WO2011071801A2 - Suivi de cible visuelle - Google Patents

Suivi de cible visuelle Download PDF

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Publication number
WO2011071801A2
WO2011071801A2 PCT/US2010/059054 US2010059054W WO2011071801A2 WO 2011071801 A2 WO2011071801 A2 WO 2011071801A2 US 2010059054 W US2010059054 W US 2010059054W WO 2011071801 A2 WO2011071801 A2 WO 2011071801A2
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WO
WIPO (PCT)
Prior art keywords
pixel
model
synthesized
pixels
silhouette
Prior art date
Application number
PCT/US2010/059054
Other languages
English (en)
Other versions
WO2011071801A3 (fr
Inventor
Ryan M. Geiss
Original Assignee
Microsoft Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Microsoft Corporation filed Critical Microsoft Corporation
Priority to CN201080055300XA priority Critical patent/CN102648032B/zh
Publication of WO2011071801A2 publication Critical patent/WO2011071801A2/fr
Publication of WO2011071801A3 publication Critical patent/WO2011071801A3/fr

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Classifications

    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F13/00—Video games, i.e. games using an electronically generated display having two or more dimensions
    • A63F13/20—Input arrangements for video game devices
    • A63F13/21—Input arrangements for video game devices characterised by their sensors, purposes or types
    • A63F13/213—Input arrangements for video game devices characterised by their sensors, purposes or types comprising photodetecting means, e.g. cameras, photodiodes or infrared cells
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F13/00—Video games, i.e. games using an electronically generated display having two or more dimensions
    • A63F13/40—Processing input control signals of video game devices, e.g. signals generated by the player or derived from the environment
    • A63F13/42—Processing input control signals of video game devices, e.g. signals generated by the player or derived from the environment by mapping the input signals into game commands, e.g. mapping the displacement of a stylus on a touch screen to the steering angle of a virtual vehicle
    • A63F13/428—Processing input control signals of video game devices, e.g. signals generated by the player or derived from the environment by mapping the input signals into game commands, e.g. mapping the displacement of a stylus on a touch screen to the steering angle of a virtual vehicle involving motion or position input signals, e.g. signals representing the rotation of an input controller or a player's arm motions sensed by accelerometers or gyroscopes
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T7/00—Image analysis
    • G06T7/20—Analysis of motion
    • G06T7/246—Analysis of motion using feature-based methods, e.g. the tracking of corners or segments
    • G06T7/251—Analysis of motion using feature-based methods, e.g. the tracking of corners or segments involving models
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
    • G06V20/00—Scenes; Scene-specific elements
    • G06V20/60—Type of objects
    • G06V20/64—Three-dimensional objects
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
    • G06V40/00—Recognition of biometric, human-related or animal-related patterns in image or video data
    • G06V40/20—Movements or behaviour, e.g. gesture recognition
    • G06V40/23—Recognition of whole body movements, e.g. for sport training
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F13/00—Video games, i.e. games using an electronically generated display having two or more dimensions
    • A63F13/80—Special adaptations for executing a specific game genre or game mode
    • A63F13/833—Hand-to-hand fighting, e.g. martial arts competition
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F2300/00—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game
    • A63F2300/10—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game characterized by input arrangements for converting player-generated signals into game device control signals
    • A63F2300/1012—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game characterized by input arrangements for converting player-generated signals into game device control signals involving biosensors worn by the player, e.g. for measuring heart beat, limb activity
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F2300/00—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game
    • A63F2300/10—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game characterized by input arrangements for converting player-generated signals into game device control signals
    • A63F2300/1087—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game characterized by input arrangements for converting player-generated signals into game device control signals comprising photodetecting means, e.g. a camera
    • A63F2300/1093—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game characterized by input arrangements for converting player-generated signals into game device control signals comprising photodetecting means, e.g. a camera using visible light
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F2300/00—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game
    • A63F2300/50—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game characterized by details of game servers
    • A63F2300/55—Details of game data or player data management
    • A63F2300/5546—Details of game data or player data management using player registration data, e.g. identification, account, preferences, game history
    • A63F2300/5553—Details of game data or player data management using player registration data, e.g. identification, account, preferences, game history user representation in the game field, e.g. avatar
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F2300/00—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game
    • A63F2300/60—Methods for processing data by generating or executing the game program
    • A63F2300/6045—Methods for processing data by generating or executing the game program for mapping control signals received from the input arrangement into game commands
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F2300/00—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game
    • A63F2300/60—Methods for processing data by generating or executing the game program
    • A63F2300/66—Methods for processing data by generating or executing the game program for rendering three dimensional images
    • A63F2300/6607—Methods for processing data by generating or executing the game program for rendering three dimensional images for animating game characters, e.g. skeleton kinematics
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F2300/00—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game
    • A63F2300/60—Methods for processing data by generating or executing the game program
    • A63F2300/66—Methods for processing data by generating or executing the game program for rendering three dimensional images
    • A63F2300/6615—Methods for processing data by generating or executing the game program for rendering three dimensional images using models with different levels of detail [LOD]
    • A—HUMAN NECESSITIES
    • A63—SPORTS; GAMES; AMUSEMENTS
    • A63F—CARD, BOARD, OR ROULETTE GAMES; INDOOR GAMES USING SMALL MOVING PLAYING BODIES; VIDEO GAMES; GAMES NOT OTHERWISE PROVIDED FOR
    • A63F2300/00—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game
    • A63F2300/80—Features of games using an electronically generated display having two or more dimensions, e.g. on a television screen, showing representations related to the game specially adapted for executing a specific type of game
    • A63F2300/8029—Fighting without shooting
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T2207/00—Indexing scheme for image analysis or image enhancement
    • G06T2207/10—Image acquisition modality
    • G06T2207/10016—Video; Image sequence
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T2207/00—Indexing scheme for image analysis or image enhancement
    • G06T2207/10—Image acquisition modality
    • G06T2207/10028—Range image; Depth image; 3D point clouds
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T2207/00—Indexing scheme for image analysis or image enhancement
    • G06T2207/30—Subject of image; Context of image processing
    • G06T2207/30196—Human being; Person

Definitions

  • One disclosed embodiment includes representing a human target with a machine-readable model configured for adjustment into a plurality of different poses and receiving an observed depth image of the human target from a source.
  • One or more push force vectors are applied to one or more force-receiving locations of the model to push the model in an XY plane towards a silhouette of the human target in the observed depth image when portions of the model are shifted away from the silhouette of the human target in the observed depth image.
  • One or more pull force vectors are applied to one or more force-receiving locations of the model to pull the model in an XY plane towards the silhouette of the human target in the observed depth image when portions of the observed depth image are shifted away from the silhouette of the model.
  • FIG. 1A shows an embodiment of an exemplary target recognition, analysis, and tracking system tracking a game player playing a boxing game.
  • FIG. IB shows the game player of FIG. 1A throwing a punch that is tracked and interpreted as a game control that causes a player avatar to throw a punch in game space.
  • FIG. 2 schematically shows a computing system in accordance with an embodiment of the present disclosure.
  • FIG. 3 shows an exemplary body model used to represent a human target.
  • FIG. 4 shows a substantially frontal view of an exemplary skeletal model used to represent a human target.
  • FIG. 5 shows a skewed view of an exemplary skeletal model used to represent a human target.
  • FIG. 6 shows an exemplary mesh model used to represent a human target.
  • FIG. 7 shows a flow diagram of an example method of visually tracking a target.
  • FIG. 8 shows an exemplary observed depth image.
  • FIG. 9 shows an exemplary synthesized depth image.
  • FIG. 10 schematically shows some of the pixels making up a synthesized depth image.
  • FIG. 11A schematically shows the application of a force to a force-receiving location of a model.
  • FIG. 11B schematically shows a result of applying the force to the force-receiving location of the model of FIG. 11A.
  • FIG. 12A shows a player avatar rendered from the model of FIG.
  • FIG. 12B shows a player avatar rendered from the model of FIG.
  • FIG. 13 schematically shows comparing a synthesized depth image to a corresponding observed depth image.
  • FIG. 14 schematically shows identifying regions of mismatched synthesized pixels of the comparison of FIG. 13.
  • FIG. 15 schematically shows another comparison of a synthesized depth image and a corresponding observed depth image, wherein regions of mismatched pixels correspond to various pixel cases.
  • FIG. 16 schematically shows an example embodiment of a pull pixel case.
  • FIG. 17 schematically shows an example embodiment of a push pixel case.
  • FIG. 18 shows a table detailing example relationships between various pixel cases and skeletal model joints.
  • FIG. 19 illustrates application of constraints to a model representing a target.
  • FIG. 20 illustrates another application of constraints to a model representing a target.
  • FIG. 21 illustrates yet another application of constraints to a model representing a target.
  • the present disclosure is directed to target recognition, analysis, and tracking.
  • the use of a depth camera or other source for acquiring depth information for one or more targets is disclosed.
  • Such depth information may then be used to efficiently and accurately model and track the one or more targets, as described in detail below.
  • the target recognition, analysis, and tracking described herein provides a robust platform in which one or more targets can be consistently tracked at a relatively fast frame rate, even when the target(s) move into poses that have been considered difficult to analyze using other approaches (e.g., when two or more targets partially overlap and/or occlude one another!
  • FIG. 1A shows a nonlimiting example of a target recognition, analysis, and tracking system 10.
  • FIG. 1A shows a computer gaming system 12 that may be used to play a variety of different games, play one or more different media types, and/or control or manipulate non-game applications.
  • FIG. 1A also shows a display 14 in the form of a high-definition television, or HDTV 16, which may be used to present game visuals to game players, such as game player 18.
  • FIG. 1A shows a high-definition television, or HDTV 16, which may be used to present game visuals to game players, such as game player 18.
  • FIG. 1A shows a nonlimiting example of a target recognition, analysis, and tracking system 10.
  • FIG. 1A shows a computer gaming system 12 that may be used to play a variety of different games, play one or more different media types, and/or control or manipulate non-game applications.
