Abstract:
A system and method enables animators to pose character models&#39; feet. An initial foot model position is received. The initial foot model position specifies a foot model contact point. One or more foot roll parameters are specified that change the relative angle between at least a portion of the foot model and an initial orientation of an alignment plane. Foot roll parameters specify the rotation of the foot model around foot model contact points. Foot roll parameters can include heel roll, ball roll, and toe roll, which specify the rotation of the foot model around contact points on the heel, ball, and toe, respectively, of a foot model. To maintain the position of the foot model contact point, the foot model position is adjusted based on the foot roll parameter. The repositioned foot model is realigned with alignment plane, which restores contact at the foot model contact point.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 60/572,008, filed May 17, 2004, which is incorporated by reference herein for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of computer graphics, and in particular to methods and apparatus for animating computer generated characters. Many computer graphic images are created by mathematically modeling the interaction of light with a three dimensional scene from a given viewpoint. This process, called rendering, generates a two-dimensional image of the scene from the given viewpoint, and is analogous to taking a photograph of a real-world scene. Animated sequences can be created by rendering a sequence of images of a scene as the scene is gradually changed over time. A great deal of effort has been devoted to making realistic looking rendered images and animations. 
     Animation, whether hand-drawn or computer generated, is as much an art as it is a science. Animators must not only make a scene look realistic, but must also convey the appropriate dramatic progression and emotional impact required by the story. This is especially true when animating characters. Characters drive the dramatic progression of the story and establish an emotional connection with the audience. 
     Effective walk animations are often an important contribution to the expressiveness of a character&#39;s animation. A character&#39;s walk or gait can be used to express the character&#39;s emotions. Additionally, walking, running, or other types of character motion can add excitement to a scene, as compared with scenes having motionless characters. At the very least, effective and realistic walk animations reinforce an audience&#39;s suspension of disbelief. However, creating convincing walk animations with the appropriate emotional expression and level of energy is particularly challenging and time consuming. 
     One of the difficulties in creating walk animations arises from the kinematic complexity of walking itself. During a typical walk animation for a bipedal character model, the foot first touches the ground at the heel. As the character&#39;s weight shifts forward, the foot rotates around the heel contact point until it is flat against the ground surface. Then, as the character&#39;s weight shifts further forward, the foot begins to lift off the ground, typically by bending and rotating around the ball of the foot. Finally, the foot lifts off the ground entirely and the character&#39;s weight is transferred to the other foot. 
     Many animation tools make it difficult to mimic these kinematic attributes of walking. Typically, animation tools enable animators only to rotate the foot around specifically defined locations, such as the ankle or ball of the foot. As animators apply rotations to these locations, the foot of a character model often slides forward or backwards relative to the ground plane. Additionally, these rotations can also cause the foot to lift off the ground plane prematurely, or to penetrate below the ground plane. 
     As a result of these effects, the correct positioning of the foot of a character model during a walk animation is often an iterative process. First, the animator places the foot at the desired location relative to the ground plane. The animator then specifies the desired foot rotation around the heel and/or ball. This causes the contact point of the foot to shift position relative to the ground; thus the animator must then reposition the foot back to the desired location. As adjustments are made to the foot rotation, the animator must make further adjustments to the position of the foot. Because of the complexity and time required for these iterative adjustments, animators tend to construct scenes in which character models&#39; feet are hidden, so as to avoid this issue entirely. 
     It is therefore desirable for a system and method to enable animators to efficiently specify the positions and rotations of the feet of character models. It is further desirable that the system and method automatically adjust the position of the foot of a character model in response to a rotation to eliminate unwanted shifts in position of the foot contact point. It is also desirable that the system and method be suitable for rotations of the foot of a character model around the heel contact point, the ball contact, and any other foot contact point. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the invention includes a system and method that enables animators to efficiently specify the positions and rotations of the feet of character models. In an embodiment, an animator specifies an initial foot model position. The initial foot model position specifies a foot model contact point. Animators specify one or more foot roll parameters that change the relative angle between at least a portion of the foot model and an initial orientation of an alignment plane. Foot roll parameters specify the rotation of the foot model around foot model contact points. Foot roll parameters can include heel roll, ball roll, and toe roll, which specify the rotation of the foot model around contact points on the heel, ball, and toe, respectively, of a foot model. To maintain the position of the foot model contact point, the foot model position is adjusted based on the foot roll parameter. The repositioned foot model is realigned with alignment plane, which restores contact at the foot model contact point. 
