Patent Publication Number: US-2021192764-A1

Title: Method and system for detecting and tracking objects using characteristic points

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 16/426,921, filed on May 30, 2019, which is a continuation of International Application No. PCT/CN2016/108281, filed on Dec. 1, 2016, the entire contents of all of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosed embodiments relate generally to operating a movable platform and more particularly, but not exclusively, to detecting and tracking objects based on characteristic points. 
     BACKGROUND 
     Movable platforms (e.g., movable objects) such as unmanned aerial vehicles (UAVs) can be used for performing surveillance, reconnaissance, and exploration tasks for military and civilian applications. A movable platform may carry a payload configured to perform a specific function. For example, the payload may include an imaging device for capturing image data of the surrounding environment for avoiding obstacles and tracking targets in the surrounding environment. It is important to efficiently and accurately detect and track objects included in image frames captured by the imaging device. 
     SUMMARY 
     There is a need for systems and methods for devices that process image data including disparity depth data for detecting and tracking objects using image frames captured by stereoscopic cameras and an imaging device borne by a movable platform. Such systems and methods optionally complement or replace conventional methods for processing image data. By processing image data including disparity depth data with the aid of sensor data obtained from one or more sensors borne by the movable platform, and by using characteristic points identified from the image data for objects detection and tracking, some embodiments of the present application can significantly improve the efficiency and accuracy in image data processing and objects detection and tracking. Additionally, the image processing techniques as disclosed herein can be performed after or in real time as the movable platform moves along a path and captures image data. 
     In accordance with some embodiments, a method for selecting disparity map comprises: obtaining a disparity map based on stereoscopic image frames captured by stereoscopic cameras borne on a movable platform. The method further comprises receiving a 2-dimensional mask including a plurality of projection points defining a predefined 3-dimensional volume adjacent the movable platform. Each projection point has a threshold disparity value for objects within the predefined 3-dimensional volume. The method also comprises selecting, among the disparity map, a subset of elements by comparing disparity values of the elements with the threshold disparity values on the 2-dimensional mask that correspond to projections of the elements onto the 2-dimensional mask. The subset of elements represent actual objects within the predefined 3-dimensional volume. 
     In accordance with some embodiments, a system may comprise one or more processors coupled to the imaging device; memory; and one or more programs. The one or more programs are stored in the memory and configured to be executed by the one or more processors. The one or more programs including instructions for performing the operations of the above method. In accordance with some embodiments, a non-transitory computer-readable storage medium has stored therein instructions that, when executed by the electronic device, cause the electronic device to perform the operations of the above method. 
     In accordance with some embodiments, a method for detecting objects comprises: obtaining a disparity map based on stereoscopic image frames captured by stereoscopic cameras borne on a movable platform. The method further comprises determining a plurality of continuous regions in the disparity map. Each continuous region includes a plurality of elements having disparity values within a predefined range. The method further comprises identifying, within each continuous region, a continuous sub-region including one or more elements having a highest disparity value than that of the other elements within the continuous region as an object. The method also comprises determining a distance between the object and the movable platform using at least the highest disparity value. 
     In accordance with some embodiments, an unmanned aerial vehicle (UAV) may comprise a propulsion system, one or more sensors, an imaging device, and one or more processors coupled to the propulsion system, the one or more sensors, and the imaging device. The one or more processors are configured for performing the operations of the above method. In accordance with some embodiments, a system may comprise one or more processors coupled to the imaging device; memory; and one or more programs. The one or more programs are stored in the memory and configured to be executed by the one or more processors. The one or more programs including instructions for performing the operations of the above method. In accordance with some embodiments, a non-transitory computer-readable storage medium has stored therein instructions that, when executed by the electronic device, cause the electronic device to perform the operations of the above method. 
     In accordance with some embodiments, a method for tracking objects comprises: identifying an object for tracking by a movable platform within a disparity map. The method further comprises determining a location of an element representing the object in a first image frame captured by an imaging device borne on the movable platform. The method further comprises selecting one or more characteristic points of the element representing the object as tracking points of the object on the first image frame. The method also comprises updating the locations of the tracking points of the element on a second image frame captured by the imaging device in accordance with an updated disparity map and a current location of the movable platform. 
     In accordance with some embodiments, an unmanned aerial vehicle (UAV) may comprise a propulsion system, one or more sensors, an imaging device, and one or more processors coupled to the propulsion system, the one or more sensors, and the imaging device. The one or more processors are configured for performing the operations of the above method. In accordance with some embodiments, a system may comprise one or more processors coupled to the imaging device; memory; and one or more programs. The one or more programs are stored in the memory and configured to be executed by the one or more processors. The one or more programs including instructions for performing the operations of the above method. In accordance with some embodiments, a non-transitory computer-readable storage medium has stored therein instructions that, when executed by the electronic device, cause the electronic device to perform the operations of the above method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a movable platform environment, in accordance with some embodiments. 
         FIG. 2A  illustrates a movable platform, in accordance with some embodiments. 
         FIG. 2B  illustrates an exemplary carrier of a movable platform, in accordance with embodiments. 
         FIG. 2C  illustrates an exemplary sensing system of a movable platform, in accordance with some embodiments. 
         FIGS. 3A and 3B  illustrate a process for preparing an exemplary 2-dimensional mask from a predefined 3-dimensional space, in accordance with some embodiments. 
         FIGS. 3C and 3D  illustrate a process for using an exemplary 2-dimensional mask for selecting a disparity map, in accordance with some embodiments. 
         FIG. 4A  is a diagram illustrating a method of processing image data including disparity depth map to detect objects when a movable platform is in a static-hover mode, in accordance with some embodiments. 
         FIGS. 4B and 4C  illustrate exemplary processes of processing disparity map for detecting objects in disparity maps when a movable platform is in a static-hover mode, in accordance with some embodiments. 
         FIG. 4D  illustrates an exemplary image frame captured by the imaging device borne on the movable platform, in accordance with some embodiments. 
         FIG. 5A  is a diagram illustrating a method of processing image data including disparity map to detect objects when a movable platform is in an in-flight mode, in accordance with some embodiments. 
         FIGS. 5B-5D  illustrate exemplary processes of processing disparity map for detecting objects when a movable platform is in an in-flight mode, in accordance with some embodiments. 
         FIG. 5E  illustrates an exemplary image frame captured by the imaging device borne on the movable platform, in accordance with some embodiments. 
         FIG. 6A  is a diagram illustrating a method of processing image data including disparity map to track objects with a movable platform, in accordance with some embodiments. 
         FIG. 6B  illustrates a process of processing disparity map for tracking objects with a movable platform, in accordance with some embodiments. 
         FIG. 6C  illustrates an exemplary image frame captured by the imaging device borne on the movable platform, in accordance with some embodiments. 
         FIGS. 7A-7B  are a flow diagram illustrating a method for selecting disparity map, in accordance with some embodiments. 
         FIGS. 8A-8C  are a flow diagram illustrating a method for processing image data for detecting objects by a movable platform, in accordance with some embodiments. 
         FIGS. 9A-9C  are a flow diagram illustrating a method for processing image data for tracking objects by a movable platform, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the various described embodiments. However, it will be apparent to one of ordinary skill in the art that the various described embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components, circuits, and networks have not been described in detail so as not to unnecessarily obscure aspects of the embodiments. 
     The following description uses an unmanned aerial vehicle (UAV) as an example of a movable object (or a movable platform). UAVs include, e.g., fixed-wing aircrafts and rotary-wing aircrafts such as helicopters, quadcopters, and aircraft having other numbers and/or configurations of rotors. In some embodiments, the movable platform also includes, but is not limited to, a self-driving car (i.e., an autonomous car, a driverless car), a virtual reality (VR) headset, an augmented reality (AR) headset, a handheld gimbal with a camera and image processing capabilities. It will be apparent to those skilled in the art that other types of movable platforms may be substituted for UAVs as described below, such as a mobile phone, a tablet, or a remote control. 
     The present disclosure provides techniques related to processing image data including disparity depth map captured by stereoscopic cameras borne on a movable for detecting and tracking objects. In some embodiments, a disparity map is obtained based on stereoscopic image frames captured by stereoscopic cameras borne on a movable platform. A plurality of continuous regions are determined to have elements with disparity values within a predefined range. Within each continuous region, a continuous sub-region is identified with elements having the highest disparity value, i.e., a continuous sub-region being closest to the movable platform, as an object for detection. A distance between the object and the movable platform is determined. Furthermore, characteristic points are identified for tracking the objects. Locations of the characteristic points are identified on image frames captured by an imaging device borne on the movable platform, and the locations are updated in accordance with updated disparity map and updated spatial information of the movable platform. Efficient and accurate image processing and objects detection and tracking can be achieved using the image processing techniques disclosed in the present application. 
       FIG. 1  illustrates a movable platform environment  100 , in accordance with some embodiments. The movable platform environment  100  includes a movable platform  102 . In some embodiments, the movable platform  102  includes a carrier  104  and/or a payload  106 . 
     In some embodiments, the carrier  104  is used to couple the payload  106  to the movable platform  102 . In some embodiments, the carrier  104  includes an element (e.g., a gimbal and/or damping element) to isolate the payload  106  from movement of the movable platform  102  and/or the movement mechanism  114 . In some embodiments, the carrier  104  includes an element for controlling movement of the payload  106  relative to the movable platform  102 . 
