Robust attribute transfer for character animation

One embodiment of the invention disclosed herein provides techniques for transferring attributes from a source animated character to a target character. A character attribute transfer system identifies a first set of markers corresponding to the source animated character. The character attribute transfer system identifies a second set of markers corresponding to the target character. The character attribute transfer system generates a geometric correspondence between the source animated character and the target character based on the first set of markers and the second set of markers independent of differences in geometry between the source animated character and the target character. The character attribute transfer system transfers a first attribute from the source animated character to the target character based on the geometric correspondence.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates generally to three-dimensional (3D) computer graphics and, more specifically, to robust attribute transfer for character animation.

Description of the Related Art

Among other things, computer animation involves designing and creating animated characters for use in various computer games and films. Typically, each animated character includes a joint hierarchy, a geometric object, and a set of controllers and constraints that define the rig of the character. The joint hierarchy, also referred to herein as a “skeleton,” defines the bones and related structure of the animated character and how the bones are connected to one another. The geometric object defines the skin of the character and gives the animated character a unique appearance. Typically, the geometric object consists of a 3D mesh of geometry primitives, such as triangles, although other types of geometric objects can be specified. Accordingly, the geometric object is also referred to herein as a “mesh representation.” The rig allows an animator to pose and move the character during animation. The animated character also includes skinning weights that define how the geometric object corresponding to the skin of the character is bound to or attaches to the rig. To create multiple characters for a game or film, computer animation artists create a joint hierarchy, geometric object, rig, and skinning weights separately for each separate animated character.

One drawback to the above approach is that generating animated characters for any given application, such as a game or a film, is time consuming and tedious because each character has to be designed separately. This problem is substantially exacerbated in applications involving hundreds or thousands of different characters. For example, to create a crowd of one hundred animated characters, a computer animation artist would have to create a separate joint hierarchy, geometric object, rig, and set of skinning weights for each of the one hundred animated characters. Again, such manual techniques are both time consuming and tedious, resulting in increased costs and delays.

One potential solution is to create a common geometric object used by each character, sculpted to have a unique appearance, and to apply the same common joint hierarchy, rig, and set of skinning weights across the different sculpted geometric objects. Such an approach could be more efficient in that the computer animation artist would not need to create a unique joint hierarchy, rig, and set of skinning weights for each separate character. However, such an approach arguably restricts the geometric object to be of the same topology for all characters. Otherwise, the common joint hierarchy, rig, and set of skinning weights being applied across the geometric objects would not “fit” properly. With these limitations, computer animation artists would be constrained in generating desired appearances for the animation characters being designed, which would hinder overall artistic freedom and reduce the overall visual quality of applications.

As the foregoing illustrates, what is needed in the art are more effective ways to design animated characters.

SUMMARY OF THE INVENTION

Various embodiments of the present application set forth a method for transferring characteristics from a source animated character to a target character. The method includes identifying a first set of markers corresponding to the source animated character. The method includes identifying a second set of markers corresponding to the target character. The method further includes generating a geometric correspondence between the source animated character and the target character based on the first set of markers and the second set of markers independent of differences in geometry between the source animated character and the target character. The method further includes transferring a first characteristic from the source animated character to the target character based on the geometric correspondence.

Other embodiments of the present invention include, without limitation, a computer-readable medium including instructions for performing one or more aspects of the disclosed techniques as well as a computing device for performing one or more aspects of the disclosed techniques.

At least one advantage of the disclosed techniques is that new animated characters can be created from a mesh representation without the need to create a custom joint hierarchy, rig, and set of skinning weights for the new animated character. The joint hierarchy, rig, and set of skinning weights from an existing animated character to the new animated character without placing constraints on the design of the mesh representation of the new animated character. The time and effort to generate a new animated character is reduced relative to prior approaches, and the computer animation artist can design the appearance of the new animated character without undue limits on artistic freedom.

DETAILED DESCRIPTION

Hardware Overview

FIG. 1illustrates a computing system100configured to implement one or more aspects of the present invention. This figure in no way limits or is intended to limit the scope of the present invention. Computing system100may be a personal computer, video game console, personal digital assistant, mobile phone, mobile device, or any other device suitable for implementing one or more aspects of the present invention.

As shown, computing system100includes, without limitation, a processor102, display processor112, input/output (I/O) bridge107, and system memory104, coupled together and communicating via a bus path that may include a memory bridge105. Processor102may be any technically feasible form of processing device configured to process data and execute program code. Processor102could be, for example, a central processing unit (CPU), a graphics processing unit (GPU), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), and so forth. Likewise, display processor112may be any technically feasible form of processing device configured to process data and execute program code. Display processor112could be, for example, a central processing unit (CPU), a graphics processing unit (GPU), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), and so forth.

Processor102and display processor112include one or more processing cores. In operation, one or both of processor102and display processor112is the master processor of computing system100, controlling and coordinating operations of other system components. System memory104stores software applications and data for use by processor102. Either or both of processor102and display processor112execute software applications stored within system memory104and optionally an operating system. In particular, either or both of processor102and display processor112executes software and then performs one or more of the functions and operations set forth in the present application. Memory bridge105, which may be, e.g., a Northbridge chip, is connected via a bus or other communication path (e.g., a HyperTransport link) to an I/O (input/output) bridge107. I/O bridge107, which may be, e.g., a Southbridge chip, receives user input from one or more user input devices108(e.g., keyboard, mouse, joystick, digitizer tablets, touch pads, touch screens, still or video cameras, motion sensors, and/or microphones) and forwards the input to processor102via memory bridge105.