     In an embodiment, a method of posing a foot model includes receiving a first orientation of an alignment plane; receiving a foot position specifying the position of the foot model; and receiving a foot roll parameter for the foot model. The foot roll parameter specifies an angle between an alignment plane and a reference frame associated with the foot model. The method further includes changing the relative angle between at least a portion of the foot model and the alignment plane based on the foot roll parameter; specifying a new foot position for the foot model based on the foot roll parameter; and realigning the foot model with the alignment plane. 
     In a further embodiment, changing the relative angle between at least a portion of the foot model and the alignment plane includes applying a transformation to the alignment plane. The transformation includes a rotation proportional to the foot roll parameter, which rotates the alignment plane to a second orientation. Specifying a new foot position includes applying the transformation to the foot position. In an additional embodiment, realigning the foot model with the alignment plane includes rotating the foot model such that the reference frame associated with the foot model is aligned with the second orientation of the alignment plane. 
     In another embodiment, changing the relative angle between at least a portion of the foot model and the alignment plane includes applying a transformation to the reference frame associated with the foot model. The transformation includes a rotation proportional to the foot roll parameter, which rotates the reference frame associated with the foot model around a first joint. Specifying a new foot position includes applying an inverse of the transformation to the foot position. In an additional embodiment, changing the relative angle between at least a portion of the foot model and the alignment plane also includes applying a transformation to a predetermined portion of the foot model, thereby rotating the predetermined portion of the foot model around the first joint. In still another embodiment, realigning the foot model with the alignment plane includes rotating the foot model such that the reference frame associated with the foot model is aligned with the first orientation of the alignment plane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the drawings, in which: 
         FIGS. 1A-1B  illustrate two different phases of a walk animation suitable for an application of an embodiment of the invention; 
         FIG. 2  illustrates a method of repositioning the foot of a character model to compensate for heel roll according to an embodiment of the invention; 
         FIGS. 3A-3E  illustrate an example application of the method of  FIG. 2  according to an embodiment of the invention; 
         FIG. 4  illustrates a method of repositioning the foot of a character model to compensate for ball roll according to an embodiment of the invention; 
         FIGS. 5A-5E  illustrate an example application of the method of  FIG. 4  according to an embodiment of the invention; and 
         FIG. 6  illustrates an example computer system suitable for implementing an embodiment of the invention. 
     
    
    
     In the drawings, the use of like reference numbers indicates similar elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1A-1B  illustrate two different phases of a walk animation suitable for an application of an embodiment of the invention.  FIG. 1A  illustrates a first phase  100  of a typical walk animation. In phase  100 , the foot  105  of a character model contacts the ground plane  110  at heel contact point  115 . As the character model moves forward, the foot  105  rotates around the heel contact point  115  until it is flat against the ground surface. The rotation  120  of the foot  105  around heel contact point  115  is referred to as heel roll. 
       FIG. 1B  illustrates a second phase  150  of a typical walk animation. In phase  150 , the foot  155  of a character model is lifted from the ground plane  160 . In phase  150 , the foot  155  rotates around ball contact point  165 . Additionally, the toe portion  175  of the foot  155  bends so as to remain in contact with the ground plane  160 . The rotation  170  of foot  155  around ball contact point  165  is referred to as ball roll. 
