrinkage tensor fields in 4D space-time domain for the original cloth animation sequences. Based on the assumption of cloth developability, the stretch/shrinkage information is utilized to recover fold shapes and directions. Finally, a space-time consistent deformation method is applied to recover the high frequency folds. The work is generally applicable to most of the cloth capture methods applied either on scanned point clouds or video sequences.

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