  • FIG. 1A also shows a display 14 in the form of a high-definition television, or HDTV 16, which may be used
  • FIG. 1A shows a capture device in the form of a depth camera 20, which may be used to visually monitor one or more game players, such as game player 18.
  • the example shown in FIG. 1A is nonlimiting. As described below with reference to FIG. 2, a variety of different types of target recognition, analysis, and tracking systems may be used without departing from the scope of this disclosure.
  • a target recognition, analysis, and tracking system may be used to recognize, analyze, and/or track one or more targets, such as game player 18.
  • FIG. 1A shows a scenario in which game player 18 is tracked using depth camera 20 so that the movements of game player 18 may be interpreted by gaming system 12 as controls that can be used to affect the game being executed by gaming system 12. In other words, game player 18 may use his movements to control the game. The movements of game player 18 may be interpreted as virtually any type of game control.
  • FIG. 1A shows game player
  • FIG. IB shows HDTV 16 visually presenting player avatar 24 throwing a punch that strikes boxing opponent 22 responsive to game player 18 throwing a punch in physical space.
  • Other movements by game player 18 may be interpreted as other controls, such as controls to bob, weave, shuffle, block, jab, or throw a variety of different power punches.
  • some movements may be interpreted into controls that serve purposes other than controlling player avatar 24. For example, the player may use movements to end, pause, or save a game, select a level, view high scores, communicate with a friend, etc.
  • a target may include a human and an object.
  • a player of an electronic game may be holding an object, such that the motions of the player and the object are utilized to adjust and/or control parameters of the electronic game.
  • the motion of a player holding a racket may be tracked and utilized for controlling an on-screen racket in an electronic sports game.
  • the motion of a player holding an object may be tracked and utilized for controlling an on-screen weapon in an electronic combat game.
  • Target recognition, analysis, and tracking systems may be used to interpret target movements as operating system and/or application controls that are outside the realm of gaming.
  • the illustrated boxing scenario is provided as an example, but is not meant to be limiting in any way. To the contrary, the illustrated scenario is intended to demonstrate a general concept, which may be applied to a variety of different applications without departing from the scope of this disclosure.
  • FIGs. 1A and IB show a nonlimiting example in the form of gaming system 12, HDTV 16, and depth camera 20.
  • FIG. 2 schematically shows a computing system 40 that may perform one or more of the target recognition, tracking, and analysis methods and processes described herein.
  • Computing system 40 may take a variety of different forms, including, but not limited to, gaming consoles, personal computing gaming systems, military tracking and/or targeting systems, and character acquisition systems offering green- screen or motion-capture functionality, among others.
  • Computing system 40 may include a logic subsystem 42, a data- holding subsystem 44, a display subsystem 46, and/or a capture device 48.
  • the computing system may optionally include components not shown in FIG. 2, and/or some components shown in FIG. 2 may be peripheral components that are not integrated into the computing system.
  • Logic subsystem 42 may include one or more physical devices configured to execute one or more instructions.
  • the logic subsystem may be configured to execute one or more instructions that are part of one or more programs, routines, objects, components, data structures, or other logical constructs. Such instructions may be implemented to perform a task, implement a data type, transform the state of one or more devices, or otherwise arrive at a desired result.
  • the logic subsystem may include one or more processors that are configured to execute software instructions. Additionally or alternatively, the logic subsystem may include one or more hardware or firmware logic machines configured to execute hardware or firmware instructions.
  • the logic subsystem may optionally include individual components that are distributed throughout two or more devices, which may be remotely located in some embodiments.
  • Data-holding subsystem 44 may include one or more physical devices configured to hold data and/or instructions executable by the logic subsystem to implement the herein described methods and processes. When such methods and processes are implemented, the state of data-holding subsystem 44 may be transformed (e.g., to hold different data).
  • Data-holding subsystem 44 may include removable media and/or built-in devices.
  • Data- holding subsystem 44 may include optical memory devices, semiconductor memory devices (e.g., RAM, EEPROM, flash, etc.), and/or magnetic memory devices, among others.
  • Data-holding subsystem 44 may include devices with one or more of the following characteristics : volatile, nonvolatile, dynamic, static, read/write, read-only, random access, sequential access, location addressable, file addressable, and content addressable.
  • logic subsystem 42 and data-holding subsystem 44 may be integrated into one or more common devices, such as an application specific integrated circuit or a system on a chip.
  • FIG. 2 also shows an aspect of the data-holding subsystem in the form of computer-readable removable media 50, which may be used to store and/or transfer data and/or instructions executable to implement the herein described methods and processes.
  • Display subsystem 46 may be used to present a visual representation of data held by data-holding subsystem 44. As the herein described methods and processes change the data held by the data-holding subsystem, and thus transform the state of the data-holding subsystem, the state of display subsystem 46 may likewise be transformed to visually represent changes in the underlying data. As a nonlimiting example, the target recognition, tracking, and analysis described herein may be reflected via display subsystem 46 in the form of a game character that changes poses in game space responsive to the movements of a game player in physical space. Display subsystem 46 may include one or more display devices utilizing virtually any type of technology. Such display devices may be combined with logic subsystem 42 and/or data-holding subsystem 44 in a shared enclosure, or such display devices may be peripheral display devices, as shown in FIGs. 1A and IB.
  • Computing system 40 further includes a capture device 48 configured to obtain depth images of one or more targets.
  • Capture device 48 may be configured to capture video with depth information via any suitable technique (e.g., time-of-flight, structured light, stereo image, etc.).
  • capture device 48 may include a depth camera, a video camera, stereo cameras, and/or other suitable capture devices.
  • the capture device 48 may emit infrared light to the target and may then use sensors to detect the backscattered light from the surface of the target.
  • pulsed infrared light may be used, wherein the time between an outgoing light pulse and a corresponding incoming light pulse may be measured and used to determine a physical distance from the capture device to a particular location on the target.
  • the phase of the outgoing light wave may be compared to the phase of the incoming light wave to determine a phase shift, and the phase shift may be used to determine a physical distance from the capture device to a particular location on the target.
  • time-of-flight analysis may be used to indirectly determine a physical distance from the capture device to a particular location on the target by analyzing the intensity of the reflected beam of light over time, via a technique such as shuttered light pulse imaging.
  • structured light analysis may be utilized by capture device 48 to capture depth information.
  • patterned light i.e., light displayed as a known pattern such as grid pattern or a stripe pattern
  • the pattern may become deformed in response, and this deformation of the pattern may be studied to determine a physical distance from the capture device to a particular location on the target.
  • the capture device may include two or more physically separated cameras that view a target from different angles, to obtain visual stereo data.
  • the visual stereo data may be resolved to generate a depth image.
  • capture device 48 may utilize other technologies to measure and/or calculate depth values. Additionally, capture device 48 may organize the calculated depth information into "Z layers," i.e., layers perpendicular to a Z axis extending from the depth camera along its line of sight to the viewer.
  • two or more different cameras may be incorporated into an integrated capture device.
  • a depth camera and a video camera e.g., RGB video camera
  • two or more separate capture devices may be cooperatively used.
  • a depth camera and a separate video camera may be used.
  • a video camera it may be used to provide target tracking data, confirmation data for error correction of target tracking, image capture, face recognition, high-precision tracking of fingers (or other small features), light sensing, and/or other functions.
  • a capture device may include one or more onboard processing units configured to perform one or more target analysis and/or tracking functions.
  • a capture device may include firmware to facilitate updating such onboard processing logic.
  • Computing system 40 may optionally include one or more input devices, such as controller 52 and controller 54.
  • Input devices may be used to control operation of the computing system.
  • input devices such as controller 52 and/or controller 54 can be used to control aspects of a game not controlled via the target recognition, tracking, and analysis methods and procedures described herein.
  • input devices such as controller 52 and/or controller 54 may include one or more of accelerometers, gyroscopes, infrared target/sensor systems, etc., which may be used to measure movement of the controllers in physical space.
  • the computing system may optionally include and/or utilize input gloves, keyboards, mice, track pads, trackballs, touch screens, buttons, switches, dials, and/or other input devices.
  • target recognition, tracking, and analysis may be used to control or augment aspects of a game, or other application, conventionally controlled by an input device, such as a game controller.
  • the target tracking described herein can be used as a complete replacement to other forms of user input, while in other embodiments such target tracking can be used to complement one or more other forms of user input.
  • Computing system 40 may be configured to perform the target tracking methods described herein. However, it should be understood that computing system 40 is provided as a nonlimiting example of a device that may perform such target tracking. Other devices are within the scope of this disclosure. [0051] Computing system 40, or another suitable device, may be configured to represent each target with a model. As described in more detail below, information derived from such a model can be compared to information obtained from a capture device, such as a depth camera, so that the fundamental proportions or shape of the model, as well as its current pose, can be adjusted to more accurately represent the modeled target.
  • the model may be represented by one or more polygonal meshes, by a set of mathematical primitives, and/or via other suitable machine representations of the modeled target.
  • FIG. 3 shows a nonlimiting visual representation of an example body model 70.
  • Body model 70 is a machine representation of a modeled target (e.g., game player 18 from FIGs. 1A and IB).
  • the body model may include one or more data structures that include a set of variables that collectively define the modeled target in the language of a game or other application/operating system.
  • a model of a target can be variously configured without departing from the scope of this disclosure.
  • a model e.g., a machine- readable model
  • Each body part may be characterized as a mathematical primitive, examples of which include, but are not limited to, spheres, anisotropically- scaled spheres, cylinders, anisotropic cylinders, smooth cylinders, boxes, beveled boxes, prisms, and the like.
  • the target may be represented by a model including a plurality of portions, each portion associated with a part index corresponding to a part of the target.
  • the part index may be a body-part index corresponding to a part of the human target.
  • body model 70 of FIG. 3 includes body parts bpl through bpl4, each of which represents a different portion of the modeled target.
  • Each body part is a three-dimensional shape.
  • bp3 is a rectangular prism that represents the left hand of a modeled target
  • bp 5 is an octagonal prism that represents the left upper-arm of the modeled target.