     Phases  100  and  150  are provided for the purposes of illustration, and embodiments of the invention can be applied to any type of animation in which a foot or other portion of a character model is positioned with respect to and/or rotated around a contact point. Additionally, the heel roll and ball roll rotations can be employed in any phase of a walk animation. For example, a tip-toeing walk animation may use ball roll rotation as the foot of character model makes initial contact with a surface. 
     Additionally, computer-generated animation of characters is typically accomplished by manipulating a three-dimensional model of a character into a series of bodily positions, or poses, over a sequence of frames. A realistic looking character model is often extremely complex, having millions of surfaces and hundreds or thousands of attributes. Due to the complexity involved with animating such complex models, animation tools often rely on armatures and animation variables to define character animation. 
     An armature is a “stick figure” representing the character&#39;s pose, or bodily position. By moving the armature segments, which are the “sticks” of the “stick figure,” the armature can be manipulated into a desired pose. As the armature is posed by the animator, the animation tools modify character model so that the bodily attitude of the character roughly mirrors that of the armature. 
     Animation variables are another way of defining the character animation of a complex character model. Animation variables are parameters for functions that modify the appearance of a character model. In their simplest form, animation variables may manipulate armature segments, thereby altering the appearance of the character model indirectly, or manipulate the character model directly, bypassing the armature. 
     Animation variables can be used to abstract complicated modifications to a character model to a relatively simple control. For example, a single animation variable can define the degree of opening of a character&#39;s mouth. In this example, the value of the animation variable may manipulate several different parts of the armature and/or modify portions of the character model directly to create a modified character model having a mouth opened to the desired degree. For each animation variable, there are often one or more functions that specify how the value of the animation variable affects the character model. The set of functions defining the relationship between animation variables and a character model is sometimes referred to as the rigging of the character model. 
     The values of various foot roll parameters, such as heel roll and ball roll, can be specified as animation variables. In an embodiment of the invention, the rigging of the character model includes functions that automatically reposition the feet of the character model in response to the values of foot roll parameters, so as to keep the foot contact points in a fixed position with respect to a ground plane. 
       FIG. 2  illustrates a method  200  of repositioning the foot of a character model to compensate for heel roll according to an embodiment of the invention. At optional step  205 , an alignment plane is specified for one or more feet of the character model. In an embodiment, an animator uses an animation software tool to manually specify the orientation of the alignment plane. A horizontal alignment plane can be used to represent level ground. In an embodiment, this can be set as the default orientation of the alignment plane absent an animator specifying a different orientation. Sloping ground, such as hills, can be represented by changing the orientation of the alignment plane to a non-horizontal orientation. In an embodiment, the animation software tool assumes that the foot of the character model has been placed in contact with the ground based on the foot position specified by the animator; thus the alignment plane is automatically positioned so as to pass through a specific point of the foot model. This point, referred to as a heel contact point, can be defined as part of the foot model prior to the foot model&#39;s use in the animation software tool. In additional embodiments, the location of the heel contact point can be adjusted to meet the artistic demands of a particular scene. In another embodiment, the alignment plane can be automatically determined from the position and orientation of surfaces in the scene that are in close proximity to the foot of the character model. In this latter embodiment, step  205  may be performed after step  210 , which is described below. 
     The position of the foot of a character model is specified in step  210 . In an embodiment, an animator enters the value of one or more animation variables into an animation software tool to specify the position of the foot of the character model. In a further embodiment, the animator specifies the position of the foot of the character model by specifying the position and orientation of the parts of the associated leg of the character model, such as the thigh and calf portions of the character model&#39;s leg. In an alternate embodiment, the animator can specify the location of the foot of the character model directly, for example by specifying the position of a specific point of the foot model, for example the ankle joint, and orientation of the foot model around this joint. The animation system then determines the appropriate position and orientation of the associated leg of the character model using techniques such as inverse kinematics. 