A display processor112is coupled to memory bridge105via a bus or other communication path (e.g., a PCI Express, Accelerated Graphics Port, or HyperTransport link); in one embodiment display processor112is a graphics subsystem that includes at least one graphics processing unit (GPU) and graphics memory. Graphics memory includes a display memory (e.g., a frame buffer) used for storing pixel data for each pixel of an output image. Graphics memory can be integrated in the same device as the GPU, connected as a separate device with the GPU, and/or implemented within system memory104.

Display processor112periodically delivers pixels to a display device110(e.g., a screen or conventional CRT, plasma, OLED, SED or LCD based monitor or television). Additionally, display processor112may output pixels to film recorders adapted to reproduce computer generated images on photographic film. Display processor112can provide display device110with an analog or digital signal.

A system disk114is also connected to I/O bridge107and may be configured to store content and applications and data for use by processor102and display processor112. System disk114provides non-volatile storage for applications and data and may include fixed or removable hard disk drives, flash memory devices, and CD-ROM, DVD-ROM, Blu-ray, HD-DVD, or other magnetic, optical, or solid state storage devices.

A switch116provides connections between I/O bridge107and other components such as a network adapter118and various add-in cards120and121. Network adapter118allows computing system100to communicate with other systems via an electronic communications network, and may include wired or wireless communication over local area networks and wide area networks such as the Internet.

Other components (not shown), including USB or other port connections, film recording devices, and the like, may also be connected to I/O bridge107. For example, an audio processor may be used to generate analog or digital audio output from instructions and/or data provided by processor102, system memory104, or system disk114. Communication paths interconnecting the various components inFIG. 1may be implemented using any suitable protocols, such as PCI (Peripheral Component Interconnect), PCI Express (PCI-E), AGP (Accelerated Graphics Port), HyperTransport, or any other bus or point-to-point communication protocol(s), and connections between different devices may use different protocols, as is known in the art.

In one embodiment, display processor112incorporates circuitry optimized for graphics and video processing, including, for example, video output circuitry, and constitutes a graphics processing unit (GPU). In another embodiment, display processor112incorporates circuitry optimized for general purpose processing. In yet another embodiment, display processor112may be integrated with one or more other system elements, such as the memory bridge105, processor102, and I/O bridge107to form a system on chip (SoC). In still further embodiments, display processor112is omitted and software executed by processor102performs the functions of display processor112.

Pixel data can be provided to display processor112directly from processor102. In some embodiments of the present invention, instructions and/or data representing a scene are provided to a render farm or a set of server computers, each similar to computing system100, via network adapter118or system disk114. The render farm generates one or more rendered images of the scene using the provided instructions and/or data. These rendered images may be stored on computer-readable media in a digital format and optionally returned to computing system100for display. Similarly, stereo image pairs processed by display processor112may be output to other systems for display, stored in system disk114, or stored on computer-readable media in a digital format.

Alternatively, processor102provides display processor112with data and/or instructions defining the desired output images, from which display processor112generates the pixel data of one or more output images, including characterizing and/or adjusting the offset between stereo image pairs. The data and/or instructions defining the desired output images can be stored in system memory104or graphics memory within display processor112. In an embodiment, display processor112includes 3D rendering capabilities for generating pixel data for output images from instructions and data defining the geometry, lighting shading, texturing, motion, and/or camera parameters for a scene. Display processor112can further include one or more programmable execution units capable of executing shader programs, tone mapping programs, and the like.

Processor102, render farm, and/or display processor112can employ any surface or volume rendering technique known in the art to create one or more rendered images from the provided data and instructions, including rasterization, scanline rendering REYES or micropolygon rendering, ray casting, ray tracing, image-based rendering techniques, and/or combinations of these and any other rendering or image processing techniques known in the art.

It will be appreciated that the system shown herein is illustrative and that variations and modifications are possible. The connection topology, including the number and arrangement of bridges, may be modified as desired. For instance, in some embodiments, system memory104is connected to processor102directly rather than through a bridge, and other devices communicate with system memory104via memory bridge105and processor102. In other alternative topologies display processor112is connected to I/O bridge107or directly to processor102, rather than to memory bridge105. In still other embodiments, I/O bridge107and memory bridge105might be integrated into a single chip. The particular components shown herein are optional; for instance, any number of add-in cards or peripheral devices might be supported. In some embodiments, switch116is eliminated, and network adapter118and add-in cards120,121connect directly to I/O bridge107.

Robust Attribute Transfer for Character Animation

FIG. 2is a block diagram of a character attribute transfer system200that may be implemented via the computing system100ofFIG. 1, according to various embodiments of the present invention. In some embodiments, at least a portion of the character attribute transfer system200may be implemented via the computing system100ofFIG. 1. As shown, the character attribute transfer system200includes, without limitation, a central processing unit (CPU)202, storage204, an input/output (I/O) devices interface206, a network interface208, an interconnect210, and a system memory212. The computing system100ofFIG. 1can be configured to implement the character attribute transfer system200. The processor202, storage204, I/O devices interface206, network interface208, interconnect210, and system memory212function substantially the same as described in conjunction withFIG. 1except as further described below.