  • Body model 70 is exemplary in that a body model may contain any number of body parts, each of which may be any machine-understandable representation of the corresponding part of the modeled target.
  • a model including two or more body parts may also include one or more joints. Each joint may allow one or more body parts to move relative to one or more other body parts.
  • a model representing a human target may include a plurality of rigid and/or deformable body parts, wherein some body parts may represent a corresponding anatomical body part of the human target.
  • each body part of the model may comprise one or more structural members (i.e., "bones"), with joints located at the intersection of adjacent bones. It is to be understood that some bones may correspond to anatomical bones in a human target and/or some bones may not have corresponding anatomical bones in the human target.
  • a human target may be modeled as a skeleton including a plurality of skeletal points, each skeletal point having a three- dimensional location in world space.
  • the various skeletal points may correspond to actual joints of a human target, terminal ends of a human target's extremities, and/or points without a direct anatomical link to the human target.
  • Each skeletal point has at least three degrees of freedom (e.g., world space x, y, z).
  • the skeleton can be fully defined by 3 x ⁇ values, where ⁇ is equal to the total number of skeletal points included in the skeleton.
  • a skeleton with 33 skeletal points can be defined by 99 values, for example.
  • some skeletal points may account for axial roll angles.
  • the bones and joints may collectively make up a skeletal model, which may be a constituent element of the model.
  • the skeletal model may include one or more skeletal members for each body part and a joint between adjacent skeletal members.
  • Exemplary skeletal model 80 and exemplary skeletal model 82 are shown in FIGs. 4 and 5, respectively.
  • FIG. 4 shows a skeletal model 80 as viewed from the front, with joints jl through j33.
  • FIG. 5 shows a skeletal model 82 as viewed from a skewed view, also with joints jl through j33.
  • Skeletal model 82 further includes roll joints j34 through j47, where each roll joint may be utilized to track axial roll angles.
  • an axial roll angle may be used to define a rotational orientation of a limb relative to its parent limb and/or the torso.
  • roll joint j40 may be used to indicate the direction the associated wrist is pointing (e.g., palm facing up).
  • joints can receive forces and adjust the skeletal model, as described below, roll joints may instead be constructed and utilized to track axial roll angles.
  • an axial roll angle may be determined. For example, if examining a lower leg, the orientation of the lower leg relative to the associated upper leg and hips may be examined in order to determine an axial roll angle.
  • some models may include a skeleton and/or body parts that serve as a machine representation of a modeled target.
  • a model may alternatively or additionally include a wireframe mesh, which may include hierarchies of rigid polygonal meshes, one or more deformable meshes, or any combination of the two.
  • FIG. 6 shows a model 90 including a plurality of triangles (e.g., triangle 92) arranged in a mesh that defines the shape of the body model. Such a mesh may include bending limits at each polygonal edge.
  • the number of triangles, and/or other polygons, that collectively constitute the mesh can be selected to achieve a desired balance between quality and computational expense. More triangles may provide higher quality and/or more accurate models, while fewer triangles may be less computationally demanding.
  • a body model including a polygonal mesh need not include a skeleton, although it may in some embodiments.
  • the above described body part models, skeletal models, and polygonal meshes are nonlimiting example types of models that may be used as machine representations of a modeled target.
  • Other models are also within the scope of this disclosure.
  • some models may include patches, non-uniform rational B-splines, subdivision surfaces, or other high-order surfaces.
  • a model may also include surface textures and/or other information to more accurately represent clothing, hair, and/or other aspects of a modeled target.
  • a model may optionally include information pertaining to a current pose, one or more past poses, and/or model physics. It is to be understood that any model that can be posed and then rasterized to (or otherwise rendered to or expressed by) a synthesized depth image, is compatible with the herein described target recognition, analysis, and tracking.
  • a model serves as a representation of a target, such as game player 18 in FIGs. 1A and IB.
  • information from a capture device such as depth camera 20 in FIGs. 1A and IB, can be used to adjust a pose and/or the fundamental size/shape of the model so that it more accurately represents the target.
  • one or more forces may be applied to one or more force-receiving aspects of the model to adjust the model into a pose that more closely corresponds to the pose of the target in physical space.
  • the force may be applied to a joint, a centroid of a body part, a vertex of a triangle, or any other suitable force-receiving aspect of the model.
  • two or more different calculations may be used when determining the direction and/or magnitude of the force.
  • differences between an observed image of the target, as retrieved by a capture device, and a rasterized (i.e., synthesized) image of the model may be used to determine the forces that are applied to the model in order to adjust the body into a different pose.
  • FIG. 7 shows a flow diagram of an example method 100 of tracking a target using a model (e.g., body model 70 of FIG. 3).
  • the target may be a human, and the human may be one of two or more targets being tracked.
  • method 100 may be executed by a computing system (e.g., gaming system 12 shown in FIG. 1 and/or computing system 40 shown in FIG. 2) to track one or more players interacting with an electronic game being played on the computing system.
  • a computing system e.g., gaming system 12 shown in FIG. 1 and/or computing system 40 shown in FIG. 2
  • tracking of the players allows physical movements of those players to act as a real-time user interface that adjusts and/or controls parameters of the electronic game.
  • the tracked motions of a player may be used to move an on-screen character or avatar in an electronic role-playing game.
  • the tracked motions of a player may be used to control an on-screen vehicle in an electronic racing game.
  • the tracked motions of a player may be used to control the building or organization of objects in a virtual environment.
  • method 100 includes receiving an observed depth image of the target from a source.
  • the source may be a depth camera configured to obtain depth information about the target via a suitable technique such as time-of-flight analysis, structured light analysis, stereo vision analysis, or other suitable techniques.
  • the observed depth image may include a plurality of observed pixels, where each observed pixel has an observed depth value.
  • the observed depth value includes depth information of the target as viewed from the source. Knowing the depth camera's horizontal and vertical field of view, as well as the depth value for a pixel and the pixel address of that pixel, the world space position of a surface imaged by that pixel can be determined. For convenience, the world space position of a surface imaged by the pixel may be referred to as the world space position of the pixel.
  • FIG. 8 shows a visual representation of an exemplary observed depth image 140.
  • observed depth image 140 captures an exemplary observed pose of a person (e.g., game player 18) standing with his arms raised.
  • method 100 may optionally include downsampling the observed depth image to a lower processing resolution. Downsampling to a lower processing resolution may allow the observed depth image to be more easily utilized and/or more quickly processed with less computing overhead.
  • method 100 may optionally include removing non-player background elements from the observed depth image. Removing such background elements may include separating various regions of the observed depth image into background regions and regions occupied by the image of the target. Background regions can be removed from the image or identified so that they can be ignored during one or more subsequent processing steps. Virtually any background removal technique may be used, and information from tracking (and from the previous frame) can optionally be used to assist and improve the quality of background-removal.
  • method 100 may optionally include removing and/or smoothing one or more high-variance and/or noisy depth values from the observed depth image.
  • high-variance and/or noisy depth values in the observed depth image may result from a number of different sources, such as random and/or systematic errors occurring during the image capturing process, defects and/or aberrations resulting from the capture device, etc. Since such high- variance and/or noisy depth values may be artifacts of the image capturing process, including these values in any future analysis of the image may skew results and/or slow calculations. Thus, removal of such values may provide better data integrity for future calculations.
  • Other depth values may also be filtered.
  • the accuracy of growth operations described below with reference to step 118 may be enhanced by selectively removing pixels satisfying one or more removal criteria. For instance, if a depth value is halfway between a hand and the torso that the hand is occluding, removing this pixel can prevent growth operations from spilling from one body part onto another during subsequent processing steps.
  • method 100 may optionally include filling in and/or reconstructing portions of missing and/or removed depth information. Such backfilling may be accomplished by averaging nearest neighbors, filtering, and/or any other suitable method.
  • method 100 may include obtaining a model (e.g., body model 70 of FIG. 3).
  • the model may include a skeleton comprising a plurality of skeletal points, one or more polygonal meshes, one or more mathematical primitives, one or more high- order surfaces, and/or other features used to provide a machine representation of the target.
  • the model may exist as an instance of one or more data structures existing on a computing system.
  • the model may be a posed model obtained from a previous time step (i.e., frame).
  • a posed model resulting from a previous iteration of method 100 may be obtained.
  • the model may be adjusted from one frame to the next based on the observed depth image for the current frame and the model from the previous frame.
  • the previous frame's model may be projected by a momentum calculation to yield an estimated model for comparison to the current observed depth image. This may be done without looking up a model from a database or otherwise starting from scratch every frame. Instead, incremental changes may be made to the model in successive frames.
  • a pose may be determined by one or more algorithms, which can analyze a depth image and identify, at a coarse level, where the target(s) of interest (e.g., human(s)) are located and/or the pose of such target(s). Algorithms can be used to select a pose during an initial iteration or whenever it is believed that the algorithm can select a pose more accurate than the pose calculated during a previous time step.
  • the target(s) of interest e.g., human(s)
  • Algorithms can be used to select a pose during an initial iteration or whenever it is believed that the algorithm can select a pose more accurate than the pose calculated during a previous time step.
  • the model may be obtained from a database and/or other program.
  • a model may not be available during a first iteration of method 100, in which case the model may be obtained from a database including one or more models. In such
  • model from the database may be chosen using a searching algorithm designed to select a model exhibiting a pose similar to that of the target. Even if a model from a previous time step is available, a model from a database may be used. For example, a model from a database may be used after a certain number of frames, if the target has changed poses by more than a predetermined threshold, and/or according to other criteria.
  • the model, or portions thereof may be synthesized.
  • the target's body core torso, midsection, and hips
  • that model may be originally constructed using the contents of an observed depth image, where the outline of the target in the image (i.e., the silhouette) may be used to shape the mesh in the X and Y dimensions.
  • the observed depth value(s) in that area of the observed depth image may be used to "mold" the mesh in the XY direction, as well as in the Z direction, of the model to more favorably represent the target's body shape.
  • Method 100 may further include representing any clothing appearing on the target using a suitable approach.