     In an example application of step  210 ,  FIG. 3A  illustrates the position of a foot model  305 . In an embodiment, the position of the foot model is specified by the position of ankle joint  307 . In addition to the ankle joint, two additional coordinate spaces are associated with the foot model  305 : toe space coordinate system  309  and align space coordinate system  311 . In  FIG. 3A , the toe space  309  and align space  311  are aligned to the same position and orientation. The align space  311  represents the position and orientation of alignment plane  313 , specified for example in step  205 . It should be noted that the heel of the foot model  305  contacts the alignment plane  313  at heel contact point  314 . The toe space  309  represents the position of the toe of the foot model relative to the ankle joint  307 . 
     Returning the method  200 , the amount of heel roll is specified in step  215 . In an embodiment, an animator specifies the heel roll as an animation variable associated with a foot of the character model using an animation tool. In response to the heel roll specified in step  215 , step  220  rotates the align space defining the orientation of the alignment plane by the amount of heel roll specified in step  215 . In an embodiment, this rotation is expressed as a transformation matrix that rotates the align space around a heel contact point. 
       FIG. 3B  illustrates an example application of steps  215  and  220 . A heel roll amount  315  is specified for the foot model  305 . The align space  311  is rotated about the heel contact point  314  by the heel roll amount  315 . This in turn rotates the orientation of the alignment plane  313  to the position shown. For the purposes of illustration, the plane  313 ′ shows the original unrotated position of the alignment plane along with contact point  314 . 
     Method  200  continues with step  225 , in which the foot position is changed to compensate for the heel roll. In an embodiment, the foot position, as specified for example by the position of the ankle joint, is moved to a new position by applying the same transformation that was applied to move the align space. For example, this can be accomplished by applying the same transformation matrix to the position of the foot that was previously used to rotate the align space by the heel roll amount. In an embodiment, this transformation moves the foot position by rotating the ankle joint, or other reference point of the foot model, around the heel contact point. 
       FIG. 3C  illustrates an example application of step  225 . In this example, the ankle joint  307  of foot model  305  is moved to a new position based upon the heel roll transformation  315 . For the purposes of illustration, the ankle joint  307 ′ shows the original position of the ankle joint prior to the application of the transformation. 
     Step  230  poses the leg and foot model according to the new position and orientations specified by method  200 . In an embodiment, the foot model is rotated to align with the rotated alignment plane specified in step  220 . Additionally, the position of the foot model is changed to that specified in step  225 . For example, the foot model can be moved so that its ankle joint aligns with the ankle joint position specified in step  225 . In further embodiments, additional unrelated animation variables specifying other aspects of the foot model can be applied at this point as well. Additionally, an embodiment can determine the pose of the leg associated with the foot model using other animation variables and/or other techniques such as inverse kinematics. 
       FIGS. 3D and 3E  illustrate an example application of step  225 . In  FIG. 3D , the foot model  305  is shifted from its original position to the position specified by the newly moved ankle joint  307 . For the purposes of illustration, an outline  305 ′ shows the original position of the foot model  305  as specified by the ankle joint  307 ′. As can be seen in  FIG. 3D , the repositioning of the foot model  305  causes the heel contact point  314  to break contact with the alignment plane  313 ; however, contact will be restored when the foot model is rotated to align with the rotated align space  311 . 
       FIG. 3E  illustrates the rotation of the foot model  305  to the orientation specified by the heel roll. In this example, this is accomplished by rotating the foot model  305  around the heel contact point  314  to align the toe space  309  with the rotated align space  311 . As can be seen in  FIG. 3E , this rotation also has the effect of placing the heel contact point  314  of the foot model back in contact with alignment plane  313 . 