The processor202retrieves and executes programming instructions stored in the system memory212. Similarly, the processor202stores and retrieves application data residing in the system memory212. The interconnect210facilitates transmission, such as of programming instructions and application data, between the processor202, input/output (I/O) devices interface206, storage204, network interface208, and system memory212. The I/O devices interface206is configured to receive input data from user I/O devices222. Examples of user I/O devices222may include one of more buttons, a keyboard, and a mouse or other pointing device. The I/O devices interface206may also include an audio output unit configured to generate an electrical audio output signal, and user I/O devices222may further includes a speaker configured to generate an acoustic output in response to the electrical audio output signal. Another example of a user I/O device222is a display device that generally represents any technically feasible means for generating an image for display. For example, the display device could be a liquid crystal display (LCD) display, CRT display, or DLP display. The display device may be a TV that includes a broadcast or cable tuner for receiving digital or analog television signals.

Processor202is included to be representative of a single CPU, multiple CPUs, a single CPU having multiple processing cores, and the like. And the system memory212is generally included to be representative of a random access memory. The storage204may be a disk drive storage device. Although shown as a single unit, the storage204may be a combination of fixed and/or removable storage devices, such as fixed disc drives, floppy disc drives, tape drives, removable memory cards, or optical storage, network attached storage (NAS), or a storage area-network (SAN). Processor202communicates to other computing devices and systems, via network interface208, where network interface208is configured to transmit and receive data via a communications network.

The system memory212includes, without limitation, a geometric correspondence engine232, a skeleton transfer engine234, a skinning weights transfer engine236, source character representations238, and target character representations240.

The geometric correspondence engine232establishes the geometric correspondence between a mesh representation of a source animated character and a mesh representation of a target character. The geometric correspondence engine232establishes this geometric correspondence by generating a mapping between a set of markers on the source animated character and a set of markers on the source animated character. The geometric correspondence is generated independent of differences in geometry and geometric details between the source and target mesh representations of the source animated character and the target character, respectively. More specifically, the correspondence is generated independent of differences of geometric details such as the number of geometry primitives in each mesh representation or how the mesh representation has been divided into geometry primitives. The geometry primitives of a mesh representation are also referred to herein as faces.

Each source animated character is represented as CS=(MS, RS, WS), where CSis the source character representation238, MSis the source mesh representation, RSis the source joint hierarchy, or skeleton, and WSis the set of source skinning weights that bind or attach the mesh representation to the joint hierarchy. Similarly, each target character is represented as CT=(MT, RT, WT), where CTis the target character representation240, MTis the target mesh representation, RTis the target joint hierarchy, or skeleton, and WTis the set of target skinning weights that bind or attach the mesh representation to the joint hierarchy. Using this nomenclature, the geometric correspondence engine232establishes the geometric correspondence between the source mesh representation MSand the target mesh representation MT. Once geometric correspondence is established between the source mesh representation MSand the target mesh representation MT, the source animated character CScan populate the target joint hierarchy RTand the set of target skinning weights WTbased on the source joint hierarchy RSand set of source skinning weights WS, respectively.

The geometric correspondence engine232determines the point-to-point correspondence between MSand MT, linking together points with similar semantic meanings, such as the tip of the right finger and the top of the head. The geometric correspondence engine232produces a geometric correspondence at a desired level of smoothness and has the property of bijection. Bijection is a one-to-one correspondence between the elements of two sets, where every element of one set is paired with exactly one element of the other set, and every element of the other set is paired with exactly one element of the first set, and there are no unpaired elements.

The geometric correspondence engine232computes a dense correspondence from a set of markers associated with the source target mesh representations, where either or both of the mesh representations may be non-isometric. The geometric correspondence engine232performs a three-step process: (1) identifying markers on the source mesh representation MSand the target mesh representation MT; (2) deforming the source mesh representation MSso that the source mesh representation MSmatches the target mesh representation MT; and (3) extracting the dense geometric correspondence based on closest locations between the deformed source mesh representation MSand the target mesh representation MT.

In the first step, the geometric correspondence engine232identifies the markers that are placed on the source mesh representation MSand the target mesh representation MT. The markers may be manually placed or otherwise specified by a user or, in the alternative, produced via an automated process. Areas of the mesh representation exhibiting a high level detail, such as the hands and feet, may have a higher density of marker placement relative to areas of the mesh representation exhibiting a lower level of detail. In some embodiments, a set of approximately 325 markers produces a geometric correspondence model with an acceptable level of quality for transferring the skeleton and skinning weights.