     In some embodiments, the payload  106  is coupled (e.g., rigidly coupled) to the movable platform  102  (e.g., coupled via carrier  104 ) such that the payload  106  remains substantially stationary relative to movable platform  102 . For example, the carrier  104  is coupled to the payload  106  such that the payload is not movable relative to the movable platform  102 . In some embodiments, the payload  106  is mounted directly to the movable platform  102  without requiring the carrier  104 . In some embodiments, the payload  106  is located partially or fully within the movable platform  102 . 
     In some embodiments, a remote control  108  communicates with the movable platform  102 , e.g., to provide control instructions to the movable platform  102  and/or to display information received from the movable platform  102  on a display (not shown) of the remote control  108 . Although the remote control  108  is typically a portable (e.g., handheld) device, the remote control  108  need not be portable. In some embodiments, the remote control  108  is a dedicated control device (e.g., for the movable platform  102 ), a laptop computer, a desktop computer, a tablet computer, a gaming system, a wearable device (e.g., glasses, a glove, and/or a helmet), a microphone, a portable communication device (e.g., a mobile telephone) and/or a combination thereof 
     In some embodiments, an input device of the remote control  108  receives user input to control aspects of the movable platform  102 , the carrier  104 , the payload  106 , and/or a component thereof. Such aspects include, e.g., orientation, position, orientation, velocity, acceleration, navigation, and/or tracking. For example, a position of an input device of the remote control  108  (e.g., a position of a component of the input device) is manually set by a user to a position corresponding to an input (e.g., a predetermined input) for controlling the movable platform  102 . In some embodiments, the input device is manipulated by a user to input control instructions for controlling the navigation of the movable platform  102 . In some embodiments, an input device of remote control  108  is used to input a flight mode for the movable platform  102 , such as auto pilot or navigation according to a predetermined navigation path. 
     In some embodiments, the display (not shown) of the remote control  108  displays information generated by the movable platform sensing system  210 , the memory  204 , and/or another system of the movable platform  102 . For example, the display displays information about the movable platform  102 , the carrier  104 , and/or the payload  106 , such as position, orientation, orientation, movement characteristics of the movable platform  102 , and/or distance between the movable platform  102  and another object (e.g., a target and/or an obstacle). In some embodiments, information displayed by the display of remote control  108  includes images captured by an imaging device  216  ( FIG. 2A ), tracking data (e.g., a graphical tracking indicator applied to a representation of a target), and/or indications of control data transmitted to the movable platform  102 . In some embodiments, information displayed by the display of the remote control  108  is displayed in substantially real-time as information is received from the movable platform  102  and/or as image data is acquired. In some embodiments, the display of the remote control  108  is a touchscreen display. 
     In some embodiments, the movable platform environment  100  includes a computing device  110 . The computing device  110  is, e.g., a server computer, a cloud server, a desktop computer, a laptop computer, a tablet, or another portable electronic device (e.g., a mobile telephone). In some embodiments, the computing device  110  is a base station that communicates (e.g., wirelessly) with the movable platform  102  and/or the remote control  108 . In some embodiments, the computing device  110  provides data storage, data retrieval, and/or data processing operations, e.g., to reduce the processing power and/or data storage requirements of the movable platform  102  and/or the remote control  108 . For example, the computing device  110  is communicatively connected to a database and/or the computing device  110  includes a database. In some embodiments, the computing device  110  is used in lieu of or in addition to the remote control  108  to perform any of the operations described with regard to the remote control  108 . 
     In some embodiments, the movable platform  102  communicates with a remote control  108  and/or a computing device  110 , e.g., via wireless communications  112 . In some embodiments, the movable platform  102  receives information from the remote control  108  and/or the computing device  110 . For example, information received by the movable platform  102  includes, e.g., control instructions for controlling movable platform  102 . In some embodiments, the movable platform  102  transmits information to the remote control  108  and/or the computing device  110 . For example, information transmitted by the movable platform  102  includes, e.g., images and/or video captured by the movable platform  102 . 
     In some embodiments, communications between the computing device  110 , the remote control  108  and/or the movable platform  102  are transmitted via a network (e.g., Internet  116 ) and/or a wireless signal transmitter (e.g., a long range wireless signal transmitter) such as a cellular tower  118 . In some embodiments, a satellite (not shown) is a component of Internet  116  and/or is used in addition to or in lieu of the cellular tower  118 . 
     In some embodiments, information communicated between the computing device  110 , the remote control  108  and/or the movable platform  102  include control instructions. Control instructions include, e.g., navigation instructions for controlling navigational parameters of the movable platform  102  such as position, orientation, orientation, and/or one or more movement characteristics of the movable platform  102 , the carrier  104 , and/or the payload  106 . In some embodiments, control instructions include instructions directing movement of one or more of the movement mechanisms  114 . For example, control instructions are used to control flight of a UAV. 
     In some embodiments, control instructions include information for controlling operations (e.g., movement) of the carrier  104 . For example, control instructions are used to control an actuation mechanism of the carrier  104  so as to cause angular and/or linear movement of the payload  106  relative to the movable platform  102 . In some embodiments, control instructions adjust movement of the carrier  104  relative to the movable platform  102  with up to six degrees of freedom. 
     In some embodiments, control instructions are used to adjust one or more operational parameters for the payload  106 . For example, control instructions include instructions for adjusting an optical parameter (e.g., an optical parameter of the imaging device  216 ). In some embodiments, control instructions include instructions for adjusting imaging properties and/or image device functions, such as capturing an image, initiating/ceasing video capture, powering an imaging device  216  on or off, adjusting an imaging mode (e.g., capturing still images or capturing video), adjusting a distance between left and right components of a stereographic imaging system, and/or adjusting a position, orientation, and/or movement (e.g., pan rate, pan distance) of a carrier  104 , a payload  106  and/or an imaging device  216 . 
     In some embodiments, when control instructions are received by movable platform  102 , the control instructions change parameters of and/or are stored by memory  204  ( FIG. 2A ) of movable platform  102 . 
       FIG. 2A  illustrates an exemplary movable platform  102 , in accordance with some embodiments. The movable platform  102  typically includes one or more processor(s)  202 , a memory  204 , a communication system  206 , a movable platform sensing system  210 , and one or more communication buses  208  for interconnecting these components. 
     In some embodiments, the movable platform  102  is a UAV and includes components to enable flight and/or flight control. In some embodiments, the movable platform  102  includes communication system  206  with one or more network or other communications interfaces (e.g., via which flight control instructions are received), one or more movement mechanisms  114 , and/or one or more movable platform actuators  212  (e.g., to cause movement of movement mechanisms  114  in response to received control instructions). Although the movable platform  102  is depicted as an aircraft, this depiction is not intended to be limiting, and any suitable type of movable platform can be used. Actuator  212  is, e.g., a motor, such as a hydraulic, pneumatic, electric, thermal, magnetic, and/or mechanical motor. 
     In some embodiments, the movable platform  102  includes movement mechanisms  114  (e.g., propulsion mechanisms). Although the plural term “movement mechanisms” is used herein for convenience of reference, “movement mechanisms  114 ” refers to a single movement mechanism (e.g., a single propeller) or multiple movement mechanisms (e.g., multiple rotors). The movement mechanisms  114  include one or more movement mechanism types such as rotors, propellers, blades, engines, motors, wheels, axles, magnets, nozzles, and so on. The movement mechanisms  114  are coupled to the movable platform  102  at, e.g., the top, bottom, front, back, and/or sides. In some embodiments, the movement mechanisms  114  of a single movable platform  102  include multiple movement mechanisms of the same type. In some embodiments, the movement mechanisms  114  of a single movable platform  102  include multiple movement mechanisms with different movement mechanism types. The movement mechanisms  114  are coupled to the movable platform  102  using any suitable means, such as support elements (e.g., drive shafts) and/or other actuating elements (e.g., the movable platform actuators  212 ). For example, a movable platform actuator  212  receives control signals from the processor(s)  202  (e.g., via the control bus  208 ) that activates the movable platform actuator  212  to cause movement of a movement mechanism  114 . For example, the processor(s)  202  include an electronic speed controller that provides control signals to a movable platform actuator  212 . 
     In some embodiments, the movement mechanisms  114  enable the movable platform  102  to take off vertically from a surface or land vertically on a surface without requiring any horizontal movement of the movable platform  102  (e.g., without traveling down a runway). In some embodiments, the movement mechanisms  114  are operable to permit the movable platform  102  to hover in the air at a specified position and/or orientation. In some embodiments, one or more of the movement mechanisms  114  are controllable independently of one or more of the other movement mechanisms  114 . For example, when the movable platform  102  is a quadcopter, each rotor of the quadcopter is controllable independently of the other rotors of the quadcopter. In some embodiments, multiple movement mechanisms  114  are configured for simultaneous movement. 