     Similar to method  200 ,  FIG. 4  illustrates a method  400  of repositioning the foot of a character model to compensate for ball roll according to an embodiment of the invention. At optional step  405 , an alignment plane is specified for one or more feet of the character model. In an embodiment, an animator uses an animation software tool to manually specify the orientation of the alignment plane to represent level or sloping ground. In an embodiment, the animation software tool assumes that the foot of the character model has been placed in contact with the ground based on the foot position specified by the animator; thus the alignment plane is automatically positioned so as to pass through a specific point of the foot model. This point, referred to as a ball contact point, can be defined as part of the foot model prior to the foot model&#39;s use in the animation software tool. In additional embodiments, the location of the ball contact point can be adjusted to meet the artistic demands of a particular scene. In another embodiment, the alignment plane can be automatically determined from the position and orientation of surfaces in the scene that are in close proximity to the foot of the character model. In this latter embodiment, step  405  may be performed after step  410 , which is described below. 
     The position of the foot of a character model is specified in step  410 . In an embodiment, an animator enters the value of one or more animation variables into an animation software tool to specify the position of the foot of the character model. In a further embodiment, the animator specifies the position of the foot of the character model by specifying the position and orientation of the parts of the associated leg of the character model, such as the thigh and calf portions of the character model&#39;s leg. In an alternate embodiment, the animator can specify the location of the foot of the character model directly, for example by specifying the position of a specific point of the foot model, for example the ankle joint, and orientation of the foot model around this joint. The animation system then determines the appropriate position and orientation of the associated leg of the character model using techniques such as inverse kinematics. 
     In an example application of step  410 ,  FIG. 5A  illustrates the position of a foot model  505 . In an embodiment, the position of the foot model is specified by the position of ankle joint  507 . In addition to the ankle joint, two additional coordinate spaces are associated with the foot model  505 : toe space coordinate system  509  and align space coordinate system  511 . In  FIG. 5A , the toe space  509  and align space  511  are aligned to the same position and orientation. The align space  511  represents the position and orientation of alignment plane  513 , specified for example in step  505 . It should be noted that the ball of the foot model  505  contacts the alignment plane  513  at ball contact point  520 . The toe space  509  represents the position of the toe of the foot model relative to the ankle joint  507 . 
     Returning the method  400 , the amount of ball roll is specified in step  415 . In an embodiment, an animator specifies the ball roll as an animation variable associated with a foot of the character model using an animation tool. In response to the ball roll specified in step  415 , step  420  modifies the foot model to reflect the specified amount of ball roll. In an embodiment, step  420  rotates the toe space of the foot model around a ball contact point by the amount of ball roll specified in step  415 . In an embodiment, this rotation is expressed as a transformation matrix. In an additional embodiment, the toe portion of the foot model is deformed to reflect the bending of the foot model around the foot ball joint. This deformation can be accomplished by rotating one or more control points defining the shape of the toe portion of the foot model by all or a portion of the amount of ball rotation specified in step  415 . Alternatively, this deformation can be accomplished by any other technique known in the art for modifying character models in response to animation variables specifying joint rotations, control points, or other attributes of a model. 
       FIG. 5B  illustrates an example application of steps  415  and  420 . A ball roll amount  515  is specified for the foot model  505 . The toe space  509  is rotated about the ball contact point  520  by the ball roll amount  515 . Additionally, the toe portion  530  of the foot model  505  is deformed to reflect the bending of the foot model around the ball joint  525 . 
     Method  400  continues with step  425 , in which the foot position is changed to compensate for the ball roll. In an embodiment, the foot position, as specified for example by the position of the ankle joint, is moved to a new position by applying the inverse of the transformation that was applied to move the toe space. For example, this can be accomplished by inverting the transformation matrix applied to the toe space and then applying the inverted transformation to the position of the foot. 
       FIG. 5C  illustrates an example application of step  425 . In this example, the ankle joint  507  of foot model  505  is moved to a new position based upon the inverse  535  of the ball roll transformation  515 . For the purposes of illustration, the ankle joint  507 ′ shows the original position of the ankle joint prior to the application of the inverse transformation  535 . 