  • a suitable approach may include adding to the model auxiliary geometry in the form of primitives or polygonal meshes, and optionally adjusting the auxiliary geometry based on poses to reflect gravity, cloth simulation, etc.
  • Such an approach may facilitate molding the models into more realistic representations of the targets.
  • method 100 may optionally comprise applying a momentum algorithm to the model. Because the momentum of various parts of a target may predict change in an image sequence, such an algorithm may assist in obtaining the pose of the model.
  • the momentum algorithm may use a trajectory of each of the joints or vertices of a model over a fixed number of a plurality of previous frames to assist in obtaining the model.
  • knowledge that different portions of a target can move a limited distance in a time frame can be used as a constraint in obtaining a model.
  • a constraint may be used to rule out certain poses when a prior frame is known.
  • method 100 may also include rasterizing the model into a synthesized depth image. Rasterization allows the model described by mathematical primitives, polygonal meshes, or other objects to be converted into a synthesized depth image described by a plurality of pixels.
  • Rasterizing may be carried out using one or more different techniques and/or algorithms.
  • rasterizing the model may include projecting a representation of the model onto a two-dimensional plane.
  • rasterizing may include projecting and rasterizing the collection of body-part shapes onto a two-dimensional plane. For each pixel in the two dimensional plane onto which the model is projected, various different types of information may be stored.
  • FIG. 9 shows a visual representation 150 of an exemplary synthesized depth image corresponding to body model 70 of FIG. 3.
  • FIG. 10 shows a pixel matrix 160 of a portion of the same synthesized depth image.
  • each synthesized pixel in the synthesized depth image may include a synthesized depth value.
  • the synthesized depth value for a given synthesized pixel may be the depth value from the corresponding part of the model that is represented by that synthesized pixel, as determined during rasterization. In other words, if a portion of a forearm body part (e.g., forearm body part bp4 of FIG.
  • a corresponding synthesized pixel (e.g., synthesized pixel 162 of FIG. 10) may be given a synthesized depth value (e.g., synthesized depth value 164 of FIG. 10) equal to the depth value of that portion of the forearm body part.
  • synthesized pixel 162 has a synthesized depth value of 382 cm.
  • a neighboring hand body part e.g., hand body part bp3 of FIG. 3
  • a corresponding synthesized pixel (e.g., synthesized pixel 166 of FIG.
  • synthesized depth value 168 of FIG. 10 may be given a synthesized depth value (e.g., synthesized depth value 168 of FIG. 10) equal to the depth value of that portion of the hand body part.
  • synthesized pixel 166 has a synthesized depth value of 383 cm.
  • the corresponding observed depth value is the depth value observed by the depth camera at the same pixel address. It is to be understood that the above is provided as an example. Synthesized depth values may be saved in any unit of measurement or as a dimensionless number.
  • each synthesized pixel in the synthesized depth image may include an original body-part index determined during rasterization. Such an original body-part index may indicate to which of the body parts of the model that pixel corresponds.
  • synthesized pixel 162 has an original body-part index of bp4
  • synthesized pixel 166 has an original body-part index of bp3.
  • the original body-part index of a synthesized pixel may be nil if the synthesized pixel does not correspond to a body part of the target (e.g., if the synthesized pixel is a background pixel).
  • synthesized pixels that do not correspond to a body part may be given a different type of index.
  • a body-part index may be a discrete value or a probability distribution indicating the likelihood that a pixel belongs to two or more different body parts.
  • each synthesized pixel in the synthesized depth image may include an original player index determined during rasterization, where the original player index corresponds to the target. For example, if there are two targets, synthesized pixels corresponding to the first target will have a first player index and synthesized pixels corresponding to the second target will have a second player index.
  • the pixel matrix 160 corresponds to only one target, therefore synthesized pixel 162 has an original player index of PI, and synthesized pixel 166 has an original player index of PI.
  • Other types of indexing systems may be used without departing from the scope of this disclosure.
  • each synthesized pixel in the synthesized depth image may include a pixel address.
  • the pixel address may define the position of a pixel relative to other pixels.
  • synthesized pixel 162 has a pixel address of [5,7]
  • synthesized pixel 166 has a pixel address of [4,8] . It is to be understood that other addressing schemes may be used without departing from the scope of this disclosure.
  • each synthesized pixel may optionally include other types of information, some of which may be obtained after rasterization.
  • each synthesized pixel may include an updated body-part index, which may be determined as part of a snap operation performed during rasterization, as described below.
  • Each synthesized pixel may include an updated player index, which may be determined as part of a snap operation performed during rasterization.
  • Each synthesized pixel may include an updated body-part index, which may be obtained as part of a grow/fix operation, as described below.
  • Each synthesized pixel may include an updated player index, which may be obtained as part of a grow/fix operation, as described above.
  • Each synthesized pixel may include an updated synthesized depth value, which may be obtained as part of the snap operation.
  • pixel information provided above are not limiting.
  • Various different types of information may be stored as part of each pixel.
  • Such information may include information obtained from the depth image, information obtained from rasterizing the machine-readable model, and/or information derived from one or more processing operations (e.g., snap operation, grow operation, etc.).
  • processing operations e.g., snap operation, grow operation, etc.
  • Such information can be stored as part of a common data structure, or the different types of information may be stored in different data structures that can be mapped to particular pixel locations (e.g., via a pixel address).
  • player indices and/or body-part indices obtained as part of a snap operation during rasterization may be stored in a rasterization map and/or a snap map
  • player indices and/or body-part indices obtained as part of a grow/fix operation after rasterization may be stored in a grow map, as described below.
  • Nonlimiting examples of other types of pixel information that may be assigned to each pixel include, but are not limited to, joint indices, bone indices, vertex indices, triangle indices, centroid indices, and the like.
  • observed pixels may be used to represent observed information obtained from a depth camera or other source.
  • data may be used to represent information that is rasterized, derived, calculated, or otherwise synthesized.
  • observed data e.g., an observed depth value
  • synthesized data e.g., a synthesized depth value
  • a comparison can be made between an observed pixel and a synthesized pixel at the same pixel address by comparing observed data at that pixel address with synthesized data at that pixel address.
  • method 100 of FIG. 7 may optionally include snapping and/or growing the body-part indices and/or player indices.
  • the synthesized depth image may be augmented so that the body-part index and/or player index of some pixels are changed in an attempt to more closely correspond to the modeled target.
  • the indices derived from the snap operation, or the indices derived from the grow operation it is to be understood that any one or more of these indices may be used, as well as other indices obtained from other suitable methods of estimating the player and/or body part to which that pixel belongs.
  • one or more Z- Buffers and/or body-part/player index maps may be constructed.
  • a first version of such a buffer/map may be constructed by performing a Z-test in which a forward-most surface closest to the viewer (e.g., depth camera) at each pixel is selected and a body-part index and/or player index associated with that surface is written to the corresponding pixel.
  • This map may be referred to as the rasterization map or the original synthesized depth map, and this map may include the original body-part index for each pixel.
  • a second version of such a buffer/map may be constructed by performing a Z-test in which a surface of the model that is closest to an observed depth value at that pixel is selected and a body-part index and/or player index associated with that surface is written to the corresponding pixel.
  • This may be referred to as the snap map, and this map may include the snap body-part index for each pixel.
  • Such tests may be constrained so as to reject a Z- distance between a synthesized depth value and an observed depth value that is beyond a predetermined threshold.
  • two or more Z-buffers and/or two or more body-part/player index maps may be maintained, thus allowing two or more of the above described tests to be carried out.
  • a third version of a buffer/map may be constructed by growing and/or correcting a body-part/player index map. This may be referred to as a grow map.
  • the values may be grown over any "unknown" values within a predetermined Z- distance, so that a space being occupied by the target, but not yet occupied by the body model, may be filled with proper body-part/player indices.
  • Such an approach may further include overtaking a known value if a more favorable match is identified.
  • the grow map may begin with a pass over synthesized pixels of the snap map to detect pixels having neighboring pixels with a different body- part/player index. These may be considered “edge" pixels, i.e., frontiers along which values may optionally be propagated.
  • growing the pixel values may include growing into either "unknown” or "known” pixels.
  • the body-part/player index value for example, in one scenario, may have been zero before, but may now have a non-zero neighboring pixel.
  • the four direct neighboring pixels may be examined, and the neighboring pixel having an observed depth value more closely resembling that of the pixel of interest may be selected and assigned to the pixel of interest.
  • a pixel with a known nonzero body-part/player index value may be overtaken, if one of its neighboring pixels has a depth value written during rasterization that more closely matches the observed depth value of the pixel of interest than that of the synthesized depth value for that pixel.
  • updating a body-part/player index value of a synthesized pixel may include adding its neighboring four pixels to a queue of pixels to be revisited on a subsequent pass. As such, values may continue to be propagated along the frontiers without doing an entire pass over all the pixels.
  • different NxN blocks of pixels e.g., 16x16 blocks of pixels
  • Such an optimization may be applied at any point during the target analysis after rasterization in various forms.
  • grow operations may take a variety of different forms.
  • various flood-fills may first be performed to identify regions of like values, and then it can be decided which regions belong to which body parts.
  • the number of pixels that anybody-part/player index object e.g., left forearm body part bp4 of FIG. 3
  • the aforementioned approaches may include adding advantages or disadvantages, for certain poses, to bias the growth for certain body parts so that the growth may be correct.
  • a progressive snap adjustment can be made to the snap map if it is determined that a distribution of pixels from a body part is grouped at one depth, and another distribution of pixels from the same body part is grouped at another depth, such that a gap exists between these two distributions. For example, an arm waving in front of a torso, and near to that torso, may "spill into” the torso. Such a case may yield a group of torso pixels with a body-part index indicating that they are arm pixels, when in fact they should be torso pixels. By examining the distribution of synthesized depth values in the lower arm, it may be determined that some of the arm pixels may be grouped at one depth, and the rest may be grouped at another depth.
  • the gap between these two groups of depth values indicates a jump between arm pixels and what should be torso pixels.
  • the spillover may then be remedied by assigning torso body-part indices to the spillover pixels.
  • a progressive snap adjustment can be helpful in an arm-over-background-object case.