     In some embodiments, the movement mechanisms  114  include multiple rotors that provide lift and/or thrust to the movable platform  102 . The multiple rotors are actuated to provide, e.g., vertical takeoff, vertical landing, and hovering capabilities to the movable platform  102 . In some embodiments, one or more of the rotors spin in a clockwise direction, while one or more of the rotors spin in a counterclockwise direction. For example, the number of clockwise rotors is equal to the number of counterclockwise rotors. In some embodiments, the rotation rate of each of the rotors is independently variable, e.g., for controlling the lift and/or thrust produced by each rotor, and thereby adjusting the spatial disposition, velocity, and/or acceleration of the movable platform  102  (e.g., with respect to up to three degrees of translation and/or up to three degrees of rotation). 
     In some embodiments, the memory  204  stores one or more instructions, programs (e.g., sets of instructions), modules, controlling systems and/or data structures, collectively referred to as “elements” herein. One or more elements described with regard to the memory  204  are optionally stored by the remote control  108 , the computing device  110 , and/or another device. In some embodiments, imaging device  216  includes memory that stores one or more parameters described with regard to the memory  204 . 
     In some embodiments, the memory  204  stores a controlling system configuration that includes one or more system settings (e.g., as configured by a manufacturer, administrator, and/or user). For example, identifying information for the movable platform  102  is stored as a system setting of the system configuration. In some embodiments, the controlling system configuration includes a configuration for the imaging device  216 . The configuration for the imaging device  216  stores parameters such as position, zoom level and/or focus parameters (e.g., amount of focus, selecting autofocus or manual focus, and/or adjusting an autofocus target in an image). Imaging property parameters stored by the imaging device configuration include, e.g., image resolution, image size (e.g., image width and/or height), aspect ratio, pixel count, quality, focus distance, depth of field, exposure time, shutter speed, and/or white balance. In some embodiments, parameters stored by the imaging device configuration are updated in response to control instructions (e.g., generated by processor(s)  202  and/or received by the movable platform  102  from remote control  108  and/or the computing device  110 ). In some embodiments, parameters stored by the imaging device configuration are updated in response to information received from the movable platform sensing system  210  and/or the imaging device  216 . 
     In some embodiments, a controlling system performs imaging device adjustment. 
     The imaging device adjustment module stores, e.g., instructions for adjusting a distance between an image sensor and an optical device of an imaging device  216 , e.g., instructions for controlling an imaging device actuator. In some embodiments, one or more instructions for performing imaging device adjustment are stored in the memory  204 . 
     In some embodiments, the controlling system performs an autofocus operation. For example, the autofocus operation is performed, e.g., periodically, when a device determines from image analysis that a focus level has fallen below a focus level threshold, in response a determination that movable platform  102  and/or an image subject (e.g., a target or a remote object) has moved by more than a threshold distance, and/or in response to user input. In some embodiments, user input (e.g., received at remote control  108  and/or computing device  110 ) initiates and/or adjusts an autofocus mode. In some embodiments, user input indicates one or more regions (e.g., in an image captured by imaging device  216 , such as an image displayed by remote control  108  and/or computing device  110 ) to be used and/or prioritized for an autofocus operation. In some embodiments, the autofocus module generates control instructions for moving an optical device relative to an image sensor in accordance with an image distance value determined by an image distance determination module. In some embodiments, one or more instructions for performing an autofocus operation are stored in the memory  204 . 
     In some embodiments, the controlling system performs image distance determination, e.g., to determine an object distance and/or an image distance in accordance with the operations described herein. For example, the image distance determination module uses sensor data from one or more depth sensors and one or more orientation sensors of a movable platform to determine an image distance and generate a control instruction for moving an optical device relative to an image sensor in accordance with the determined image distance. In some embodiments, one or more instructions for performing image distance determination are stored in the memory  204 . 
     The above identified controlling system, modules, and/or programs (e.g., sets of instructions) need not be implemented as separate software programs, procedures or modules, and thus various subsets of these modules may be combined or otherwise re-arranged in various embodiments, and stored in the memory  204 . In some embodiments, the controlling system includes a subset of the modules and data structures identified above. Furthermore, the memory  204  may store additional modules and data structures not described above. In some embodiments, the programs, modules, and data structures stored in the memory  204 , or a non-transitory computer readable storage medium of memory  204 , provide instructions for implementing respective operations in the methods described below. In some embodiments, some or all of these modules may be implemented with specialized hardware circuits that subsume part or all of the module functionality. One or more of the above identified elements may be executed by one or more processors  202  of the movable platform  102 . In some embodiments, one or more of the above identified modules are stored on one or more storage devices of a device remote from the movable platform (such as memory of the remote control  108 , the computing device  110 , and/or the imaging device  216 ) and/or executed by one or more processors of a device remote from the movable platform  102  (such as processor(s) of the remote control  108 , the computing device  110 , and/or the imaging device  216 ). 
     The communication system  206  enables communication with the remote control  108  and/or the computing device  110 , e.g., via wireless signals  112 . The communication system  206  includes, e.g., transmitters, receivers, and/or transceivers for wireless communication. In some embodiments, the communication is one-way communication, such that data is only received by the movable platform  102  from the remote control  108  and/or the computing device  110 , or vice-versa. In some embodiments, communication is two-way communication, such that data is transmitted in both directions between the movable platform  102  and the remote control  108  and/or the computing device  110 . In some embodiments, the movable platform  102 , the remote control  108 , and/or the computing device  110  are connected to the Internet  116  or other telecommunications network, e.g., such that data generated by the movable platform  102 , the remote control  108 , and/or the computing device  110  is transmitted to a server for data storage and/or data retrieval (e.g., for display by a website). 
     In some embodiments, the sensing system  210  of the movable platform  102  includes one or more sensors, as described further with reference to  FIG. 3 . In some embodiments, movable platform  102  and/or control unit  104  use sensing data generated by sensors of sensing system  122  to determine information such as a position of movable platform  102 , an orientation of movable platform  102 , movement characteristics of movable platform  102  (e.g., angular velocity, angular acceleration, translational velocity, translational acceleration and/or direction of motion along one or more axes), proximity of movable platform  102  to potential obstacles, weather conditions, locations of geographical features and/or locations of manmade structures. 
       FIG. 2B  illustrates an exemplary carrier  108  in a target tracking system  100 , in accordance with embodiments. In some embodiments, carrier  108  couples a payload  106  to a movable platform  102 . 
     In some embodiments, carrier  108  includes a frame assembly including one or more frame members  252 . In some embodiments, frame member  252  is coupled with movable platform  102  and payload  106 . In some embodiments, frame member  252  supports payload  106 . 
     In some embodiments, carrier  108  includes one or more mechanisms, such as one or more actuators  254 , to cause movement of carrier  108  and/or payload  106 . Actuator  254  is, e.g., a motor, such as a hydraulic, pneumatic, electric, thermal, magnetic, and/or mechanical motor. In some embodiments, actuator  254  causes movement of frame member  252 . In some embodiments, actuator  254  rotates payload  106  about one or more axes, such as three axes: X axis (“pitch axis”), Z axis (“roll axis”), and Y axis (“yaw axis”), relative to movable platform  102 . In some embodiments, actuator  254  translates payload  106  along one or more axes relative to movable platform  102 . 
     In some embodiments, carrier  108  includes one or more carrier sensing system  256 , e.g., for determining a state of carrier  108  or payload  106 . Carrier sensing system  256  includes, e.g., motion sensors (e.g., accelerometers), rotation sensors (e.g., gyroscopes), potentiometers, and/or inertial sensors. In some embodiments, carrier sensing system  256  includes one or more sensors of movable platform sensing system  210  as described below with regard to  FIG. 3 . Sensor data determined by carrier sensing system  256  includes, e.g., spatial disposition (e.g., position, orientation, or attitude) and/or movement information such as velocity (e.g., linear or angular velocity) and/or acceleration (e.g., linear or angular acceleration) of carrier  108  and/or payload  106 . In some embodiments, sensing data and/or state information calculated from the sensing data are used as feedback data to control the movement of one or more components (e.g., frame member  252 , actuator  254 , and/or damping element  258 ) of carrier  108 . Carrier sensor  206  is coupled to, e.g., frame member  252 , actuator  254 , damping element  258 , and/or payload  106 . In an embodiment, a carrier sensor  256  (e.g., a potentiometer) measures movement of actuator  254  (e.g., the relative positions of a motor rotor and a motor stator) and generates a position signal representative of the movement of the actuator  254  (e.g., a position signal representative of relative positions of the motor rotor and the motor stator). In some embodiments, data generated by a carrier sensor  256  is received by processor(s)  116  and/or memory  204  of movable platform  102 . 
     In some embodiments, the coupling of carrier  108  to movable platform  102  includes one or more damping elements  258 . Damping elements  258  are configured to reduce or eliminate movement of the load (e.g., payload  106  and/or carrier  108 ) caused by movement of movable platform  102 . Damping elements  258  include, e.g., active damping elements, passive damping elements, and/or hybrid damping elements having both active and passive damping characteristics. The motion damped by the damping elements  258  can include one or more of vibrations, oscillations, shaking, or impacts. Such motions may originate from motions of movable platform that are transmitted to the load. For example, the motion may include vibrations caused by the operation of a propulsion system and/or other components of a movable platform  101 . 
     In some embodiments, a damping element  258  provides motion damping by isolating the load from the source of unwanted motion by dissipating or reducing the amount of motion transmitted to the load (e.g., vibration isolation). In some embodiments, damping element  258  reduces the magnitude (e.g., amplitude) of the motion that would otherwise be experienced by the load. In some embodiments the motion damping applied by a damping element  258  is used to stabilize the load, thereby improving the quality of images captured by the load (e.g., image capturing device), as well as reducing the computational complexity of image stitching steps required to generate a panoramic image based on the captured images. 