     Step  430  poses the leg and foot model according to the new position and orientations specified by method  400 . In an embodiment, the foot model is rotated to align with the alignment plane specified in step  405 . Additionally, the position of the foot model is changed to that specified in step  425 . For example, the foot model can be moved so that its ankle joint aligns with the ankle joint position specified in step  425 . In further embodiments, additional unrelated animation variables specifying other aspects of the foot model can be applied at this point as well. Additionally, an embodiment can determine the pose of the leg associated with the foot model using other animation variables and/or other techniques such as inverse kinematics. 
       FIGS. 5D and 5E  illustrate an example application of step  425 . In  FIG. 5D , the foot model  505  is shifted from its original position to the position specified by the newly moved ankle joint  507 . For the purposes of illustration, an outline  505 ′ shows the original position of the foot model  505  as specified by the ankle joint  507 ′. As can be seen in  FIG. 5D , the repositioning of the foot model  505  causes the ball of the foot model to break contact with contact point  520 ; however, contact will be restored when the foot model is rotated to align with the alignment plane  513 . 
       FIG. 5E  illustrates the rotation of the foot model  505  to the orientation specified by the ball roll. In this example, this is accomplished by rotating the foot model  505  around the ball contact point  520  to align the toe space  509  with the align space  511 . As can be seen in  FIG. 5E , this rotation also has the effect of placing the ball contact point  520  in contact with the alignment plane  513 . 
     Although the foot roll rotation has been discussed with reference to examples of heel roll and ball roll, additional embodiments of the invention can implement additional foot rotations. For example, a toe roll rotation, defined as the rotation of the foot around a toe contact point at the toe of a foot model, can be implemented using a similar method to that described for heel roll, with the main difference being rotating the alignment plane in the opposite direction. Additionally, although the above discussion has assumed that an animator specifies the foot position and foot roll, in further embodiments, these parameters can be specified automatically by a software application, for example using a simulation or referencing a predetermined animation cycle. 
       FIG. 6  illustrates an example computer system suitable for implementing an embodiment of the invention.  FIG. 6  illustrates an example computer system  1000  capable of implementing an embodiment of the invention. Computer system  1000  typically includes a monitor  1100 , computer  1200 , a keyboard  1300 , a user input device  1400 , and a network interface  1500 . User input device  1400  includes a computer mouse, a trackball, a track pad, graphics tablet, touch screen, and/or other wired or wireless input devices that allow a user to create or select graphics, objects, icons, and/or text appearing on the monitor  1100 . Embodiments of network interface  1500  typically provides wired or wireless communication with an electronic communications network, such as a local area network, a wide area network, for example the Internet, and/or virtual networks, for example a virtual private network (VPN). 
     Computer  1200  typically includes components such as one or more general purpose processors  1600 , and memory storage devices, such as a random access memory (RAM)  1700 , disk drives  1800 , and system bus  1900  interconnecting the above components. RAM  1700  and disk drive  1800  are examples of tangible media for storage of data, audio/video files, computer programs, applet interpreters or compilers, virtual machines, embodiments of the herein described invention including geometric scene data, object data files, shader descriptors, a rendering engine, output image files, texture maps, and displacement maps. Further embodiments of computer  1200  can include specialized audio and video subsystems for processing and outputting audio and graphics data. Other types of tangible media include floppy disks; removable hard disks; optical storage media such as DVD-ROM, CD-ROM; non-volatile memory devices such as flash memories; read-only-memories (ROMS); battery-backed volatile memories; and networked storage devices. 
     It should be noted that once the posed or deformed model has been created using one or more of the above discussed embodiments, any rendering technique, for example ray-tracing or scanline rendering, can create a final image or frame from the model in combination with lighting, shading, texture mapping, and any other image processing information. 
     Further embodiments can be envisioned to one of ordinary skill in the art after reading the attached documents. In other embodiments, combinations or sub-combinations of the above disclosed invention can be advantageously made. The block diagrams of the architecture and flow charts are grouped for ease of understanding. However it should be understood that combinations of blocks, additions of new blocks, re-arrangement of blocks, and the like are contemplated in alternative embodiments of the present invention. 
     The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims.