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Transposition Based Blendshape Direct Manipulation
Swaps the pseudo-inverse at the heart of blendshape direct manipulation for a transposition step, mapping dragged points onto the deformation directions to stop the unexpected jumps.
Abstract
The blendshape approach is a predominant technique for creating high quality facial animation. Facial poses are generated by altering the corresponding weight parameters manually for each key-frame by using traditional slider interfaces. However, authoring a production quality facial animation with this process requires time-consuming, labor intensive and iterative work from the artists. Direct manipulation interfaces address this problem with a pin-and-drag operation inspired by the inverse kinematics approaches. The mathematical frameworks of the direct manipulation techniques are mostly based on the pseudo-inverse of the blendshape matrices which include all target shapes' vertex positions. However, the pseudo-inverse approaches often give unexpected results during the facial pose editing process because of unstable behavior. To this end, we propose the transposition approach to enhance direct manipulation by reducing unexpected movements during weight editing. Our approach extracts the deformation directions from the blendshape matrix, and directly maps the sparse constrained point movements to the extracted directions. Our experiments show that, instead of pseudo-inverse based formulations, a transposition based framework gives smoother and more reliable facial poses during the weight editing process. The proposed approach improves the fidelity of the generated facial expressions by keeping hazardous movements to a minimum. It is robust, efficient, easy to implement and operates on any blendshape model.
How to read this
- Category
- Conference paper on the numerics of blendshape direct manipulation.
- Contributions
- A diagnosis: existing direct manipulation methods invert the blendshape matrix with a pseudo-inverse, and that is what makes editing unstable.
- A transposition based alternative that extracts deformation directions from the blendshape matrix and maps dragged points onto them.
- Experiments reporting smoother and more reliable poses than the pseudo-inverse formulation.
- A method presented as robust, easy to implement, and applicable to any blendshape model.
- Context
- The direct ancestor is lewis-direct-manipulation-blendshapes-2010, which established the pin-and-drag idea for blendshapes and whose pseudo-inverse this paper is explicitly arguing against. Lewis co-authors here, which is worth noticing: this is the same researcher revisiting his own formulation. The result is extended three years later in cetinaslan-jacobian-transpose-2020.Builds on: Direct Manipulation Blendshapes
- Correctness
- The argument is a familiar one from inverse kinematics, where transpose methods trade exactness for stability, and the same trade applies here. The transpose does not solve the constraint, it descends toward it, so pins are satisfied approximately. Whether that matters depends on your use, and the paper's own comparisons are the place to judge it. The evaluation is qualitative, on the authors' own examples.
- Clarity
- Clear if you are comfortable with the blendshape matrix formulation and with pseudo-inverse versus transpose in inverse kinematics. Read lewis-anjyo-blendshape-star-2014 first if you are not.
- How to read it
- First pass: abstract and the comparison figures against the pseudo-inverse. Second pass: the derivation of the deformation directions, which is the core. Third pass: skip it and read cetinaslan-jacobian-transpose-2020 instead, which is the same argument with the hybrid and the locality term added.
Builds on
Related work
- Stabilized blendshape editing using localized Jacobian transpose descent 2020 / GMOD
- Position Manipulation Techniques for Facial Animation 2016 / PhD Thesis
- Sketch-Based Controllers for Blendshape Facial Animation 2015 / Eurographics
- Direct manipulation of blendshapes using a sketch-based interface 2018 / Web3D
Keywords
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