     Damping element  258  described herein can be formed from any suitable material or combination of materials, including solid, liquid, or gaseous materials. The materials used for the damping elements may be compressible and/or deformable. For example, the damping element  258  is made of, e.g. sponge, foam, rubber, gel, and the like. For example, damping element  258  includes rubber balls that are substantially spherical in shape. The damping element  258  is, e.g., substantially spherical, rectangular, and/or cylindrical. In some embodiments, damping element  208  includes piezoelectric materials or shape memory materials. In some embodiments, damping elements  258  include one or more mechanical elements, such as springs, pistons, hydraulics, pneumatics, dashpots, shock absorbers, isolators, and the like. In some embodiments, properties of the damping element  258  are selected so as to provide a predetermined amount of motion damping. In some instances, the damping element  208  has viscoelastic properties. The properties of damping element  258  are, e.g., isotropic or anisotropic. In some embodiments, damping element  258  provides motion damping equally along all directions of motion. In some embodiments, damping element  258  provides motion damping only along a subset of the directions of motion (e.g., along a single direction of motion). For example, the damping element  258  may provide damping primarily along the Y (yaw) axis. In this manner, the illustrated damping element  258  reduces vertical motions. 
     In some embodiments, carrier  108  includes controller  260 . Controller  260  includes, e.g., one or more controllers and/or processors. In some embodiments, controller  260  receives instructions from processor(s)  116  of movable platform  102 . For example, controller  260  is connected to processor(s)  202  via control bus  208 . In some embodiments, controller  260  controls movement of actuator  254 , adjusts one or more parameters of carrier sensor  256 , receives data from carrier sensor  256 , and/or transmits data to processor  202 . 
       FIG. 2C  illustrates an exemplary sensing system  210  of a movable platform  102 , in accordance with some embodiments. In some embodiments, one or more sensors of the movable platform sensing system  210  are mounted to the exterior, located within, or otherwise coupled to the movable platform  102 . In some embodiments, one or more sensors of the movable platform sensing system  210  are components of and/or coupled to the carrier  104  (e.g.,  FIG. 2B ), the payload  106 , and/or the imaging device  216 . Where sensing operations are described herein as being performed by the movable platform sensing system  210 , it will be recognized that such operations are optionally performed by one or more sensors of the carrier  104 , the payload  106 , and/or the imaging device  216  in addition to and/or in lieu of one or more sensors of the movable platform sensing system  210 . 
     Movable platform sensing system  210  generates static sensing data (e.g., a single image captured in response to a received instruction) and/or dynamic sensing data (e.g., a series of images captured at a periodic rate, such as a video). 
     In some embodiments, movable platform sensing system  210  includes one or more image sensors  262 . In some embodiments, the one or more image sensors  262  include a plurality of stereoscopic cameras, such as a pair of stereoscopic cameras including a left stereographic image sensor  264  and a right stereographic image sensor  266 . The image sensors  262  capture images, image streams (e.g., videos), stereographic images (e.g., stereoscopic images), and/or stereographic image streams (e.g., stereographic videos). In some embodiments, the image sensors  262  include multiple pairs of stereoscopic cameras located at different parts (e.g., sides, areas, etc.) of the movable platform  102 , such as one or more parts of the top part, bottom part, front part, back part, left part, and right part of the movable platform  102 . For example, movable platform sensing system  210  includes a pair of stereoscopic cameras located at the front of the movable platform  102 , another pair of stereoscopic cameras at the back of the movable platform  102 , and yet another pair of stereoscopic cameras at the bottom of the movable platform  102 . Image sensors  262  detect light, such as visible light, infrared light, and/or ultraviolet light. In some embodiments, movable platform sensing system  210  includes one or more optical devices (e.g., lenses) to focus or otherwise alter the light onto one or more image sensors  262 . In some embodiments, image sensors  262  include, e.g., semiconductor charge-coupled devices (CCD), active pixel sensors using complementary metal-oxide-semiconductor (CMOS) or N-type metal-oxide-semiconductor (NMOS, Live MOS) technologies, or any other types of sensors. 
     In some embodiments, movable platform sensing system  210  includes one or more audio transducers  268 . For example, an audio detection system includes audio output transducer  270  (e.g., a speaker), and audio input transducer  272  (e.g. a microphone, such as a parabolic microphone). In some embodiments, microphone and a speaker are used as components of a sonar system. In some embodiments, a sonar system is used to detect current location information of an object (e.g., an obstacle and/or a target) in the environment. 
     In some embodiments, movable platform sensing system  210  includes one or more infrared sensors  274 . In some embodiments, a distance measurement system includes a pair of infrared sensors, e.g., infrared sensor  276  (such as a left infrared sensor) and infrared sensor  278  (such as a right infrared sensor) or another sensor or sensor pair. The distance measurement system can be used to measure a distance to an object in the environment (e.g., a target and/or an obstacle). 
     In some embodiments, a system to produce a disparity map includes one or more sensors or sensor pairs of movable platform sensing system  210  (such as left stereographic image sensor  264  and right stereographic image sensor  266 ; audio output transducer  270  and audio input transducer  272 ; and/or left infrared sensor  276  and right infrared sensor  278 . In some embodiments, a pair of sensors in a stereo data system (e.g., a stereographic imaging system) simultaneously captures data from different positions. In some embodiments, a depth map is generated by a stereo data system using the simultaneously captured data. In some embodiments, a depth map is used for positioning and/or detection operations, such as detecting an obstacle, detecting current location information of an obstacle, detecting a target, and/or detecting current location information for a target. In some embodiments, movable platform sensing system  210  includes one or more depth sensors, such as time-of-flight (TOF) cameras. For example, movable platform sensing system  210  includes TOF cameras located at left and right sides of the movable platform  102 . One or more TOF cameras may also locate other parts of the movable platform  102 . A TOF camera calculates a distance of each point/pixel in an image frame based on the speed of light. For example, the time-of-flight of a light signal between the imaging system and the subject for a pixel in the image frame is measured to determine the distance (e.g., depth) of the corresponding pixel. 
     In some embodiments, movable platform sensing system  210  further includes, but is not limited to, one or more global positioning system (GPS) sensors  280 , motion sensors (e.g., accelerometers)  282 , rotation sensors (e.g., gyroscopes), inertial sensors  284 , proximity sensors (e.g., infrared sensors) and/or weather sensors  286  (e.g., pressure sensor, temperature sensor, moisture sensor, and/or wind sensor), visual odometry (VO) system  288 , Lidar system  290 , and ultrasonic sensor  292 . In some embodiments, the movable platform sensing system  210  includes an inertial measurement unit (IMU) that may include the motion sensors  282 , the rotation sensors, and optionally magnetometers. 
     In some embodiments, sensing data generated by one or more sensors of movable platform sensing system  210  and/or information determined using sensing data from one or more sensors of movable platform sensing system  210  are transmitted to remote control  108  (e.g., via communication system  206 ). In some embodiments, data generated one or more sensors of movable platform sensing system  210  and/or information determined using sensing data from one or more sensors of movable platform sensing system  122  is stored by memory  204 . 
       FIGS. 3A and 3B  illustrate a process for preparing an exemplary 2-dimensional mask  320  from a predefined 3-dimensional space  300  (also referred to as 3-dimensional volume  300 ), in accordance with some embodiments. In some embodiments, an image sensor  302  of the movable platform  102 , such as the left stereographic image sensor  264  and/or the right stereographic image sensor  266  of the movable platform sensing system  210  or the imaging device  216 , has a valid detection range corresponding to a predefined range (e.g., a predefined 3-dimensional space) within which the image data, e.g., disparity information, of the object(s) are more accurately captured by the image sensor  302 . In some embodiments, the valid detection range of the image sensor  302  along the Z dimension in  FIG. 3A  is between 0.5 meters to 15 meters. Due to a limitation of the shooting angle of the image sensor  302 , the 3-dimensional space  300  also has limitations along the X and Y dimensions. In some embodiments, the valid detection range can be represented using a predefined 3-dimensional space  300 , e.g., a cuboid ABCDEFGH, located along the movement trajectory of the movable platform  102  (or the image sensor  302 ). In some embodiments, the cuboid ABCDEFGH has a dimension of 10 m×10 m×15 m as shown in  FIG. 3A . In some embodiments, the 3-dimensional space  300  can be represented using other suitable shapes (not shown) including, but not limited to, a cylinder, a sphere, or a cone. 
