Curve-skeletons are powerful shape descriptors able to provide higher level information on topology, structure and semantics of a given digital object. Their range of application is wide and encompasses computer animation, shape matching, modelling and remeshing. While a universally accepted definition of curve-skeleton is still lacking, there are currently many algorithms for the curve-skeleton computation (or skeletonization) as well as different techniques for building a mesh around a given curve-skeleton (inverse skeletonization). Despite their widespread use, automatically extracted skeletons usually need to be processed in order to be used in further stages of any pipeline, due to different requirements. We present here an advanced tool, named SkeletonLab, that provides simple interactive techniques to rapidly and automatically edit and repair curve skeletons generated using different techniques proposed in literature, as well as handcrafting them. The aim of the tool is to allow trained practitioners to manipulate the curve-skeletons obtained with skeletonization algorithms in order to fit their specific pipelines or to explore the requirements of newly developed techniques.
ProblemSketch as the most intuitive and powerful 2D design method has been used by artists for decades. However it is not fully integrated into current 3D animation pipeline as the difficulties of interpreting 2D line drawing into 3D. Posing 3D characters from 2D input is a complex and open problem.
Related WorkGuay and colleagues use the line of action to pose the character with a single line to constrain the orientation of the skeleton structure.
Our ApproachWe propose a new sketch based character posing system which is more flexible, efficient and it requires less input from the user. The character can be easily posed no matter the sketch represents a skeleton structure or shape contours.
3D Rigged Character Subset of vertices for matchingInput:-Character model V and its rigging p.-Sampled sketch:-Subset of vertices from the character mesh:including:* the points are around the outlines; * the points are lying close to the projection of the skeleton.To match the two point sets V and Y and meanwhile deform V to Y as closely as possible (finding p), we formulate it as solving the following optimization problem:such that:is the correspondance matrix, where the last row and column are introduced to handle outliers;-the second regularization term is used to add further constraints for searching candidate solutions in limited space;-the third term is used to prevent treating too many points as outliers.The objective function consists of:-a linear discrete assignment problem for correspondence;-a non-linear continuous problem for deformation.We can adopt an alternating strategy to solve the correspondence parameter ω and the rig parameter p:-By fixing p, we relax the binary ω to be a continuous valued matrix in [0, 1] and solve the relaxed subproblem by using Softassign and deterministic annealing.-By fixing ω, the enery function can be solved using a Newton-Raphson scheme.
Results and Future workThe current system solves the posing parameters to match the input sketch and the rigged mesh. We intend to incorporate shape deformation into our energy function in the future, offering a more complete tool for character animation.
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