Character animation : constant volume skinning

Authors: Damien Rohmer, Stefanie Hahmann, Marie-Paule Cani

Contribution 1: Local volume preservation for skinned characters

Local volume preservation for skinned characters
D. Rohmer, S. Hahmann, M.-P. Cani
Computer Graphics Forum (Proceedings of Pacific Graphics 2008), Volume 27, Number 7, 2008   

Generating plausible deformations of a character skin within the standard production pipeline is a challenge. This paper presents a volume preservation method dedicated to skinned characters. As usual, the character is defined by a skin mesh at some rest pose and an animation skeleton. At each animation step, skin deformations are first computed using standard SSD. Our method then corrects the result using a set of local deformations which model the fold-over-free, constant volume behavior of soft tissues. This is done geometrically, without the need of any physically-based simulation. To make the method easily applicable, we also provide automatic ways to extract the local regions where volume is to be preserved and to compute adequate skinning weights, both based on the character's morphology.

Illustration of our method in a complex case. a) Input data: skinned mesh and skeleton.
b) Automatic segmentation. c) Standard SSD.
d) Our method where the volume is locally preserved (see belly and trunk).


Results for the organic effect. First line: Classical SSD.
Second line: Our organic effect, the left picture illustrates the gamma-intensity in red
(note that gamma values are shown for each local region).

Illustration of the rubber effect.
a) & b) Deformation with classical SSD.
c) gamma-map for the rubber effect.
d) & e) Our rubbery effect.
Local volume preservation for skinned characters f) Real rubber giraf toy with bent neck.

Contribution 2: Exact volume preserving skinning with shape control

D. Rohmer, S. Hahmann, M.-P. Cani
Exact volume preserving skinning with shape control
ACM/EG Symposium on Computer Animation (SCA) August, 2009. New Orleans.   

Constant volume deformation of the belly, modeled as a two folds bulge.
The cross-section views depicted at the bottom are cut along the left legs.

In the real world, most objects do not loose volume when they deform: they may for instance compensate a local compression by inflating in the orthogonal direction, or, in the case of a character, preserve volume through specific bulges and folds. This paper presents a novel extension to smooth skinning, which not only offers an exact control of the object volume, but also enables the user to specify the shape of volume-preserving deformations through intuitive 1D profile curves. The method, a geometric post-processing to standard smooth skinning, perfectly fits into the usual production pipeline. It can be used whatever the desired locality of volume correction and does not bring any constraint on the original mesh. Several behaviors mimicking the way rubber-like materials and organic shapes respectively deform can be modeled. An improved algorithm for robustly computing skinning weights is also provided, making the method directly usable on complex characters, even for non-experts.


Various profiles can be used to preserve volume while offering some local shape control.
a) SSD reference.
b) Isotropic inflation (0,12) = (1,1,1)/3.
c) Rubber effect (0,1,2) = (0,1,0).
d) Biceps-like muscle (0,12) = (0.1,0.1,0.8).
e) Folds effect (0,12) = (0.0,0.5,0.5). The curves represents the g distribution multiplied by the values.

Complete pipe-line of skinning weights computation. Left: Initial mesh + skeleton. Second picture: Voxelized domain bounded by the mesh. Third picture: Color encoded result after volumetric distance propagation (only the main influencing bone is shown). Right: Final result after surfacic diffusion.

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