In the second step the geometric correspondence engine232deforms the source mesh representation MSso that the source mesh representation MSmatches the target mesh representation MT. The geometric correspondence engine232performs a series of substeps to align, scale, and deform the source mesh representation MSso that the position, orientation, scale, and shape of the source mesh representation MSbecome similar to the position, orientation, scale, and shape, respectively, of the target mesh representation MT. The geometric correspondence engine232performs energy minimizations computations based on marker positions, vertex Laplacian, and Voronoi areas. The geometric correspondence engine232computes per-vertex closest location between the deformed and the initial target mesh representations, while rejecting locations in the mesh representation that have inconsistent normal vectors. When computing the closest location for a specific vertex, the geometric correspondence engine232inspects the normal vectors of each candidate polygon. The geometric correspondence engine232rejects the candidate polygon if the angle with the vertex normal is greater than 90°. Because the Laplacian operator can severely distort the vertex and face normals, the geometric correspondence engine232relies on the input mesh normal to determine whether to reject polygons. As a result, this rejection test may improve the robustness of the geometric correspondence by reducing the likelihood of triangle flipping and self-intersecting geometry.

In the third step, after the source mesh representation MShas been deformed to match the target mesh representation MT, the geometric correspondence engine232extracts the dense correspondence based on closest locations between the deformed source mesh representation MSand the target mesh representation MT. In so doing, the geometric correspondence engine232rejects locations on the mesh representations associated with inconsistent normal vectors. In the resulting geometric correspondence, each vertex included in the deformed source is bijectively linked to a barycentric coordinate on a polygon of the target mesh representation MT, and vice versa.

The skeleton transfer engine234calculates the target joint hierarchy RTvia an optimization technique, while preserving the relationships between vertices in the source mesh representation MSand joints in the source joint hierarchy RS. The skeleton transfer engine234also performs a pose normalization procedure to ensure that joints in the target joint hierarchy RTand the source joint hierarchy RSare correctly aligned. After the geometric correspondence engine232computes the geometric correspondence between MSand MT, the skeleton transfer engine234determines the position of the source skeleton RSwithin MT. In so doing, the skeleton transfer engine234performs a two-step process by first positioning and then orienting joints, while considering the relationship among joints, vertices, and skinning weights.

In the first step, the skeleton transfer engine234transfers the joint positions from the source skeleton RSto the target skeleton RT, while preserving the relationships found in the source character representation238. The skeleton transfer engine234inspects the source skinning weights WSto extract the joint-vertices relationships. Typically, each joint or element of the source skeleton RSaffects only a subset of the vertices included in the source mesh representation MS. For example, the shoulder joint would affect a subset of the vertices near the shoulder, such as the vertices representing the arm, hand, and upper back. These vertices would be associated with non-zero skinning weights with respect to the shoulder joint. Other vertices that are not affected by the shoulder joint would be associated with zero skinning weights with respect to the shoulder joint.

With respect to a particular joint, the vertices associated with non-zero skinning weights {viS|wiS>0} are referred to herein as weighted vertices. The skeleton transfer engine234extracts the joint-vertices relationship from the source character representation238and then apply the joint-vertices relationship to the target character representation240to compute the new joint position in the target character. Considering a joint jSand the m weighted vertices viSassociated with the joint jS, the joint positions are expressed as a linear combination of vertex positions, as given by Equation 1 below:

Of all possible linear combinations of vertex positions, the skeleton transfer engine234selects the linear combination satisfying the following criteria: (1) the linear combination is very close to the joint position; (2) the value of each coefficient ciis proportional to the corresponding skinning weight; and (3) the sum of the coefficients ciis equal to 1. If these three criteria are satisfied, then the joint location is enclosed in the convex envelope of the weighted vertices and the vertices with larger weights are given more importance. To determine the coefficients ci, the skeleton transfer engine234minimizes the energy function for each source joint jS, as given by Equations 2a-2c below:

E⁡(ci,…⁢,cm)=ω1⁢(∑i=1m⁢⁢ci⁢vi⁢⁢𝒮)-J𝒮2(2⁢a)+ω2⁢∑i=1m⁢⁢(ci-wi⁢⁢𝒮∑j=1m⁢⁢wj𝒮)2(2⁢b)+ω3⁡((∑i=1m⁢⁢ci)-1)2(2⁢c)
where wiSis the skinning weight of vertex viSfor joint jS, and the parameters ω1, ω2, and ω3adjust the influence of each part of the energy function. The first term (Equation 2a) minimizes the distance between the linear combination of the vertex positions and the joint position. Although the first term (Equation 2a) of the equation has the quality of yielding the joint position, Equation 2a may be under-constrained, resulting several combinations of vertex positions that may yield the same joint position. Further, the first term is likely to result in including vertices with low skinning weight values, which are typically farther from the joint and, correspondingly, of relatively low value.

To correct this issue, the skeleton transfer engine234computes a second term (Equation 2b) to ensure that each coefficient ciis proportional to the corresponding skinning weight wiS. However, the magnitude of the second term (Equation 2b) may yield large errors for the first term (Equation 2a). In order to mitigate this issue, the skeleton transfer engine234sets ω1>>ω2, causing the first term (Equation 2a) to have a high absolute value relative to the second term (Equation 2b). Finally, the skeleton transfer engine234computes a third term (Equation 2c) to keep the joint within the convex envelope of the vertices, and, correspondingly, to penalize “out-of-the-mesh” joint positions. In some embodiments, a good balance among the three terms is achieved where ω1=1,000, ω2=1, and ω3=10. The energy function of Equations 2a-2c is quadratic, and the skeleton transfer engine234minimizes this energy function by solving the corresponding linear system. The skeleton transfer engine234then uses the resulting coefficients cito determine the new target joint position jT, as given by Equation 3 below:

where ρiTis the point on the target mesh representation corresponding to vertex viSbased on the geometric correspondence.