  • a histogram can be used to identify a gap in the observed depth of the pixels of interest (i.e., pixels thought to belong to the arm). Based on such a gap, one or more groups of pixels can be identified as properly belonging to an arm and/or other group(s) can be rejected as background pixels.
  • the histogram can be based on a variety of metrics, such as absolute depth; depth error (synthesized depth— observed depth), etc.
  • the progressive snap adjustment may be performed in ⁇ line during rasterization, prior to any grow operations.
  • method 100 of FIG. 7 may optionally include creating a height map from the observed depth image, the synthesized depth image, and the body-part/player index maps at the three stages of processing described above.
  • the gradient of such a height map, and/or a blurred version of such a height map may be utilized when determining the directions of adjustments that are to be made to the model, as described hereafter.
  • the height map is merely an optimization, however! alternatively or additionally, a search in all directions may be performed to identify nearest joints where adjustments may be applied and/or the direction in which such adjustments are to be made.
  • a height map it may be created before, after, or in parallel to the pixel class determinations described below.
  • the height map When used, the height map is designed to set the player's actual body at a low elevation and the background elements at a high elevation.
  • a watershed- style technique can then be used to trace "downhill” in the height map, to find the nearest point on the player from the background, or vice versa (i.e., seek "uphill” in the height map to find the nearest background pixel to a given player pixel).
  • the synthesized depth image and the observed depth image may not be identical, and thus the synthesized depth image can use adjustments and or modifications so that it more closely matches an observed depth image and can thus more accurately represent the target. It is to be understood that adjustments can be made to the synthesized depth image by first making adjustments to the model (e.g., change the pose of the model), and then synthesizing the adjusted model into a new version of the synthesized depth image.
  • adjustments can be made to the synthesized depth image by first making adjustments to the model (e.g., change the pose of the model), and then synthesizing the adjusted model into a new version of the synthesized depth image.
  • a number of different approaches may be taken to modify a synthesized depth image.
  • two or more different models may be obtained and rasterized to yield two or more synthesized depth images.
  • Each synthesized depth image may then be compared to the observed depth image by a predetermined set of comparison metrics.
  • the synthesized depth image demonstrating a closest match to the observed depth image may be selected, and this process may be optionally repeated in order to improve the model.
  • this process can be particularly useful for refining the body model to match the player's body type and/or dimensions.
  • the two or more synthesized depth images may be blended via interpolation or extrapolation to yield a blended synthesized depth image.
  • two or more synthesized depth images may be blended in such a way that the blending techniques and parameters vary across the blended synthesized depth image. For example, if a first synthesized depth image is favorably matched to the observed depth image in one region, and a second synthesized depth image is favorably matched in a second region, the pose selected in the blended synthesized depth image could be a mixture resembling the pose used to create the first synthesized depth image in the first region, and the pose used to create the second synthesized depth image in the second region.
  • the synthesized depth image may be compared to the observed depth image.
  • Each synthesized pixel of the synthesized depth image may be classified based on the results of the comparison. Such classification may be referred to as determining the pixel case for each pixel.
  • the model used to create the synthesized depth image e.g., body model 70 of FIG. 3 may be systematically adjusted in accordance with the determined pixel cases.
  • one or more pixel cases may be selected for each synthesized pixel based on a comparison to a corresponding pixel of the observed image having a same pixel address as the synthesized pixel.
  • the comparison may be based on one or more factors, which include, but are not limited to - the difference between an observed depth value and a synthesized depth value for that synthesized pixel; the difference between the original body-part index, the (snap) body-part index, and/or the (grow) body/part index for that synthesized pixel; and/or the difference between the original player index, the (snap) player index, and/or the (grow) player index for that synthesized pixel.
  • the pixel case may be selected from a set of defined pixel cases, as described in more detail with reference to 124-136 of FIG. 7.
  • FIG. 13 shows an example of a synthesized depth image (e.g., synthesized depth image 150 of FIG. 9) analytically compared with a corresponding observed depth image (e.g., observed depth image 140 of FIG. 8), for determining pixel mismatches and thus identifying pixel cases.
  • Synthesized pixels of synthesized depth image 150 corresponding to the model are represented in FIG. 13 by a synthesized silhouette 200 depicted in solid line
  • the observed pixels of observed depth image 140 corresponding to the target are represented in FIG. 13 by an observed silhouette 202 depicted in dashed line.
  • an observed silhouette 202 depicted in dashed line may be an analytical comparison of information corresponding to each pixel address such as depicted at FIG. 10.
  • each synthesized pixel may be associated with a pixel case.
  • a pixel case may be selected from a set of defined pixel cases such as a refine-z pixel case, a magnetism pixel case, a push pixel case, a pull pixel case, a self-occluding push and/or pull pixel case, etc.
  • FIG. 14 shows example regions of synthesized pixels of synthesized silhouette 200 having pixel mismatches (e.g., depth value of observed depth image different than depth value of synthesized image by more than a threshold amount), indicated in diagonal-line shading such as shown at 204.
  • pixel mismatches e.g., depth value of observed depth image different than depth value of synthesized image by more than a threshold amount
  • the model represented in the synthesized depth image may be adjusted so that the model better represents the target.
  • FIG. 15 shows another example comparison 206 of a synthesized depth image and a corresponding observed depth image, wherein different pixel cases have been selected for different regions of synthesized pixels.
  • Region 208 includes one or more portions of the model that are shifted forward or backward in a depth direction (i.e., Z-shifted) from a corresponding portion or portions of the observed depth image.
  • region 208 may correspond to pixels having a refine-z pixel case.
  • the regions identified by diagonal shading, such as example region 210 indicate portions of the model shifted away from a silhouette of the human target in the observed depth image.
  • region 210 may correspond to pixels having a push pixel case.
  • regions identified by horizontal shading such as example region 212
  • region 212 may correspond to pixels having a pull pixel case.
  • regions identified by cross-hatch shading such as example region 214, indicate portions of the model such as arms and/or hands that correspond to pixels having a magnetism pixel case.
  • a pixel case may be selected from a set of defined pixel cases such as a refine-z pixel case, a magnetism pixel case, a push pixel case, a pull pixel case, a self-occluding push and/or pull pixel case, etc.
  • the synthesized pixels of the model having these pixel mismatches may then be corrected by adjusting the model to more closely match the observed image.
  • Such adjustments may be made, for example, by applying forces to the model to reposition the model into a different pose that more closely matches the observed image.
  • forces may be applied via force vectors having a magnitude and a direction, which may be applied to a force-receiving location of the model, as indicated at 141, 142 and 144 of FIG. 7.
  • the computation and application of each force vector may be based on the pixel case.
  • Such a force vector may be derived from a single pixel address or from a group of two or more related pixel addresses (e.g., neighboring pixel addresses with matching values— body-part indices, player indices, etc.). Examples of the pixel cases and associated force vectors are discussed hereafter in more detail.
  • determining a pixel case may include selecting a refine-z pixel case.
  • the refine-z pixel case may be selected when the observed depth value of an observed pixel (or in a region of observed pixels) of the observed depth image does not match the synthesized depth value(s) in the synthesized depth image, but is close enough to likely belong to the same object in both images, and the body-part indices match (or, in some cases, correspond to neighboring body parts or regions).
  • a refine-z pixel case may be selected for a synthesized pixel if a difference between an observed depth value and a synthesized depth value for that synthesized pixel is within a predetermined range and, optionally, if that synthesized pixel's (grow) body part index corresponds to a body part that has not been designated for receiving magnetism forces.
  • a synthesized pixel of interest may be classified with a refine-z pixel case if the synthesized depth value does not match the observed depth value, and an absolute difference between the synthesized depth value and the observed depth value is less than a predetermined threshold.
  • the refine-z pixel case corresponds to a computed force vector that may exert a force on the model to move the model into the correct position.
  • a refine-z force vector may be applied to one or more force-receiving locations of the model to move a portion of the model towards a corresponding portion of the observed depth image (e.g., in a direction along the Z axis and perpendicular to an image plane).
  • the computed force vector may be applied along the Z axis perpendicular to the image plane, along a vector normal to an aspect of the model (e.g., face of the corresponding body part), and/or along a vector normal to nearby observed pixels.
  • the computed force vector may be applied to a combination of a vector normal to the face of the corresponding body part and a vector normal to nearby observed pixels.
  • a combination may be an average, a weighted average, a linear interpolation, etc.
  • the magnitude of the force vector is based on the difference in the observed and synthesized depth values, with greater differences corresponding to larger forces.
  • the force vector may increase in proportion to the absolute difference between the synthesized depth value and the observed depth value.
  • the force-receiving location to which the force is applied can be selected to be the nearest qualifying force-receiving location to the pixel of interest (e.g., nearest torso joint), or the force can be distributed among a weighted blend of the nearest qualifying force-receiving locations.
  • the nearest qualifying force-receiving location may be chosen, however, in some cases, the application of biases can be helpful. For example, if a pixel lies halfway down the upper leg, and it has been established that the hip joint is less mobile (or agile) than the knee, it may be helpful to bias the joint forces for mid-leg pixels to act on the knee rather than the hip. Additional examples of biases are described hereafter.
  • the nearest qualifying force-receiving location for a refine-z pixel case may be determined by comparing a distance between the synthesized pixel of interest and each qualifying force-receiving location.
  • the nearest qualifying force-receiving location may be determined, for example, by comparing a distance between the synthesized pixel of interest and each qualifying force-receiving location on a body part that is associated with a body-part index of the synthesized pixel of interest.
  • the force vector may be one of a plurality of force vectors applied to a weighted blend of nearest qualifying force-receiving locations.
  • the force vector may be biased, for example, towards relatively more mobile qualifying force-receiving locations. For example, application of the force vector may be biased to a less-than-nearest qualifying force-receiving location that is more mobile than the nearest qualifying force-receiving location.
  • the determination of which force-receiving location is nearest to the pixel of interest can be found by a brute-force search, with or without the biases mentioned above.
  • the set of force-receiving locations searched may be limited to only those on or near the body part that is associated with the body-part index of this pixel.