     In some embodiments, an electronic device (e.g., the computing device  110 , the remote control  108 , or the movable platform  102 ,  FIG. 1 ) obtains a 2-dimensional mask  350 , as shown in  FIG. 3B . In some embodiments, the 2-dimensional mask  350  is obtained by projecting the 3-dimensional volume  300  onto a 2-dimensional plane. The points A′, B′, C′, D′, E′, F′, G′, and H′ on the 2-dimensional mask  350  correspond to projections of the points A, B, C, D, E, F, G, and H from the 3-dimensional volume onto the 2-dimensional plane respectively. Each point of the points A′, B′, C′, D′, E′, F′, G′, and H′ on the 2-dimensional mask  350  has a threshold value determined by disparity values for objects located at points A, B, C, D, E, F, G, and H respectively in the 3-dimensional volume. As for other points on the 2-dimensional mask, each also has a threshold value determined by a disparity value of an object located at a corresponding location of the 3-dimensional volume  300 . In some embodiments, the threshold values on the 2-dimensional mask  350  define the minimum disparity values (corresponding to one or more farthest distances in the world coordinate system) for selecting objects within the valid detection range (e.g., the 3-dimensional volume  300 ) of the image sensor  302 . 
     In some embodiments, the threshold values of such points are determined by projecting the points on the boundaries (e.g., including on the planes EFGH, EFBA, FBCG, DHGC, ADHE, and ABCD, and the edges of these planes) of the 3-dimensional volume  300  onto to the 2-dimensional mask  350  to identify the disparity values at the corresponding points. In some other embodiments, the threshold values of the points on the 2-dimensional mask  350  are obtained by (1) determining disparity values of points corresponding to points on the edges of the 3-dimensional volume; and (2) estimating disparity values of other points at locations other than on the edges using a recurrence relation. For example, after determining the disparity values of points on edges EF and FB, BA, and AE, a threshold value of a point within the region A′E′F′B′ is estimated using a linear recurrence relation based on the threshold values of points on E′F′ and F′B′, B′A′, and A′E′. 
     In one example as shown in  FIGS. 3A-3B , a point I (e.g., an object located at the location I) located on the plane ABFE of the 3-dimensional volume  300  corresponds to point I′ within the region A′B′F′E′ of the 2-dimensional mask  350 , and the threshold value of point I′ is determined by the disparity value for an object located at the point I. In another example, a point J located within the body of the 3-dimensional volume  300  (e.g., point J is located between planes ABCD and EFGH, between planes ADHE and BCGF, and between planes ABFE and DCJH) can be projected by the image sensor  302  onto point K which is located on the plane EFGH. Point K of the 3-dimensional volume  300  corresponds to point K′ on the 2-dimensional mask  350 . Accordingly, the threshold value of K′ on the 2-dimensional mask  350  is determined by the disparity value for an object located at the point K. It is noted that disparity value of point K may or may not be the same as the disparity values of point E, F, G, or H, depending on the orientation of the image sensor  302 . 
       FIGS. 3C and 3D  illustrate a process for using an exemplary 2-dimensional mask  350  for selecting (or filtering) a disparity map, in accordance with some embodiments. In some embodiments, the movable platform sensing system  210 , such as left stereographic image sensor  264  and right stereographic image sensor  266 , are used to capture a pair of stereoscopic grayscale images respectively. A disparity map can be generated based on the pair of stereoscopic grayscale images. In some embodiments, the disparity map includes points P, Q, M, and N which are to be evaluated using the 2-dimensional mask  350 . In some examples, the disparity map is generated using semi-global block-matching (SGBM) algorithm or any other suitable processes. The disparity map includes disparity values of one or more pixels. A disparity value corresponds to a spatial difference between two locations of a single pixel (or a single point) of an object located on the left and right stereoscopic images respectively. The disparity value is related to depth information of a pixel (e.g., a distance between the object and the imaging sensor). The disparity map can be used for obtaining depth information, e.g., information related to a distance between the camera(s) and the object, of one or more objects in the image frames. In some embodiments, an electronic device (e.g., the computing device  110 , the remote control  108 , or the movable platform  102 ,  FIG. 1 ) processes the disparity map to select pixels within a more accurate range corresponding to the valid detection range of the image sensors. 
     In some embodiments, the electronic device selects, among the disparity map, a subset of elements (e.g., one or more points or one or more pixels) with respective disparity values using the 2-dimensional mask  350 . In some embodiments, the subset of elements are selected by comparing the respective disparity values of the elements with the threshold disparity values on the 2-dimensional mask that correspond to projections of the elements onto the 2-dimensional mask. The subset of elements represents actual objects within the predefined 3-dimensional volume  300 . In some embodiments, elements (such as pixels) on the disparity map having disparity values lower than the threshold values are excluded when processing the disparity map. In some embodiments, when selecting the disparity map using the 2-dimensional mask  350 , a 3-dimensional volume  300  is put (e.g., virtually) relative to the image sensor  302  in the space as shown in  FIG. 3C . The image sensor  302  is located adjacent or near (e.g., when a minimum valid detection distance is used) the plane ABCD of the 3-dimensional volume  300 . A point from the disparity map is projected from the coordinate system associated with the 3-dimensional volume  300  onto the 2-dimensional mask  350  to identify a corresponding point on the 2-dimensional mask  350 . The disparity value of this point on the disparity map is then compared with the threshold value of the projection point on the 2-dimensional mask  350  to determine whether to include or exclude this point in the valid disparity map for further processing (e.g., for object detection and/or object tracking). 
     In one example as shown in  FIGS. 3C and 3D , a pixel (or point) P from the disparity map is located within the 3-dimensional volume  300  ( FIG. 3C ). Pixel P is projected to point P′ located on the 2-dimensional mask  350  ( FIG. 3D ). For example, the coordinate x1 is within the plane boundaries of ADHE and BCGF, the coordinate y1 is within the plane boundaries of AEFB and DHGC, and the depth coordinate z1 is within the plane boundaries ABCD and EFGH. In some embodiments, it is determined that the disparity value of P in the disparity map is greater than the threshold value at P′ on the 2-dimensional mask  350 . (As discussed above with reference to  FIGS. 3A and 3B , the threshold value at P′ is determined by a disparity value of an intersection point between the plane EFGN and the projection line OP, the intersection point located farther away from the image sensor  302  compared to point P on the projection line OP.) Thus, point P is selected to be included in the valid disparity map. 
     In another example, a point (or pixel) Q from the disparity map is located outside the 3-dimensional volume  300 . For example, the depth coordinate z2 of pixel Q is outside the plane boundary EFGH. Pixel Q is projected to the point Q′ located behind the 2-dimensional mask  350 . In some embodiments, a disparity value of Q (in the 3-dimensional volume  300 ) is lower than the threshold value at Q′ (because the threshold value at Q′ is determined by a disparity value of an intersection point between the plane EFGN and the projection line OQ, such intersection point located nearer to the image sensor  302  compared to Q on the projection line OQ). Thus, point Q is excluded from the valid disparity map. 
     In yet another example, a point (or pixel) M located on the right of the 3-dimensional volume  300  (e.g., coordinate x3 is outside the plane ADHE) is projected to the point M′ located outside the 2-dimensional mask  350 . Without having to compare the disparity value against any threshold value on the 2-dimensional mask  350 , point M is excluded from the valid disparity. 
     In yet another example, a point (or pixel) N located within the 3-dimensional volume  300  is projected to point N′ located within region B′F′C′G′ of the 2-dimensional mask  350 . The threshold value of point N′ may be determined by an intersection point between the projection line ON and the plane BCGF. Thus the disparity value of point N is greater than the threshold value of point N, and point N is selected to be included on the valid disparity map. 
     In some embodiments, the electronic device further excludes a region from the disparity map corresponding to the ground within the movement trajectory of the movable platform  102 . The region corresponding to the ground may be determined in the disparity map based on spatial information, such as height and/or attitude data, of the movable platform  102 . Accordingly, the corresponding region in the image is also identified and excluded from further processing, such that the movable platform  102  will not take the ground as an object for tracking. 
     As such, the electronic device identifies one or more objects from the processed (e.g., the filtered, valid) disparity map based on the comparison results of the disparity values against the threshold values of corresponding points on the 2-dimensional mask  350 . The electronic device determines distances between the identified objects and the image sensor(s). The processed disparity map is used for object detection and/or object tracking for the movable platform  102  as discussed below. 
       FIG. 4A  is a diagram illustrating a method  400  of processing image data including disparity depth map to detect one or more objects when the movable platform  102  is in a static-hover mode, in accordance with some embodiments. In some embodiments, method  400  is performed by an electronic device such as the computing device  110 , the remote control  108 , or the movable platform  102  ( FIG. 1 ). For example, method  400  is performed by a controller of the image sensors  262 , a controller of the imaging device  216 , a controller of the movable platform  102 , or a controller of the remote control  108 . In some other embodiments, method  400  is performed by other electronic device(s), such as a mobile device or a computing device paired with the remote control  108  for operating the movable platform  102 . Operations performed in  FIG. 4A  correspond to instructions stored in computer memories or other computer-readable storage mediums of the corresponding device(s).  FIGS. 4B and 4C  illustrate exemplary processes of processing disparity map for detecting one or more objects when the movable platform  102  is in a static-hover mode, in accordance with some embodiments. One or more steps of method  400  are further illustrated in  FIGS. 4B and 4C , which are discussed in combination with  FIG. 4A  in the present disclosure. 