In some embodiments, after optimizing the joint positions, the skeleton transfer engine234may perform one or more post-process steps to further adjust joints to take advantage of straight abdomen and character symmetry. Animated characters are often created to have a relatively straight abdominal region and may exhibit bilateral, radial, or other forms of symmetry. The skeleton transfer engine234may perform spine alignment or mirroring with respect to a symmetry plane, such as a sagittal, or lateral, yz plane.

In a first technique, the skeleton transfer engine234may perform spine alignment to take advantage of a character that has a straight abdomen. The skeleton transfer engine234may employ a modified version of Equations 2a-2c, where all of the weighted vertices from the set of spine joints are considered. In such cases, the value m becomes the number of vertices with a non-zero weight for at least one spine joint. As different weights wiSexist for each joint of the spine, the second term (Equation 2b) is modified to compute the sum of the spine weights wi sumSrather than of wiS. Similarly, the second term (Equation 2b) is modified to compute the sum of the spine weights wj sumS) rather than of wjS. The skeleton transfer engine234also updates the first term (Equation 2a) to compute only the x position shared by all of the spine joints, given as (Σi=1mcivixS)−jxS)2. The skeleton transfer engine234computes the x coordinate of the target spine as jxT=Σi=1mciρixT. The skeleton transfer engine234then updates the target spine joints x coordinates with the computed jxT.

In a second technique, the skeleton transfer engine234may perform mirroring to take advantage of a character that exhibits some form of symmetry. In the case of bilateral symmetry, the skeleton transfer engine234may mirror all of the right joints to the left side of the skeleton, average the position of each symmetric joint pair, and mirror the average values back to the right side of the skeleton.

After completing the first step of positioning the joints, the skeleton transfer engine234, in a second step, transfers the joint orientations and rotations from the source skeleton RSto the target skeleton RT. The computation of the target joint positions enables the skeleton to be correctly embedded within the target mesh representation MT. Nevertheless, the skeleton RSis not defined solely by joint positions. Joint orientations and rotations are also transferred to ensure that the skeletal structure is fully preserved and articulates correctly during animation. After a successful transfer, if each skeleton is in neutral pose, with all of its joint rotations set to zero, every joint of the source and the target should have the same orientation in both the local and global spaces. Correctly transferring joint orientations and rotations helps to ensure that animations from the source character will result in similar target character poses during animation.

The skeleton transfer engine234detects differences existed in limb orientation between the source skeleton RSand the target skeleton RT. The skeleton transfer engine234computes the joint rotations that account for differences in joint orientation between the source skeleton RSand the target skeleton RT. As with the joint positioning step, the skeleton transfer engine234relies on the source weighted vertices {viS|wiS>0} and corresponding locations {ρiT}, where each viSis linked to a specific ρiTby the geometric correspondence.

The skeleton transfer engine234processes the joints one at a time, starting from the root joint, transferring joint orientations and computing joint rotations according to the process shown in Listing 1 below:

/* Take into account the rotations from parent joints */

/* Move to the next joints for both the source and target */

6 for each childS∈(jlS·children; childT∈jlT·children do

The skeleton transfer engine234directly copies the joint orientation of jlT, the orientation in local space with respect to the parent joint, from jlS(Listing 1, line 1). For each joint, the skeleton transfer engine234determines the rotation needed to align {ρiT} (Listing 1, line 2) onto {viS|wiS>0} using best-matching similarity rotation (Listing 1, line 4). The result is a rotation in global space, which the skeleton transfer engine234then converts to a Δrotation in the local space of the joint jlT(Listing 1, line 5). As the skeleton transfer engine234moves to the next joint, the skeleton transfer engine234rotates the points {ρiT} based on the cumulative inverse Δrotations of the kinematic chain from the parent up to the root joint (Listing 1, lines 3 and 7). After recursively visiting each joint (Listing 1, lines 6-8), the skeleton transfer engine234copies the entire skeleton to the target character representation240, with each joint repositioned and oriented. The skeleton transfer engine234extracts each joint Δrotation with respect to the source skeleton RS. The target character is now ready for transfer of the source skinning weights WSto the target mesh representation MT.

The skinning weights transfer engine236calculates the set of target skinning weights WTand binds or attaches the target mesh representation MTto the target joint hierarchy RT, thereby yielding a target character that is ready for animation and that preserves quality of the source character representation CS238. Skinning weights define how the mesh representation deforms and moves in response to changes in the skeleton and enable a character to animate based on the character's skeleton.

In some embodiments, each vertex included in the mesh representation is associated with a different skinning weight for each element of the skeleton. In such embodiments, if a mesh representation includes N vertices and a skeleton includes M elements, then the number of skinning weights may be N times M. In other embodiments, each vertex included in the mesh representation is associated with a set of skinning weights for a subset of the elements of the skeleton that are within a threshold distance from the vertex. In such embodiments, skinning weights would not be defined between the vertex and elements of the skeleton that are beyond the threshold distance from the vertex. In some embodiments, the skinning weights from the source animated character may be filtered based on the geometric correspondence and differences in mesh resolution between the source animated character and the target character.