  • BSP binary space partitioning
  • Each region on the body, or each body part corresponding to a body-part index, may be given its own BSP tree.
  • determining a pixel case may include selecting a magnetism pixel case.
  • the magnetism pixel case may be utilized when the synthesized pixel being examined, in the grow map, corresponds to a predetermined subset of the body parts (e.g., the arms, or bp3, bp4, bp5, bp7, bp8, and bp9 of FIG. 3). While the arms are provided as an example, other body parts, such as the legs or the entire body, may optionally be associated with the magnetism pixel case in some scenarios. Likewise, in some scenarios, the arms may not be associated with the magnetism pixel case.
  • the pixels marked for the magnetism case may be grouped into regions, each region being associated with a specific body part (such as, in this example, upper left arm, lower left arm, left hand, and so on).
  • a grow operation such as described above may be completed prior to the processing of magnetism pixels.
  • each pixel may be "tagged" with the body part of the target which most likely corresponds to that pixel.
  • one or more pixels may be tagged with the wrong body part (i.e., mis-tagged) during the grow operation.
  • a motion predictor may not be capable of completing a prediction, and as a result, fast-moving hand pixels may not be added into the snap map, whereas slower upper-arm pixels near the shoulder may still be added to the snap map.
  • limb pixels further from the shoulder may have relatively more error in the assignment of body-part indices.
  • lower-arm pixels may grow down into the hand area during the grow operation.
  • upper-arm pixels added to the snap map may be grown down into the lower-arm and the hand areas.
  • pixels corresponding to the hand of the human target may be labeled as "lower-arm” or all the arm pixels may be labeled "upper-arm.” Therefore, it may be useful to discard this information when processing magnetism, described in more detail as follows.
  • the grow operation may incorrectly identify which part of the limb the pixels belong to, the original body-part assigned to the pixels identifying the limb itself tends to be of higher confidence.
  • the subset classification assigned during the grow operation may be dropped.
  • magnetism pixels may be grouped into broader classes (i.e.
  • the pixel's location may be converted from a screen-space location, having an X, Y pixel position and a depth value, to a world-space location having coordinates identifying the location in a three-dimensional space. It can be appreciated that this is just one embodiment for processing pixels. In other embodiments, the pixel's screen-space location may not be converted to a world- space location.
  • the pixel may be projected onto the "bones" that make up the arm of the model and are represented as line segments.
  • the pixel may be projected onto a current, best-guess version of the bone.
  • This best-guess version of the bone may come from the previous frame's final pose, with or without momentum! or it may be updated with any adjustments made during the current frame (e.g., run refine-z pixel case to adjust bone, and then use adjusted bone for magnetism pixels).
  • joints may be progressively updated at any point during the processing of a frame, and the updated joints may be used for subsequent processing in the current or subsequent frames.
  • the arm may comprise three bone segments, namely an upper arm bone, a lower arm bone, and a hand.
  • a point on the finite line segment that is closest to the pixel may be analytically determined. In some embodiments, this may include comparing pixels against three-dimensional joint positions that are pulled forward in the Z-direction by the estimated radius of the limb at that joint, so that the comparison is of two surface values rather than of a surface value and an internal value.
  • the pixel may then be assigned to that closest line segment.
  • the pixel may be assigned to a different line segment if it is determined that the closest line segment may be incorrect. For example, if the target's arm is outstretched but the model's arm is in a "chicken- wing" position, a pixel that is sufficiently far from the shoulder (e.g., 1.5 times the length of the upper arm) may have the closest line segment overridden to be the lower-arm bone.
  • the pixel's location may be added to the "near" and "far" centroids of that bone, described in more detail as follows.
  • centroids of the pixels belonging to the region may be computed. These centroids can be either orthodox (all contributing pixels are weighted equally), or biased, where some pixels carry more weight than others. For example, for the upper arm, three centroids may be tracked: l) an unbiased centroid, 2) a "near" centroid, whose contributing pixels are weighted more heavily when they are closer to the shoulder! and 3) a "far" centroid, whose contributing pixels are weighted more heavily when closer to the elbow. These weightings may be linear (e.g., 2X) or nonlinear (e.g., x 2 ) or follow any curve.
  • centroids are computed, a variety of options are available (and can be chosen dynamically) for computing the position and orientation of the body part of interest, even if some are partially occluded. For example, when trying to determine the new position for the elbow, if the centroid in that area is sufficiently visible (if the sum of the weights of the contributing pixels exceeds a predetermined threshold), then the centroid itself marks the elbow (estimate #l). However, if the elbow area is not visible (perhaps because it is occluded by some other object or body part), the elbow location can still often be determined, as described in the following nonlimiting example.
  • a selection of one of the three potential estimates can be made, or a blend between the three potential estimates may be made, giving priority (or higher weight) to the estimates that have higher visibility, confidence, pixel counts, or any number of other metrics.
  • a single magnetism force vector may be applied to the model at the location of the elbow! however, it may be more heavily weighted (when accumulated with the pixel force vectors resulting from other pixel cases, but acting on this same force-receiving location), to represent the fact that many pixels were used to construct it.
  • the computed magnetism force vector may move the model so that the corresponding model more favorably matches the target shown in the observed image.
  • a model without defined joints or body parts may be adjusted using only the magnetism pixel case.
  • determining a pixel case may include selecting a push pixel case and/or a pull pixel case. These pixel cases may be invoked at the silhouette, where the synthesized and observed depth values may be severely mismatched at the same pixel address. It is noted that the pull pixel case and the push pixel case can also be used when the original player index does not match the (grow) player index.
  • the determination of push vs. pull is as follows. If the synthesized depth image contains a depth value that is greater than (farther than) the depth value in the observed depth image at that same pixel address, for example by more than a threshold amount, then the model can be pulled toward the true silhouette seen in the grown image.
  • the model may be pulled in an XY plane towards the silhouette of the target in the observed depth image.
  • pull force vectors applied to one or more force-receiving locations of the model may be used to "pull" the model.
  • FIG. 16 illustrates an example of such a pull pixel case, and is described in more detail as follows.
  • FIG. 16 schematically illustrates an example observed depth image 220 compared with an example synthesized depth image 222, as indicated at 224.
  • the pixel addresses of synthesized depth image 222 correspond to those of observed depth image 220.
  • FIG. 16 depicts an exaggerated example where observed depth image 220 and synthesized depth image 222 are clearly mismatched.
  • the two images may only be mismatched by a relatively small amount and mismatches as severe as that illustrated may be difficult to resolve.
  • Observed depth image 220 includes an image of an observed human target (i.e., game player), namely, player image 226, wherein the player image 226 has a silhouette, namely player silhouette 228, such that pixels inside player silhouette 228 are pixels of the player image 226 and pixels outside of player silhouette 228 are pixels of the observed background 230.
  • synthesized depth image 222 includes a model 232 representing the observed game player, wherein the model 232 has a silhouette, namely model silhouette 234, such that pixels inside model silhouette 234 are pixels of the model 232 and pixels outside of model silhouette 234 are pixels of the synthesized background 236.
  • synthesized depth image 222 Upon comparing synthesized depth image 222 with observed depth image 220, it becomes more apparent that a mismatch exists, such that pixels at a same pixel address correspond to different parts of each depth image.
  • an example pixel is selected for discussion, namely synthesized pixel of interest 238.
  • synthesized pixel of interest 238 corresponds to synthesized background 236 of synthesized depth image 222.
  • a same pixel address in the corresponding observed depth image corresponds to an observed pixel 240 associated with the player image 226.
  • synthesized pixel of interest 238 has a greater depth value than that of the corresponding observed pixel 240 since the background is at greater depth (i.e., farther from the depth camera) than the game player.
  • model 232 may be pulled toward the synthesized pixel of interest 238 (i.e., toward player silhouette 228) as indicated by arrow 240.
  • the model can be pushed out of the space that the player no longer occupies (and toward the real silhouette in the grown image).
  • the model may be pushed in an XY plane towards the silhouette of the human target in the observed depth image.
  • push force vectors applied to one or more force-receiving locations of the model may be used to "push" the model.
  • FIG. 17 illustrates an example of such a push pixel case, and is described in more detail as follows.
  • FIG. 17 schematically illustrates a comparison similar to that shown in FIG. 16, namely a comparison of synthesized depth image 222 and observed depth image 220.
  • a different synthesized pixel of interest is examined, namely synthesized pixel of interest 250.
  • Synthesized pixel of interest 250 corresponds to model 232 of synthesized depth image 222, whereas at a same pixel address in the corresponding observed depth image 220, observed pixel 252 is associated with observed background 230.
  • synthesized pixel of interest 250 has a lesser depth value than that of the corresponding observed pixel 252 since the model is at a lesser depth (i.e. closer to the depth camera) than the background.
  • model 222 may be pushed away from the synthesized pixel of interest 250 (i.e. toward player silhouette 228) as indicated by arrow 254.
  • a two- or three- dimensional computed force vector may be exerted on the model to correct the silhouette mismatch, either pushing or pulling parts of the body model into a position that more accurately matches the position of the target in the observed depth image.
  • the direction of such pushing and/or pulling is often predominantly in the XY plane, although a Z component can be added to the force in some scenarios. Accordingly, in some cases the push force vectors and/or the pull force vectors may be three-dimensional vectors including Z- components.
  • a pull force vector may be applied to a force-receiving location of model 232 to pull model 232 toward the player silhouette 228 in the observed depth image.
  • a magnitude of the pull force vector may be proportional to a pull-offset distance with which a portion of the observed depth image is shifted away from the silhouette of the model.
  • a pull-offset distance Dl may be defined as a distance between the synthesized pixel of interest (e.g., pixel 238) and a nearest qualifying pixel of model silhouette 234.
  • the magnitude of the pull force vector, D2 may be a function of the pull-offset distance Dl, as described in more detail hereafter.
  • a direction of the pull force vector may be parallel to a vector extending from the nearest qualifying pixel on model silhouette 234 to the synthesized pixel of interest 238.