     In some embodiments, the electronic device obtains ( 402 ) a disparity map, such as disparity map  420  in  FIG. 4B . The disparity map  420  is obtained based on stereoscopic image frames captured by stereoscopic cameras (left stereographic image sensor  264  and right stereographic image sensor  266 ) borne on the movable platform  102 . In some embodiments, the movable platform  102  statically hovers at a certain height. In some embodiments, the disparity map  420  is selected (e.g., pre-processed) using the 2-dimensional mask  350  as discussed with reference to  FIGS. 3A-3B . Only disparity map within the valid detection range of the stereoscopic imaging sensors is selected. Each point on the disparity map  420  is projected to the 2-dimensional mask  350 , and the disparity values of points on the disparity map  420  are compared with the threshold values of corresponding points on the 2-dimensional mask  350 . Pixels with disparity values lower than the corresponding predefined threshold values are excluded from the valid disparity map. 
     As shown in  FIG. 4B , method  400  proceeds to divide ( 404 ) the disparity map  420  into a plurality of areas, e.g., using a grid  422 . For example, the disparity map  420  is divided evenly into 5×4 areas. Method  400  proceeds to identify ( 406 ), in each area, one or more pixels having highest disparity values (e.g., a correlated point of an object being closest to the imaging sensors) within the valid detection range, such as pixel  424  in area  423 , pixel  426  in area  425 , and pixel  428  in area  427 . In some embodiments, within each area, a distance between each point of an object and the movable platform  102  is calculated based on a disparity value of a pixel in the disparity map that correlates to the point of the object. The distances are further ranked from the lowest disparity value towards the highest disparity value. 
     As shown in  FIG. 4C , method  400  proceeds to select ( 408 ) one or more contiguous pixels adjacent the pixels having the highest disparity values in each area to form a continuous region, such as a continuous region  432  including a plurality of pixels adjacent the pixel  426  having the highest disparity value. A plurality of continuous regions, such as regions  432 ,  434 ,  436 , and  438 , can thus be selected in the disparity map  430 . In some embodiments, the pixels within a continuous region correlate to respective points in the world coordinate system having distances to the point closest to the imaging device within a predefined range. In some examples, because a difference of disparity values between two pixels can be used for calculating a distance between two points correlated to the two pixels respectively in the world coordinate system, the one or more contiguous pixels adding up to a continuous region are selected to have disparity values with differences with the highest disparity value that are within a predefined range. In one example, a continuous region includes a plurality of pixels correlated to respective points in the world coordinate system that have distances to the closest point (correlated to the pixel having the highest disparity value) for less than 0.5 meter. In some embodiments, a continuous region, e.g., continuous region  432 , extends across multiple contiguous areas (such as area  425  and area  431 ) and covers multiple pixels with highest disparity values (such as pixel  426  and pixel  433  respectively). In some embodiments, two sub-continuous regions, e.g., sub-continuous regions  441  and  443 , are identified based on respective pixels, e.g., pixel  426  and pixel  433 , having the highest disparity values in corresponding areas, e.g., areas  425  and  431 . When the two sub-continuous regions extend over respective areas, e.g., area  425  and  431 , and overlap, the two sub-continuous regions (e.g., sub-continuous regions  441  and  443 ) are connected to form a single continuous region  432  as shown in  FIG. 4C . A plurality of continuous regions can be selected as shown in the disparity map  430  in  FIG. 4C . 
     Method  400  proceeds to identify ( 410 ), within each continuous region, a sub-region including one or more pixels having disparity values higher than that of the other pixels within the continuous region for at least a predefined threshold as an object. The sub-region is identified as an object detected by the movable platform  102 . In some embodiments, the electronic device identifies a sub-region, such as a pixel having the highest disparity value (i.e., being closest to the imaging sensors) in the continuous region. For example, as shown in  FIG. 4C , pixel  442  is detected as an object in continuous region  436 , pixel  444  is detected as an object in continuous region  438 , pixel  446  is detected as an object in continuous region  434 , and pixel  426  is detected as an object in continuous region  432 . In some embodiments, the object is an obstacle or a portion of the obstacle for avoidance by the movable platform. In some embodiments, the object is a target or a portion of the target for tracking by the movable platform. 
     Method  400  proceeds to determine ( 412 ) a distance between the identified object (e.g., the sub-region, or the pixel having the highest disparity value) in each continuous region and the movable platform  102 . In some embodiments, the distance is determined using at least the highest disparity value of the object. In some embodiments, the distance is also determined using one or more parameters of the imaging sensors, such as a focal length of the imaging sensors. 
     In some embodiments, the imaging device  216  borne on the movable platform  102  captures one or more image frames when the movable platform  102  hovers at a certain height or moves along a navigation path. Method  400  proceeds to identify ( 414 ), within an image frame captured by the imaging device  216  borne on the movable platform  102 , one or more objects corresponding to the sub-regions respectively.  FIG. 4D  illustrates an exemplary image frame  450  captured by the imaging device  216  borne on the movable platform  102 . In some embodiments, the one or more objects (e.g., pixels  452 ,  454 ,  456 , and  458 ) corresponding to the sub-regions identified in the disparity map  430  at step  410  are identified on the image frame  450 . In some embodiments, the sub-regions in the disparity map  430  are projected to respective objects or pixels in the image frame  450  based on spatial information of the movable platform  102  and spatial information of the imaging device  216 . For example, data from IMU and GPS and data from gimbal for carrying the imaging device are used for calculating and identifying the objects or pixels in the image frame  450 . In some embodiments, characteristic points and/or object matching algorithms are also used for identifying the objects/pixels in the imaging frame  450  that correspond to the sub-regions. 
     Method  400  proceeds to send ( 416 ) the image frame  450  and the determined distances associated with the one or more objects to an electronic device for display. In some embodiments as shown in  FIG. 4D , the respective distances associated with the objects are displayed in real time. 
       FIG. 5A  is a diagram illustrating a method  500  of processing image data including disparity map to detect objects when the movable platform  102  is in an in-flight mode, in accordance with some embodiments. In some embodiments, method  500  is performed by an electronic device such as the computing device  110 , the remote control  108 , or the movable platform  102  ( FIG. 1 ). For example, method  500  is performed by a controller of the image sensors  262 , a controller of the imaging device  216 , a controller of the movable platform  102 , or a controller of the remote control  108 . In some other embodiments, method  500  is performed by other electronic device(s), such as a mobile device or a computing device paired with the remote control  108  for operating the movable platform  102 . Operations performed in  FIG. 5A  correspond to instructions stored in computer memories or other computer-readable storage mediums of the corresponding device(s).  FIGS. 5B-5D  illustrate exemplary processes of processing disparity map for detecting objects when a movable platform is in an in-flight mode, in accordance with some embodiments. One or more steps of method  500  are further illustrated in  FIGS. 5B-5D , which are discussed in combination with  FIG. 5A  in the present disclosure. 
     In some embodiments, the electronic device obtains ( 502 ) a disparity map, such as disparity map  520  in  FIG. 5B . The disparity map  520  is obtained based on stereoscopic image frames captured by stereoscopic cameras (left stereographic image sensor  264  and right stereographic image sensor  266 ) borne on the movable platform  102 . In some embodiments, the movable platform  102  is in an in-flight mode. For example, the movable platform  102  moves along a navigation path. In some embodiments, the disparity map is selected (e.g., pre-processed) using the 2-dimensional mask  350  as discussed with reference to  FIGS. 3A and 3B . Only disparity map within the valid detection range of the stereoscopic imaging sensors is selected. Disparity values of the disparity map are compared with the 2-dimensional mask  350  to exclude pixels with disparity values lower than the corresponding predefined threshold values on the 2-dimensional mask. 
     As shown in  FIG. 5B , method  500  proceeds to determine ( 504 ) a plurality of continuous regions (e.g., continuous regions  522 ,  524 ,  526 , and  528 ) in the disparity map  520 . In some embodiments, each continuous region is determined to include neighboring pixels having disparity values within a first predefined range. For example, a disparity value difference between any neighboring pixels within a continuous region is no higher than 2 pixels. 
     In some embodiments, the determined continuous regions are irregular. As shown in  FIG. 5C , method  500  proceeds to determine ( 506 ) a plurality of first boxes enclosing the plurality of continuous regions respectively. In some embodiments, the first boxes are in a regular shape, such as rectangular, such as boxes  532 ,  534 ,  536 , and  538  shown in  FIG. 5C . 
     Method  500  proceeds to determine ( 508 ) a second box (an object) within each first box determined at step  506  as a sub-region. For example, as shown in  FIG. 5D , the electronic device determines the second boxes  542 ,  544 ,  546 , and  548 , within the first boxes  532 ,  534 ,  536 , and  538 . Each second box encloses one or more pixels with disparity values within a second predetermined range. For example, each second box includes one or more pixels having the highest disparity values in the corresponding first box. That is, each second box represents a portion of the first box that is closest to the movable platform  102 . In some embodiments, each second box is centered at a pixel having the highest disparity value in the corresponding first box. In some embodiments, each second box includes pixels having disparity values in a range of x %-100% (e.g., x=80%, 70%, 60%, 50%) of the highest disparity value in the corresponding first box. In one example, in order to identify the second boxes, a pixel having the highest disparity value (Dmax) and a pixel having the lowest disparity value (Dmin) within each first box are identified. Centered at the pixel having the highest disparity value, the second box is determined to include a continuous region having pixels with minimum disparity values of (Dmax+Dmin)/2. In some embodiments, each second box encloses a continuous region and is in a regular shape, such as rectangular shape. In some alternative embodiments, a sub-region is a circle that is (1) centered at the pixel having the highest disparity value and (2) having a radius of (Dmax+Dmin)/2. In some other embodiments, the sub-region encloses the pixel with Dmax but does not have to be centered at the pixel with Dmax. For example, the sub-region is identified as a rectangular region having a diagonal of (Dmax+Dmin)/2 or Dmax. The sub-region is identified as an object detected by the movable platform  102 . In some embodiments, the object is an obstacle or a portion of the obstacle for avoidance by the movable platform. In some embodiments, the object is a target or a portion of the target for tracking by the movable platform. 