The skinning weights transfer engine236obtains the target skinning weights WTbased on the geometric correspondence from the target mesh representation MTto the source mesh representation MS. For each target vertex viT, the corresponding location on the source mesh representation MSis defined as a barycentric coordinate and a triangle index.

If the source mesh representation MSand the target mesh representation MThave the same resolution, or if the source mesh representation MShas a coarser, or lower, resolution than the target mesh representation MT, then the skinning weights transfer engine236performs a linear interpolation, or linear blending, to generate a set of resampled the source skinning weights WS, and, in turn, to thereby generate the target skinning weights WS. In such cases, the new target skinning weight for a vertex included in the target mesh representation MTis computed using the corresponding barycentric coordinates and triangle on the source mesh representation MS, as given by Equation 4 below:
wiblend=βawaS+βbwbS+βcwcS(4)
where a, b, and c correspond to the three vertices included in the source triangle, waS, wbS, and wcSare the skinning weights of the vertices, and βa, βb, and βcare the barycentric coordinates of the vertices.

If the target mesh representation MThas a coarser, or lower, resolution than the source mesh representation MS, then linear blending with barycentric coordinates may result in undersampling. In such cases, vertices and weight values from the source animated character may remain unused, leading to aliasing and artifacts in the skin deformation during animation. To detect when such aliasing is likely to occur, the skinning weights transfer engine236compares the size of corresponding polygons between the target mesh representation MTand the source mesh representation MS. The skinning weights transfer engine236performs this comparison based on the re-scaling of the source mesh representation as MSto match the dimensions of the target mesh representation MTduring the geometric correspondence process. In order to prevent aliasing and artifacts, the skinning weights transfer engine236selects vertices for skinning weights filtering according to the process shown in Listing 2 below:

2 if VAblend<VA(viT) then

10 Next=vertices from next ring of neighbors

To detect vertices requiring additional filtering, the skinning weights transfer engine236computes and compares the Voronoi area of each target vertex VA(viT) with the corresponding blended Voronoi area of the source (Listing 2, lines 1 and 2). If VA(viT) is greater than VAblend, then the skinning weights transfer engine236identifies the vertices from the next ring of neighboring vertices near the location of the geometric correspondence (Listing 2, lines 5 and 10). The skinning weights transfer engine236adds these vertices to the set of selected vertices (Listing 2, line 8) The skinning weights transfer engine236adds the corresponding Voronoi areas of the selected vertices (Listing 2, line 9), until adding the Voronoi areas of the next ring of neighboring vertices would result in a total Voronoi area larger than VA(viT) (Listing 2, line 7). The resulting Voronoi area is given by Equations 5a and 5b below:

If a Voronoi area of exactly A(viT) is desired, the skinning weights transfer engine236computes a factored contribution of the vertices from the next ring of neighboring vertices that cause the Voronoi area to exceed VA(viT). The influence of this ring of neighboring vertices is, therefore, less than that of the previously selected vertices. The skinning weights transfer engine236combines the skinning weights with respect to the corresponding Voronoi areas, as given by Equations 6a-6c below:

∑l=1n⁢⁢wJl=1(7)
such that the sum of the skinning weights is equal to one. After the skinning weights are transferred, any technically feasible skinning method may be applied to bind or attach the target mesh representation MTto the target skeleton RT, including, without limitation, linear blend skinning and dual quaternion skinning.

After the skeleton and skinning weights have been transferred to the target character, the skinning weights transfer engine236binds or attaches the target mesh representation MTto the transferred skeleton RTbased on the transferred skinning weights WT. The new target character CTis then ready for animation. In some embodiments, forward and inverse kinematics animation may be used to compute joint rotations, thereby obtaining similar animation results for the source animated character and target character.

In this section, we highlight several benefits of the proposed retargeting approach. First, we show how our approach can set the skeletons and meshes from all of the target characters in the same pose as the source. Second, we validate that the retargeting works on a wide range of characters with different morphologies and different mesh topologies. Lastly, we demonstrate that the work done by an artist, when positioning joints, painting weights, and designing a specific skeleton, is faithfully transferred to target characters.

In some embodiments, the target character may be set in the same pose as the source character based on the Δrotations computed during joint orientation and rotation. This operation is referred to herein as pose normalization. In such embodiments, the skeleton transfer engine234may visit each joint of the target skeleton RTand may invert the computed Δrotations so that the actual rotation of each of the joints of the target skeleton RTmatches the corresponding rotation of the source skeleton RS. This pose normalization technique works for characters with various morphological and topological characteristics. For example, a source skeleton of an average man with 52 joints could be locally fitted to multiple target skeletons from the hips and spine up to the head, fingers, and toes. The source skeleton could be successfully fitted to target skeletons that exhibit a wide variety of morphological characteristics, such as significant differences in hand size, limb size and length, abdominal size, neck size, or head size. The pose normalization technique works well with target characters ranging from low-resolution meshes, such as 2000 polygons, to high-resolution meshes, such as 50,000 polygons. Further, animated characters with incompatible mesh topologies are processed without any change to their respective mesh representations. Such flexibility may provide more freedom to artists because artists can transfer the skeleton and skinning weights among proxy characters, including low polygon count game characters and high polygon count visual effects characters without regard to the particulars of the corresponding mesh representations.