  • a push force vector may be applied to a force-receiving location of model 232 to push model 232 towards the player silhouette 228 in the observed depth image 220.
  • a magnitude of the push force vector may be proportional to a push-offset distance with which a portion of the model is shifted away from the player silhouette 228.
  • a push-offset distance Dl may be defined as a distance between the synthesized pixel of interest, (e.g., pixel 250), and a nearest qualifying pixel of player silhouette 228.
  • the magnitude of the push force vector, D2 may be a function of the push-offset distance Dl as described in more detail hereafter.
  • a direction of the push force vector may be parallel to a vector extending from the synthesized pixel of interest 250 to the nearest qualifying pixel on the player silhouette 228.
  • the nearest qualifying point on either the player silhouette in the synthesized depth image (i.e., model silhouette) (for a pull case), or on the player silhouette in the observed depth image (for a push case) may first be found. This point can be found, for each source pixel (or for each group of source pixels), by performing a brute-force, exhaustive 2D search for the nearest point (on the desired silhouette) that meets the following criteria.
  • the closest pixel with a player index in the original map (at the seek position) that matches the player index in the grown map (at the source pixel or region) is found.
  • the closest pixel with a player index in the grown map (at the seek position) that matches the player index in the original map (at the source pixel or region) is found.
  • a nearest qualifying pixel on the model silhouette 234 and/or a nearest qualifying pixel on the player silhouette 228 may be found using a one dimensional search along the gradient of the blurred height map. Further, the nearest qualifying pixel on the model silhouette 234 may be found by testing model silhouette pixels near the silhouette pixel found using the one dimensional search. Likewise, the nearest qualifying pixel on the player silhouette 228 may be found by testing player silhouette pixels near the silhouette pixel found using the one dimensional search.
  • Other basic optimizations for this seek operation include skipping pixels, using interval halving, or using a slope-based approach; re- sampling the gradient, at intervals, as the seek progresses! as well as checking nearby for better/closer matches (not directly along the gradient) once the stopping criteria are met.
  • Some search strategies may choose the nearest qualifying pixel from a subset of candidate pixels satisfying one or more selection criteria, such as pixels having a certain body-part index.
  • D2 (Dl - 0.5 pixels) * 2.
  • Dl may either a pull- offset distance or a push-offset distance.
  • D2 may be a magnitude of a pull force vector or a push force vector, respectively.
  • the pull-offset distance and/or a push-offset distance may be found using the one dimensional search along the gradient of the blurred height map as described above.
  • each pixel in this gap may perform a small "seek" and produce a force vector.
  • the average of D2 in this case is 5, as desired - the average magnitudes of the resulting force vectors are equivalent to the distance between the silhouettes (near each force-receiving location), which is the distance that the model can be moved to put the model in the proper place.
  • the final force vector, for each source pixel may then be constructed with a direction and a magnitude (i.e., length). For pull pixels, the direction is determined by the vector from the silhouette pixel to the source pixel! for push pixels, it is the opposite vector. The length of this force vector is D2.
  • the force may be applied to a best- qualifying (e.g., nearest) force-receiving location (or distributed between several), and these forces can be averaged, at each force-receiving location, to produce the proper localized movements of the body model.
  • a best- qualifying (e.g., nearest) force-receiving location or distributed between several
  • these forces can be averaged, at each force-receiving location, to produce the proper localized movements of the body model.
  • the force-receiving location may be a joint of the model.
  • determining a pixel case may include selecting a self-occluding push and/or pull pixel case.
  • a body part may be moving in the foreground relative to a background or another target
  • the self- occluding push and pull pixel cases consider the scenarios where the body part is in front of another body part of the same target (e.g., one leg in front of another, arm in front of torso, etc.).
  • these cases may be identified when the pixel's (snap) player index matches its corresponding (grow) player index , but when the (snap) body-part index does not match its corresponding (grow) body-part index.
  • the seek direction may be derived in several ways. As nonlimiting examples, a brute-force 2D search may be performed; a second set of "occlusion" height maps may be tailored for this case so that a gradient can guide a ID search; or the direction may be set toward the nearest point on the nearest skeletal member. Details for these two cases are otherwise similar to the standard pull and push cases.
  • Push, pull, self- occluding push, and/or self-occluding pull pixel cases may be selected for a synthesized pixel if that synthesized pixel's (grow) body part index corresponds to a body part that has not been designated for receiving magnetism forces.
  • a single pixel may be responsible for one or more pixel cases.
  • a pixel may be responsible for both a self- occluding push pixel force and a refine-z pixel force, where the self-occluding push pixel force is applied to a force-receiving location on the occluding body part and the refine-z pixel force is applied to a force-receiving location on the body part being occluded.
  • determining a pixel case may include selecting no pixel case for a synthesized pixel. Oftentimes a force vector will not need to be calculated for all synthesized pixels of the synthesized depth image. For example, synthesized pixels that are farther away from the body model shown in the synthesized depth image, and observed pixels that are farther away from the target shown in the observed depth image (i.e., background pixels), may not influence any force-receiving locations or body parts. A pixel case need not be determined for such pixels, although it can be in some scenarios.
  • a difference between an observed depth value and a synthesized depth value for that synthesized pixel may be below a predetermined threshold value (e.g., the model already matches the observed image).
  • a pixel case need not be determined for such pixels, although it can be in some scenarios.
  • method 100 of FIG. 7 includes, for each synthesized pixel for which a pixel case has been determined, computing a force vector based on the pixel case selected for that synthesized pixel.
  • each pixel case corresponds to a different algorithm and/or methodology for selecting the magnitude, direction, and/or force-receiving location of a force vector.
  • a force vector (magnitude and direction) may be calculated for each synthesized pixel based on the determined pixel case and, depending on the type of model, the computed force vector can be applied to a nearest qualifying joint, a centroid of a body part, a point on a body part, a vertex of a triangle, or another predetermined force-receiving location of the model used to generate the synthesized depth image.
  • the force attributed to a given pixel can be distributed between two or more force-receiving locations on the model.
  • the force vectors may be computed and/or accumulated in any coordinate space, such as world space, screen space (pre-Z-divide), projection space (post-Z-divide), model space, and the like.
  • a magnitude of the push and/or pull force vector may be proportional to a push-offset distance and/or pull-offset distance, respectively.
  • a magnitude of the refine-z force vector may be based on the absolute difference between the synthesized depth value and the observed depth value, such that the refine-z force vector increases in proportion to that absolute difference.
  • the force vector may depend on a synthesized pixel's proximity to a bone segment, as well as the centroids of the corresponding limb.
  • method 100 includes mapping each computed force vector to one or more force-receiving locations of the model.
  • Mapping may include mapping a computed force vector to a "best-matching" force-receiving location.
  • the selection of a best-matching force-receiving location of the model is dependent on the pixel case selected for the corresponding pixel.
  • the best- matching force-receiving location may be the nearest joint, vertex, or centroid, for example.
  • moments i.e., rotational forces
  • a single pixel may be responsible for two or more different force vectors.
  • a pixel may be identified as a limb pixel occluding the torso after the snap operation, but the same pixel may then be identified as a torso pixel after the grow operation (i.e., the limb has moved from that pixel address).
  • the pixel may be responsible for a push force to the limb to push the limb out of the way and a refine-z force to the torso to move the torso to the proper depth.
  • two or more pixel forces may result from a single pixel if the pixel lies between two or more joints. For example, a mid-calf pixel may move both the ankle and the knee.
  • FIG. 18 shows a table detailing an example relationship between the pixel cases described above and the joints illustrated in skeletal model 82 of FIG. 5 to which the force vectors may be mapped.
  • Pixel cases 1-7 are abbreviated in the table as follows ⁇ 1-Pull (regular), 2-Pull (occlusion), 3-Push (regular), 4-Push (occlusion), 5-Refine-Z, 6-Magnetic Pull, and 7-Occlusion (no action).
  • a “Yes" entry in the "Receives Forces?" column indicates that the joint of that row may receive forces from a force vector.
  • An "X" entry in a pixel cases column denotes that the joint of that row may receive a force from a force vector corresponding to the pixel case of that column. It is to be understood that the table is provided as an example. It is not to be considered limiting. Other relationships between models and pixel cases may be established without departing from the scope of this disclosure.
  • translations may result from forces with similar directions acting on the force-receiving locations of a model
  • rotations may result from forces of different directions acting on the force-receiving locations of a model.
  • some of the components of the force vectors may be used to deform the model within its deformation limits, and the remaining components of the force vectors may be used to translate and/or rotate the model.
  • force vectors may be mapped to the best- matching rigid or deformable object, sub-object, and/or set of polygons of an object. Accordingly, some of the force vectors may be used to deform the model, and the remaining components of the force vectors may be used to perform rigid translation of the model. Such a technique may result in a "broken" model (e.g., an arm could be severed from the body). As discussed in more detail below, a rectification step may then be used to transform translations into rotations and/or apply constraints in order to connect body parts back together along a low-energy path.
  • 142 of method 100 includes mapping more than one force vector. For example, a first synthesized pixel having a body-part index corresponding to an arm of the human target may have been classified with a first pixel case, and a second synthesized pixel having a body-part index corresponding to a torso of the human target may have been classified with a second pixel case.
  • first force vector for the first synthesized pixel may be computed in accordance with the first pixel case
  • a second force vector for the second synthesized pixel may be computed in accordance with the second pixel case.
  • the first force vector may be mapped to a first force-receiving location of the model, where the first force-receiving location corresponds to the arm of the human target.
  • the second force vector may be mapped to a second force- receiving location of the model, where the second force-receiving location corresponds to the torso of the human target.
  • FIGs. 11A and 11B show a very simplified example of applying force vectors to a model - in the illustrated example, a skeletal model 180.
  • a skeletal model 180 For the sake of simplicity, only two force vectors are shown in the illustrated example.
  • Each such force vector may be the result of the summation of two or more different force vectors resulting from the pixel case determinations and force vector calculations of two or more different pixels.
  • a model will be adjusted by many different force vectors, each of which is the sum of many different force vectors resulting from the pixel case determinations and force vector calculations of many different pixels.