     Method  500  proceeds to determine ( 510 ) a distance between the second box (e.g., the identified object) and the movable platform  102 . In some embodiments, the distance is determined using at least the highest disparity value of the object. In some embodiments, the distance is also determined using one or more parameters of the imaging sensors, such as a focal length of the imaging sensors. 
     In some embodiments, the imaging device  216  borne on the movable platform  102  captures one or more image frames when the movable platform  102  moves along a navigation path. Method  500  proceeds to identify ( 512 ), within an image frame captured by the imaging device  216  borne on the movable platform  102 , one or more objects corresponding to the sub-regions (e.g., the second boxes) respectively.  FIG. 5E  illustrates an exemplary image frame  550  captured by the imaging device  216  borne on the movable platform  102 , in accordance with some embodiments. In some embodiments, the one or more objects (e.g., areas or boxes  552 ,  554 ,  556 , and  558 ) corresponding to the sub-regions (e.g., second boxes  542 ,  544 ,  546 , and  548 ) identified in the disparity map  520  at step  508  are identified on the image frame  550 . In some embodiments, the sub-regions in the disparity map  520  are projected to respective objects in the image frame  550  based on spatial information of the movable platform  102  and spatial information of the imaging device  216 . For example, data from IMU and GPS and data from gimbal for carrying the imaging device are used for calculating and identifying the objects within the image frame  550 . In some embodiments, characteristic points and/or object matching algorithms are also used for identifying the objects in the imaging frame  550  that correspond to the sub-regions. 
     Method  500  proceeds to send ( 514 ) the image frame  550  and the determined distances associated with the one or more objects to an electronic device for display. In some embodiments, based on the current speed of the movable platform  102  and the corresponding distance, an estimated time-to-hit value for each object within the image frame  550  is calculated. The time-to-hit values are sent to the electronic device for display. For example, as shown in  FIG. 5E , the respective distances and/or respective time-to-hit values ( FIG. 5E ) associated with the objects are displayed in real time in the image frame  550  as the movable platform  102  moves along a navigation path. 
       FIG. 6A  is a diagram illustrating a method  600  of processing image data including disparity map to track objects with the movable platform  102 , in accordance with some embodiments. In some embodiments, method  600  is performed by an electronic device such as the computing device  110 , the remote control  108 , or the movable platform  102  ( FIG. 1 ). For example, method  600  is performed by a controller of the image sensors  262 , a controller of the imaging device  216 , a controller of the movable platform  102 , or a controller of the remote control  108 . In some other embodiments, method  600  is performed by other electronic device(s), such as a mobile device or a computing device paired with the remote control  108  for operating the movable platform  102 . Operations performed in  FIG. 6A  correspond to instructions stored in computer memories or other computer-readable storage mediums of the corresponding device(s).  FIG. 6B  illustrates a process of processing disparity map  620  for tracking objects with the movable platform  102 , in accordance with some embodiments. One or more steps of method  600  are further illustrated in  FIG. 6B , which are discussed in combination with  FIG. 6A  in the present disclosure. 
     In some embodiments, the electronic device identifies/selects ( 602 ) an object within a disparity map, such as disparity map  620  in  FIG. 6B , obtained from stereoscopic cameras. In some embodiments, the disparity map  620  is obtained based on stereoscopic image frames captured by stereoscopic cameras (left stereographic image sensor  264  and right stereographic image sensor  266 ) borne on the movable platform  102 . In some embodiments, the movable platform  102  is in an in-flight mode. For example, the movable platform  102  moves along a navigation path  622 ,  FIG. 6B . In some embodiments, the disparity map is selected (e.g., pre-processed) using the 2-dimensional mask  350  as discussed with reference to  FIGS. 3A and 3B . Only disparity map within the valid detection range of the stereoscopic imaging sensors is selected. Disparity values of the disparity map are compared with the 2-dimensional mask  350  to exclude pixels with disparity values lower than the threshold values of corresponding projection points on the 2-dimensional mask. In some embodiments, the electronic device selects an element representing an object, e.g., an obstacle or a target, or a portion of an obstacle or a target, within the disparity map  620 . The element may include one or more pixels or one or more points in the disparity map. In some examples, the object, such as object  624 ,  626 , or  628 , or a nearest portion of the object, such as portion  623 ,  625 , or  627 , is selected using one or more steps of method  500  ( FIGS. 5A-5E ) or method  400  ( FIGS. 4A-4D ). 
     Method  600  proceeds to identify ( 604 ) an element representing the object in a first image frame captured by an imaging device.  FIG. 6C  illustrates an exemplary image frame  650  captured by the imaging device  216  borne on the movable platform  102 , in accordance with some embodiments. The element representing is identified in the image frame  650  corresponding to the object identified in the disparity map  620 , such as object  654  corresponding to object  624 , object  656  corresponding to object  626 , and object  658  corresponding to object  628 . In some embodiments, one or more portions identified in the image frame  650  are the nearest portions of the object(s) in disparity map  620 . The object(s) and/or portions of the object(s) may be identified in the image frame  650  using relative spatial information between the imaging device  216  and the stereoscopic imaging sensors borne on the movable platform  102 . The relative spatial information may be identified from IMU, GPS and/or the gimbal for carrying the imaging device  216 . 
     Method  600  proceeds to identify ( 606 ) one or more characteristic points, such as point  664 ,  666 , or  668 , of the corresponding element in the image frame  650  as shown in  FIG. 6C . In some embodiments, the characteristic points are pixels having different characteristics compared to neighboring pixels, such as pixels having highest disparity values or grayscale values, or pixels having drastic changes. In some embodiments, the characteristic points are identified using suitable method, such as corner/edge detection algorithms (e.g., FAST, or HARRIS algorithms). In some embodiments, the characteristic points are selected using a machine learning model trained by historical data related to characteristic point selections. In some embodiments, a navigation path  652  of the movable platform  102  is estimated based on the current speed and the attitude data (e.g., orientation angles) of the movable platform  102 . In some embodiments, the characteristic points, such as point  664 ,  666 , or  668 , are also identified to be close to the navigation path  652 , as shown in  FIG. 6C . For example, one or more points identified using the corner detection algorithm are further filtered to select the characteristic points that are within a predetermined distance range from the navigation path  652 . 
     Method  600  proceeds to track ( 608 ) the identified characteristic points in two consecutive image frames that are captured by the imaging device  216 . In some embodiments, the characteristic points are tracked using optical flow vectors from a first image frame to a second image frame. For example, motion trails of the tracking points are tracked using optical flow vectors generated from the characteristic points from the first image frame to the second image frame. The optical flow vectors can provide movement directions of the tracking points. 
     Method  600  proceeds to obtain ( 610 ) distance information between the characteristic points and the movable platform  102 . In some embodiments, the distance information is determined by integrating disparity values obtained from an updated disparity map and IMU/GPS/gimbal data (e.g., speed and/or flying distance of the movable platform  102 ) of an updated location of the movable platform  102 . In some embodiments, different weights are assigned to different data items for calculating the distance. For example, when an object is closer to the movable platform  102 , e.g., within a distance range of 10 meters, a higher weight is assigned to disparity data because the stereoscopic sensors provide more accurate data in a nearer range. When the object is farther from the movable platform  102 , e.g., outside a range of 10 meters, a higher weight is assigned to the IMU/GPS/gimbal data. 
     For example, initially an object is determined to be 20 meters from the movable platform  102  based on the disparity map. After a certain period of time of tracking, a distance determined based on an updated disparity value may be 8 meters; the IMU/GPS data shows the movable platform  102  has traveled for 15 meters. The current distance between this object and the movable platform  102  may be determined to be d=a×8+b×(20−15), where a&gt;b, a+b=100%. In some embodiments, other data obtained from other type of sensors may also be used to integrate with the disparity data and IMU/GPS data to calculate the distance. 
     Method  600  proceeds to display ( 612 ) the distance information and/or the time-to-hit value (e.g.,  FIG. 6C ) associated with each characteristic point. In some embodiments, in accordance with a determination that a distance between an object and the movable platform  102  is within a predetermined threshold value, such as closer than 15 meters or less than 4 seconds, the electronic device generates a notification to the user who is controlling the movable platform  102 . 
     In some embodiments, if at step  608 , the tracking of the characteristic points based on optical flow vectors fails, coordinates of the characteristic points can be calculated in an updated disparity map. For example, the characteristic points identified in the image frame can be projected to an updated disparity map using relative spatial information between the imaging device  216  and the movable platform  102 . The coordinates of the characteristic points can be calculated using the disparity values and/or the IMU/GPS/gimbal data. The characteristic points can then be identified in the image frame based on relative spatial information between the imaging device  216  and the movable platform  102 . Thus the tracking of the characteristic points will not be lost. 