In some embodiments, the disclosed techniques work properly with meshes representations with different resolutions. For examples, a coarse resolution animated character could be retargeted to a finer resolution target mesh representation. Likewise, a high-resolution visual effects animated character could be retargeted to a low-resolution mesh representation for mobile game applications.

In some embodiments, the disclosed techniques may be employed to propagate post-transfer artistic changes made to the source animated character to the various target characters. A computer animation artist may make changes to the mesh representation of the source animated character. Via the enclosed techniques, the transfer process may be repeated in order to propagate the changes to the target characters. For example, to approximate the anatomical movement of the knee, a computer animation artist could place knee joints close to the surface of the mesh representation. Because rigging tools typically use the projected centering technique to set the joint on the medial axis, creating a skeleton from scratch for each target character would involve additional editing operations. The disclosed approach faithfully reproduces the work of the computer animation artist.

In some embodiments, the disclosed transfer approach also preserves details in manually painted skinning weights. Computer animation artists may rely on automatic binding or attaching methods to obtain initial skinning weights. However, existing binding and attaching methods have limitations and may display artifacts during animation. To limit such artifacts, manual post-corrections may be warranted. The disclosed approach propagates the manual corrections made to the source animated character to target mesh representations. Further, even if the skinning weights have already been transferred to the target characters, additional skinning weight corrections can also be transferred to target characters. This propagation of changes from source to target characters may be performed simultaneously on multiple target characters at interactive rates.

In some embodiments, the disclosed approach is not limited to a specific skeleton structure. This flexibility provides the freedom to design skeletons with greater or fewer joints and with a hierarchy suited to a specific animation. The spatial relationships between the joints and the mesh representations may be preserved through the transfer even when the source and target joint hierarchies differ in terms of the number of joints.

It will be appreciated that the system shown herein is illustrative and that variations and modifications are possible. For example, the present disclosure describes that, after determining the geometric correspondence between the source and target mesh representations, the skeleton and the skinning weights are transferred from the source animated character to the target character. The geometric correspondence could be used to transfer other attributes of the animated characters, including, without limitation, the rig control and (u, v) coordinates for applying textures to the animated characters. The rig control includes a set of controllers and constraints that define how an animated character can pose and move. The (u, v) coordinates define positions of one or more textures that are projected onto the mesh representation, where such textures enhance the outer appearance of the animated character.

FIG. 3illustrates a set of markers placed on a mesh representation300, according to various embodiments of the present invention. As shown, a mesh representation300is populated with markers, such as marker310,312, and314. The mesh representation includes an arm320, wrist322, and hand324of an animated character (not explicitly shown). The markers on the mesh representation300are placed at different densities depending on the complexity of movement and corresponding bone structure of the various components of the mesh representation300. The bone structure and complexity of movement of the arm320is comparatively low. Therefore, the markers of the arm320, such as marker310, are placed with a relatively low density. By contrast, the bone structure and complexity of movement of the wrist322is moderate. Therefore, the markers of the wrist322, such as marker312, are placed with a medium density. Finally, the bone structure and complexity of movement of the hand324is comparatively high. Therefore, the markers of the hand324, such as marker314, are placed with a relatively high density.

FIGS. 4A-4Dillustrate a geometric correspondence mapping between a source animated character410and multiple target characters420,430, and440, according to various embodiments of the present invention. As shown, the source animated character410is divided into patches, such as patch412,414, and416, where each patch includes a portion of the mesh representation of the source animated character410. Likewise, target character420is divided into patches, such as patch422,424, and426, target character430is divided into patches, such as patch432,434, and436, and target character440is divided into patches, such as patch442,444, and446. After the geometric correspondence is performed, the patches of the source animated character410is mapped to the patches of the target characters420,430, and440. Patch412located along the left arm of the source animated character410maps to patches422,432, and442located along the left arm of target characters420,430, and440, respectively. Similarly, patch414located along the right arm of the source animated character410maps to patches424,434, and444located along the right arm of target characters420,430, and440, respectively. Finally, patch416located on the head of the source animated character410maps to patches426,436, and446located on the head of target characters420,430, and440, respectively.

FIG. 5illustrates a set of skinning weights associated with a mesh representation520and a portion of a joint hierarchy, according to various embodiments of the present invention. As shown, the mesh representation520, illustrated in green, covers the area around the left shoulder, arm, and hand of an animated character (not explicitly shown). The portion of the joint hierarchy includes an upper arm bone530and associated shoulder joint532and elbow joint534. Vertices included in the mesh representation520that are located near the upper arm bone530, such as vertex522, are highlighted in white, indicating that these vertices are associated with non-zero skinning weights with respect to the upper arm bone530. Vertices included in the mesh representation520that are located farther away the upper arm bone530, such as vertex524, are not highlighted in white, indicating that these vertices are associated with skinning weights that are at or near zero with respect to the upper arm bone530. Correspondingly, portions of the mesh representation520with a black background have vertices with skinning weights that are at or near zero. Portions of the mesh representation520with a red, orange, or yellow background have vertices with low, medium, or high skinning weights, respectively.

