Method for skinning technology based on extended position based dynamics and for weight retargeting in character animation

Patent metadata

Field	Value
Publication number	US-10275922-B2
Application number	US-201715711954-A
Country	US
Kind code	B2
Filing date	Sep 21, 2017
Priority date	Dec 26, 2016
Publication date	Apr 30, 2019
Grant date	Apr 30, 2019

Abstract

Disclosed relates to a method for skinning technology based on extended PBD and for weight retargeting in character animation. A corresponding tetrahedral mesh is generated for a given triangular mesh; a tensile constraint, a volume constraint, a self-collision constraint and an energy constraint of the extended PBD are initialized for the tetrahedral mesh, and vertex weights of the triangular mesh and the tetrahedral mesh are optimized simultaneously; the deformations of the triangular and the tetrahedral meshes are implemented through LBS technique; deformation results of the LBS are optimized through extended PBD; the joint region of the triangular mesh is smoothed through a Laplacian smoothing; bi-harmonic distance fields of the original mesh and a new mesh are calculated respectively, and two corresponding vertices of the mesh are given and a prediction function is defined; A weight retargeting algorithm is implemented in the present application and has a good practicability.

First claim

What is claimed is:

1. A method for skinning technology based on extended position based dynamics and for weight retargeting in character animation applied on a computer system, comprising the following steps:
step (1), according to an input triangular mesh, generating, by a processor of the computer system, a tetrahedral mesh corresponding to the triangular mesh, initializing a tensile constraint, a volume constraint, a self-collision constraint and an energy constraint of an extended position based dynamics (PBD) model for the tetrahedral mesh, and calculating and optimizing vertex weights of the triangular mesh and the tetrahedral mesh according to a corresponding skeleton model;
step (2), according to the vertex weights obtained in step (1), simultaneously calculating, by the processor of the computer system, transformed vertex positions of the triangular mesh and the tetrahedral mesh by using LBS (linear blend skinning) in a process of a skeletal movement to implement deformations of the triangular mesh and the tetrahedral mesh;
step (3), calculating by the processor of the computer system, the transformed vertex positions of the tetrahedral mesh through a constraint on the PBD defined by an iterative solution, and updating a deformation effect of the triangular mesh through a mapping relation between the tetrahedral mesh and the triangular mesh;
step (4), conducting, by the processor of the computer system, a Laplacian smoothing on a joint region of the triangular mesh, of which the deformation effect is currently updated in step (3), to obtain a real and natural triangular mesh deformation effect;
step (5), taking, by the processor of the computer system, the triangular mesh in step (1) as an original mesh, giving another triangular mesh having a mesh topology similar to that of the original mesh as a new mesh, calculating a bi-harmonic distance field of the original mesh and the new mesh, respectively, selecting a set of corresponding reference vertices between the original mesh and the new mesh, re-expressing all vertices on the original mesh in a form of distance vectors associated with vertices selected as the reference vertices on the original mesh, and defining a prediction function for weight retargeting calculation;
step (6), according to the bi-harmonic distance field obtained in step (5), for any vertex on a new mesh, obtaining, by the processor of the computer system through minimizing the prediction function, a corresponding vertex on the original mesh to complete a retargeting of the vertex weight from the original mesh to the new mesh.

2. The method for skinning technology based on extended position based dynamics and for weight retargeting in the character animation according to claim 1 , wherein, in step (5), in order to reduce algorithm errors, multiple corresponding vertices are acquired for each vertex on the new mesh, and then a final weight is obtained through an interpolation method; on the premise that calculation is reduced and a smallest possible error is ensured, a number of the corresponding vertices to be acquired is 5.

3. The method for skinning technology based on extended position based dynamics and for weight retargeting in the character animation according to claim 1 , wherein in step (1), the initializing a tensile constraint, a volume constraint, a self-collision constraint and an energy constraint of extended position based dynamics (PBD) model for the tetrahedral mesh is specifically:
defining an energy constraint for each small tetrahedral element in the tetrahedral mesh, wherein let P 0 be a center of gravity of the tetrahedral P 1 P 2 P 3 P 4 , then a formula is:
P 0 =Σ i=1 4 m i P i /Σ i=1 4 m i   (1)

according to the Hooke's law, the energy constraint is defined as:
C energy ( P 1 ,P 2 ,P 3 ,P 4 )=½Σ i=1 4 k i (| P i −P 0 |−d i ) 2   (2)

wherein k i is an elastic coefficient of a virtual spring P i P 0 , and d i is a static length of the spring.

4. The method for skinning technology based on extended position based dynamics and for weight retargeting in the character animation according to claim 1 , wherein in step (2), optimizing mesh deformations using extended PBD is as follows:
at a skinning stage, firstly implementing the deformations of the triangular mesh and the tetrahedral mesh through the LBS, and a formula for a position transformation of a vertex on the mesh is as follows:
P′=Σ i=1 b w i B i −1 W i P   (3)

wherein b is a number of skeleton blocks, w i is an influence weight of an ith block of the skeleton on the vertex, and Σ i=1 b w i =1, B is a constant, W i is a world matrix of the ith block of the skeleton;
using barycentric coordinates to implement a mapping between the triangular mesh and the tetrahedral mesh, a formula for a relation between each vertex P of the triangular mesh and four vertices P 1 , P 2 , P 3 and P 4 of the corresponding tetrahedral mesh is as follows:
P=Σ i=1 4 P i w i   (4)

wherein w i is a weight of vertex P corresponding to P i .

5. The method for skinning technology based on extended position based dynamics and for weight retargeting in the character animation according to claim 1 wherein in step (5), a procedure of calculating the bi-harmonic distance field of the mesh is:
step 1, calculating a bi-harmonic distance field of a new mesh M and an original mesh N, respectively;
step 2, giving a set of corresponding vertices (x i , y i )∈M×N, i=1, . . . , k, wherein k is a number of pairs of the corresponding vertices;
step 3, representing any vertex x∈M in distance vectors to a known corresponding vertex:

X
_

=

1

max
i

⁢

d
⁡

(

x
,

x
i

)

⁢

(

d
⁡

(

x
,

x
1

)

,

d
⁡

(

x
,

x
2

Assignees

• Univ Beihang

Inventors

• Pan Junjun
• Chen Lijuan

Classifications

• G06T2210/21

  Collision detection, intersection · CPC title

• G06T17/205

  Re-meshing · CPC title

• G06T13/40 Primary

  of characters, e.g. humans, animals or virtual beings · CPC title