  • FIG. 11A shows a skeletal model 180, where force vector 182 is to be applied to joint jl8 (i.e., an elbow) and force vector 184 is to be applied to joint j20 (i.e., a wrist), for the purpose of straightening one arm of skeletal model 180 to more closely match an observed depth image.
  • FIG. 11B shows skeletal model 180 after the forces are applied.
  • FIG. 11B illustrates how the applied forces adjust the pose of the model.
  • the lengths of the skeletal members may be preserved.
  • the position of joint j2 remains at the shoulder of the skeletal model, as expected for the case of a human straightening their arm.
  • the skeletal model remains intact after the forces have been applied. Maintaining the integrity of the skeletal model when applying forces results from one or more constraints being applied, as discussed in more detail hereafter. A variety of different constraints can be enforced to maintain the integrity of different possible model types.
  • method 100 of FIG. 7 optionally includes rectifying the model to a pose satisfying one or more constraints.
  • the computed force vectors may then be applied to the model. If performed without constraint, this may "break” the model, stretching it out of proportion and/or moving body parts into invalid configurations for the actual body of the target. Iterations of various functions may then be used to "relax" the new model position into a "nearby" legal configuration.
  • constraints may be gently and/or gradually applied to the pose, in order to limit the set of poses to those that are physically expressible by one or more actual bodies of one or more targets. In other embodiments, such a rectifying step may be done in a non-iterative manner.
  • the constraints may include one or more of : skeletal member length constraints, joint angle constraints, polygon edge angle constraints, and collision tests, as described hereafter.
  • skeletal member i.e., bone
  • Force vectors that can be detected i.e., force vectors at locations where joints and/or body parts are visible and not occluded
  • the propagated forces may "settle in" once all of the skeletal members are of acceptable lengths.
  • one or more of the skeletal member lengths are allowed to be variable within a predetermined range.
  • the length of skeletal members making up the sides of the torso may be variable to simulate a deformable midsection.
  • the length of skeletal members making up the upper-arm may be variable to simulate a complex shoulder socket.
  • a skeletal model may additionally or alternatively be constrained by computing a length of each skeletal member based on the target, such that these lengths may be used as constraints during rectification.
  • the desired bone lengths are known from the body model; and the difference between the current bone lengths (i.e., distances between new joint positions) and the desired bone lengths can be assessed.
  • the model can be adjusted to decrease any error between desired lengths and current lengths. Priority may be given to certain joints and/or bones that are deemed more important, as well as joints or body parts that are currently more visible than others. Also, high-magnitude changes may be given priority over low-magnitude changes.
  • FIG. 19 illustrates application of one or more constraints to a model representing the target.
  • application of the constraints may be analytical, and may include, for example, modifying pixel data such as that illustrated in FIG. 10.
  • application of a force vector to model 260 may result in a "broken" model.
  • the target may reposition itself to have an arm raised overhead.
  • a force vector may be applied to one or more force- receiving locations of the arm to mimic the motion of the target. However, doing so could potentially result in "breaking" the arm, such as depicted at 262, and/or changing the proportions of the arm, such as shown at 264.
  • constraints may be applied to ensure that adjustments to the model are physically appropriate.
  • a constraint may be applied, such as shown at 266, to ensure that the forearm and upper arm remain attached at the elbow.
  • a bone-length constraint may be applied to the forearm, such as shown at 268, to ensure that the forearm remains approximately the same length.
  • the constraints such as shown at 270, the model maintains its physical integrity.
  • Joint visibility and/or confidence may be separately tracked in the X, Y, and Z dimensions to allow more accurate application of bone length constraints. For example, if a bone connects the chest to the left shoulder, and the chest joint's Z position is high-confidence (i.e., many refine-z pixels correspond to the joint) and the shoulder's Y-position is high-confidence (many push/pull pixels correspond to the joint), then any error in the bone length may be corrected while partially or fully limiting movement of the shoulder in the Y direction or the chest in the Z direction.
  • joint positions prior to rectification may be compared to joint positions after rectification. If it is determined that a consistent set of adjustments is being made to the skeletal model in every frame, method 100 may use this information to perform a "progressive refinement" on the skeletal and/or body model. For example, by comparing joint positions before and after rectification it may be determined that in each frame the shoulders are being pushed wider apart during rectification. Such a consistent adjustment suggests that the shoulders of the skeletal model are smaller than that of the target being represented, and consequently, the shoulder width is being adjusted each frame during rectification to correct for this. In such a case, a progressive refinement, such as increasing the shoulder width of the skeletal model, may be made to correct the skeletal and/or body model to better match the target.
  • certain limbs and body parts may be limited in their range of motion relative to an adjacent body part. Additionally, this range of motion may change based on the orientation of adjacent body parts.
  • applying joint angle constraints may allow limb segments to be constrained to possible configurations, given the orientation of parent limbs and/or body parts.
  • the lower leg can be configured to bend backwards (at the knee), but not forwards. If illegal angles are detected, the offending body part(s) and/or their parents (or, in the case of a mesh model, the offending triangles and their neighbors) are adjusted to keep the pose within a range of predetermined possibilities, thus helping avoid the case where the model collapses into a pose that is deemed to be unacceptable.
  • 20 illustrates an example of a model 280 having one or more joint angle constraints applied to rectify an incorrect joint angle shown at 282, to be a within an acceptable range of motion such as shown at 284.
  • the pose may be recognized as backwards, i.e., what is being tracked as the chest is really the player's back; the left hand is really the right hand; and so on. When such an impossible angle is clearly visible (and sufficiently egregious), this can be interpreted to mean that the pose has been mapped backwards onto the player's body, and the pose can be flipped to accurately model the target.
  • Collision tests may be applied to prevent the model from interpenetrating itself. For example, collision tests may prevent any part of the forearms/hands from penetrating the torso, or prevent the forearms/hands from penetrating each other. In other examples, collision tests may prevent a leg from penetrating the other leg. In some embodiments, collision tests may be applied to models of two or more players to prevent similar scenarios from occurring between models. It is to be understood that this can be accomplished through many different representations of the model geometry — e.g., polygonal hulls may be used for the core body, and parametric capsules (rounded cylinders, which may have different radii at opposing ends) for the limb segments.
  • collision tests may be applied to a body model and/or a skeletal model. In some embodiments, collision tests may be applied to certain polygons of a mesh model.
  • FIG. 21 shows a model 290 wherein the hand and forearm of model 290 has interpenetrated the torso, as depicted at 292. Upon application of collision tests, such interpenetration can be detected and corrected, such as shown at 294.
  • Collision tests may be applied in any suitable manner.
  • One approach examines collisions of one "volumetric line segment" vs. another, where a volumetric line segment may be a line segment with a radius that extends out in 3-D.
  • An example of such a collision test may be examining a forearm vs. another forearm.
  • the volumetric line segment may have a different radius at each end of the segment.
  • Another approach examines collisions of a volumetric line segment vs. a posed polygonal object.
  • An example of such a collision test may be examining a forearm vs. a torso.
  • the posed polygonal object may be a deformed polygonal object.
  • knowledge that different portions of a target can move a limited distance in a time frame e.g., l/30 th or l/60 th of a second
  • a constraint may be used to rule out certain poses resulting from application of forces to pixel-receiving locations of the model.
  • the process can loop back to begin a new rasterization of the model into a new synthesized depth image, which may then be compared to the observed depth image so that further adjustments can be made to the model.
  • the model can be progressively adjusted to more closely represent the modeled target. Virtually any number of iterations can be completed each frame. More iterations may achieve more accurate results, but more iterations also may demand more computing overhead. It is believed that two or three iterations per frame is appropriate in many scenarios, although one iteration may be sufficient in some embodiments.
  • method 100 of FIG. 7 optionally includes changing the visual appearance of an on-screen character (e.g., player avatar 190 of FIG. 12A) responsive to changes to the model, such as changes shown in FIG. 11B.
  • an on-screen character e.g., player avatar 190 of FIG. 12A
  • a user playing an electronic game on a gaming console e.g., gaming system 12 of FIGs. 1A and IB
  • a body model e.g., body model 70 of FIG. 3
  • a skeletal model e.g., skeletal model 180 of FIG. 11A
  • the gaming console may track this motion, then in response to the tracked motion, adjust the model 180 as depicted in FIG 11B.
  • the gaming console may also apply one or more constraints, as described above.
  • the gaming console may display the adjusted player avatar 192, as shown in FIG. 12B. This is also shown by way of example in FIG. 1A, in which player avatar 24 is shown punching boxing opponent 22 responsive to game player 18 throwing a punch in real space.
  • visual target recognition can be performed for purposes other than changing the visual appearance of an on-screen character or avatar. As such, the visual appearance of an on-screen character or avatar need not be changed in all embodiments.
  • target tracking can be used for virtually limitless different purposes, many of which do not result in the changing of an on-screen character.
  • the target tracking and/or the pose of the model, as adjusted, can be used as a parameter to affect virtually any element of an application, such as a game.

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  • Processing Or Creating Images (AREA)

Abstract

L'invention concerne un procédé de suivi de cible qui consiste à représenter une cible humaine à l'aide d'un modèle lisible par machine conçu pour un ajustement en une pluralité de poses différentes, et à recevoir une image de profondeur observée de la cible humaine depuis une source. Un ou plusieurs vecteurs de force de poussée sont appliqués à un ou plusieurs emplacements de réception de force du modèle pour pousser le modèle dans un plan XY vers une silhouette de la cible humaine dans l'image de profondeur observée lorsque des parties du modèle s'éloignent de la silhouette de la cible humaine dans l'image de profondeur observée. Un ou plusieurs vecteurs de force de traction sont appliqués à un ou plusieurs emplacements de réception de force du modèle pour tirer le modèle dans un plan XY vers la silhouette de la cible humaine dans l'image de profondeur observée lorsque des parties de l'image de profondeur observée s'éloignent de la silhouette du modèle.
PCT/US2010/059054 2009-12-07 2010-12-06 Suivi de cible visuelle WO2011071801A2 (fr)

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