FIG. 6is a flow diagram of method steps for transferring attributes from a source animated character to a target character, according to various embodiments of the present invention. Although the method steps are described in conjunction with the systems ofFIGS. 1-2, persons of ordinary skill in the art will understand that any system configured to perform the method steps, in any order, is within the scope of the invention.

As shown, a method600begins at step602, where a geometric correspondence engine232included in a character attribute transfer system200identifies marker points on the mesh representation associated with a source animated character. The markers may be placed manually by a computer animation artist or via an automated process. In general, markers are placed at higher densities in areas of the mesh representation that exhibit relatively complex motion and are associated with relatively complex portions of the joint hierarchy. At step604, the geometric correspondence engine232identifies marker points on the mesh representation associated with a target character. The markers may be placed manually by a computer animation artist or via an automated process. Again, markers are placed at higher densities in areas of the mesh representation that exhibit relatively complex motion and are associated with relatively complex portions of the joint hierarchy.

At step606, the geometric correspondence engine232computes a geometric correspondence between the source mesh representation and the target mesh representation based on the markers on each of the source mesh representation and the target mesh representation. The geometric correspondence engine232establishes this geometric correspondence by generating a mapping between the set of markers on the source animated character and the set of markers on the source animated character. The geometric correspondence is generated independent of differences in geometric details between the source and target mesh representations, such as the number of geometry primitives included in each mesh representation or how the mesh representation has been divided into geometry primitives.

At step608, a skeleton transfer engine234included in the character attribute transfer system200transfers the skeleton from the source animated character to the target character based on the geometric correspondence. The skeleton transfer engine234calculates the target joint hierarchy via an optimization technique, while preserving the relationships between vertices in the source mesh representation and joints in the source joint hierarchy. The skeleton transfer engine234also performs a pose normalization procedure to ensure that joints in the target joint hierarchy and the source joint hierarchy are correctly aligned. The skeleton transfer engine234determines the position of the source skeleton within the target mesh representation. In so doing, the skeleton transfer engine234performs a two-step process by first positioning and then orienting joints, while considering the relationship among joints, vertices, and skinning weights.

At step610, a skinning weights transfer engine236included in the character attribute transfer system200transfers the skinning weights from the source animated character to the target character based on the geometric correspondence. The skinning weights transfer engine236calculates the set of target skinning weights and binds or attaches the target mesh representation to the target joint hierarchy, thereby yielding a target character that is ready for animation and that preserves quality of the source character representation. In some embodiments, the skinning weights from the source animated character may be filtered based on the geometric correspondence and differences in mesh resolution between the source animated character and the target character.

At step612, in some embodiments, additional transfer engines included in the character attribute transfer system200transfer other attributes from the source animated character to the target character based on the geometric correspondence. For example, transfer engines could transfer the rig control and/or (u, v) coordinates for applying textures to the animated characters. The rig control includes a set of controllers and constraints that define how an animated character can pose and move. The (u, v) coordinates define positions of one or more textures that are projected onto the mesh representation, where such textures enhance the outer appearance of the animated character. At step614, the skinning weights transfer engine236binds or attaches the mesh representation associated with the target character to the joint hierarchy associated with the target character based on the transferred skinning weights. The method600then terminates.

In sum, the present disclosure describes a new approach for transferring an existing skeleton, skinning weights, and other attributes from a source animated character to geometric objects that define the skin of various target characters. A set of markers is identified on the mesh representation of the source animated character. Likewise, a set of markers is the mesh representation of the target character. The source mesh representation is deformed to match the target mesh representation. The point-to-point geometric correspondence between the deformed source mesh representation and the target mesh representation is determined, linking together markers with similar semantic meanings, such as the tip of the right finger and the top of the head. The skeleton of the source animated character is transferred to the target character by positioning and orienting the joints of the target character, while considering the relationship among joints, vertices, and skinning weights. The skinning weights of the source animated character are transferred to the target character. The skinning weights from the source animated character are filtered based on the geometric correspondence and differences in mesh resolution between the two animated characters. Likewise, other attributes, including rig control information and (u, v) texture coordinates are transferred from the source animated character to the target character. After the transfer process, the mesh representation is bound to or attached to the transferred skeleton based on the transferred skinning weights. The new target character is then ready for posing and animation.

At least one advantage of the disclosed techniques is that new animated characters can be created from a mesh representation without the need to create a custom joint hierarchy, rig, and set of skinning weights for the new animated character. The joint hierarchy, rig, and set of skinning weights from an existing animated character to the new animated character without placing constraints on the design of the mesh representation of the new animated character. These techniques avoid the need to create the skeleton and skinning weights for the new animated character from scratch, a process that can be tedious and require time-consuming manual adjustments. The disclosed approach eliminates typical modeling constraints by handling characters with meshes of various topologies and morphologies. The time and effort to generate a new animated character is reduced relative to prior approaches, and the computer animation artist can design the appearance of the new animated character without undue limits on artistic freedom.

Another advantage of the disclosed techniques is that further adjustments can be made to the skinning weights of the source animated character to correct or modify the animated character. The adjusted skinning weights can be transferred to the target characters, thereby propagating the adjusted skinning weights to the relevant target.