     In some embodiments, as the movable platform  102  moves, if the object has moved to outside the field of view of the imaging device  216 , or if the object has moved outside a valid detection range (e.g., beyond 15 meters from the movable platform  102 ) of the stereoscopic imaging sensors  264  and  266 , the object tracking process may report an error in tracking. 
       FIGS. 7A and 7B  are a flow diagram illustrating a method  700  for selecting disparity map, in accordance with some embodiments. The method  700  is performed at an electronic device, such as the movable platform  102 , the imaging device  216 , the remote control  108 , and/or the computing device  110 . In some other embodiments, the method  700  is performed by other electronic device(s), such as a mobile device or a computing device paired with the remote control  108  for operating the movable platform  102 . Operations performed in  FIGS. 7A-7B  correspond to instructions stored in computer memories or other computer-readable storage mediums of the corresponding device(s). 
     The electronic device obtains ( 701 ) a disparity map based on stereoscopic image frames captured by stereoscopic cameras (e.g., stereographic image sensors  264  and  266 ,  FIG. 2C ) borne on a movable platform (e.g., the movable platform  102 ). 
     The electronic device receives ( 702 ) a 2-dimensional mask (e.g., 2-dimensional mask  350 ,  FIGS. 3B and 3D ) including a plurality of projection points defining a predefined 3-dimensional volume (e.g., 3-dimensional volume  300 ,  FIGS. 3A and 3C ) adjacent the movable platform  102 . Each projection point has a threshold disparity value for objects within the predefined 3-dimensional volume. 
     In some embodiments, the 3-dimensional volume  300  is ( 706 ) determined based on a valid detection range of the stereoscopic cameras. In some embodiments, the 3-dimensional volume is ( 708 ) a cuboid with a dimension of 10 m×10 m×15 m as shown in  FIG. 3A . 
     In some embodiments, the electronic device selects ( 710 ) a first element (e.g., point P,  FIG. 3A ) in the disparity map having a disparity value greater than a threshold disparity value on the 2-dimensional mask that corresponds to a projection of the first element onto the 2-dimensional mask. In some embodiments, the electronic device obtains ( 712 ) the 2-dimensional mask by determining a threshold value for a projection point on the 2-dimensional mask using a disparity value for an object at a corresponding point on the 3-dimensional volume. 
     The electronic device selects ( 704 ), among the disparity map, a subset of elements. The subset of elements are selected by comparing disparity values of the elements with threshold disparity values on the 2-dimensional mask that correspond to projections of the elements onto the 2-dimensional mask. The subset of elements represents actual objects within the predefined 3-dimensional volume. In some embodiments, an element corresponds to a pixel, a point, and/or a group of pixels in the disparity map. 
     In some embodiments, the electronic device excludes ( 714 ) a region from the subset of elements in the disparity map that corresponds to the ground within the movement trajectory of the movable platform based on spatial information of the movable platform. In some embodiments, the electronic device identifies ( 716 ) the objects from the subset of elements based on disparity values of the subset of elements. The electronic device determines ( 718 ) distances between the objects and the movable platform. In some embodiments, the electronic device tracks ( 720 ) the objects based on an updated disparity map and an updated location of the movable platform  102 . 
       FIGS. 8A-8C  are a flow diagram illustrating a method  800  for processing image data for detecting objects by the movable platform  102 , in accordance with some embodiments. The method  800  is performed at an electronic device, such as the movable platform  102 , the imaging device  216 , the remote control  108 , and/or the computing device  110 . In some other embodiments, the method  800  is performed by other electronic device(s), such as a mobile device or a computing device paired with the remote control  108  for operating the movable platform  102 . Operations performed in  FIGS. 8A-8C  correspond to instructions stored in computer memories or other computer-readable storage mediums of the corresponding device(s). 
     The electronic device obtains ( 802 ) a disparity map (e.g., disparity map  520 ,  FIG. 5B ) based on stereoscopic image frames captured by stereoscopic cameras (left stereographic image sensor  264  and right stereographic image sensor  266 ,  FIG. 2B ) borne on the movable platform  102 . The disparity map can be generated based on a pair of stereoscopic grayscale images. The disparity map includes disparity values of one or more pixels corresponding to distances between two corresponding pixels in the pair of left and right stereoscopic images. The disparity value is related to depth information of a pixel (e.g., a distance between the object and the imaging sensor). The disparity map can be used for obtaining depth information, e.g., information related to a distance between the camera(s) and the object, of one or more objects in the image frames. 
     In some embodiments, the electronic device selects ( 818 ), from a raw disparity map obtained directly from the stereoscopic image frames, a subset of disparity values for the disparity map between threshold disparity values of corresponding projection points on a 2-dimensional mask (e.g., 2-dimensional mask  350 ,  FIGS. 3B and 3D ). The 2-dimensional mask includes a plurality of projection points defining a predefined 3-dimensional volume (e.g., 3-dimensional volume  300 ,  FIGS. 3A and 3C ) adjacent the movable platform, each projection point having a threshold disparity value for objects within the predefined 3-dimensional volume. The subset of disparity values represents actual objects within the predefined 3-dimensional volume. 
     The electronic device determines ( 804 ) a plurality of continuous regions (e.g., regions  432 ,  434 ,  436 , and  438 ,  FIG. 4C ; boxes  532 ,  534 ,  536 , and  538 ,  FIG. 5C ) in the disparity map. Each continuous region includes a plurality of elements having disparity values within a predefined range. For example, a continuous region includes pixels within 0.5 meters or within 2 pixels. The electronic device identifies ( 806 ), within each continuous region, a continuous sub-region (e.g., pixel  426 ,  442 ,  446 , or  44 ,  FIG. 4C ; box  542 ,  544 ,  546 , or  548 ,  FIG. 5D ) including one or more elements having a highest disparity value (i.e., being closest to the movable platform  102 ) than that of the other elements within the continuous region as an object. 
     In some embodiments as illustrated in  FIGS. 4A-4D , determining the plurality of continuous regions comprises ( 820 ) dividing the disparity map into a plurality of areas using a grid (e.g., grid  422 ,  FIG. 4B ); identifying an element having a highest disparity value in each area (such as pixel  424 ,  426  in  FIG. 4B ). In some embodiments, determining the plurality of continuous regions also comprises ( 820 ) selecting, within each area, one or more contiguous elements to form a respective continuous region of the plurality of continuous regions (e.g., as regions  432 ,  434 ,  436 , and  438 ,  FIG. 4C ). The differences between respective disparity values of the contiguous elements and the highest disparity value are within the predefined range. In some examples, distances between objects in the world coordinate system corresponding to the one or more contiguous elements and an object corresponding to the element with the highest disparity value are within a predefined range, e.g., 0.5 meters. 
     In some embodiments as illustrated in  FIGS. 5A-5E , differences of disparity values between neighboring elements of the plurality of elements in each continuous region (e.g., region  522 ,  524 ,  526 , or  528 ,  FIG. 5B ) are within the predefined range, such as 2 pixels. In some embodiments, the electronic device determines ( 824 ) a plurality of first boxes enclosing the plurality of continuous regions respectively, such as boxes  532 ,  534 ,  536 , and  538 . 
     In some embodiments, the electronic device determines ( 826 ) a second box (e.g., box  542 ,  544 ,  546 , or  548 ,  FIG. 5D ) within each first box (e.g., boxes  532 ,  534 ,  536 , and  538 ,  FIG. 5C ) as a continuous sub-region. The second box encloses the one or more elements with the highest disparity values in the corresponding first box. In some embodiments, elements within a second box have disparity values within a range from (Dmax+Dmin)/2 to Dmax. Dmax and Dmin correspond to the highest disparity value and the lowest disparity value respectively within a corresponding first box enclosing the second box. In some other embodiments, elements within a second box have disparity values within a range, e.g., 80%-100% of the highest disparity value within the corresponding first box. 
     In some embodiments, the electronic device tracks ( 830 ) the objects as the movable platform  102  moves along a navigation path based on an updated disparity map and an updated location of the movable platform. In some embodiments, the object is ( 814 ) an obstacle or a portion of the obstacle for avoidance by the movable platform  102 . In some embodiments, the object is ( 816 ) a target or a portion of the target for tracking by the movable platform. 
     The electronic device determines ( 808 ) a distance between the object and the movable platform using at least the highest disparity value. In some embodiments, the electronic device identifies ( 810 ), within an image frame (e.g., image frame  450 ,  FIG. 4D ; image frame  550 ,  FIG. 5E ) captured by the imaging device  216  borne by the movable platform  102 , one or more objects corresponding to the continuous sub-regions respectively, such as pixels  452 ,  454 ,  456 , and  458  in  FIG. 4D , or boxes  552 ,  554 ,  556 , and  558  in  FIG. 5E . The one or more objects may be identified within the image frame using relative spatial information of the imaging device  216  and the movable platform  102 . The one or more objects may be identified in the image frame using characteristic points and/or object matching. In some embodiments, the electronic device sends ( 812 ) the image frame and the determined distances associated with the one or more objects to an electronic device (such as a display of the remote control  108  or a mobile device coupled to the remote control  108 ) for display in real time. 
       FIGS. 9A-9C  are a flow diagram illustrating a method  900  for processing image data for tracking objects by the movable platform  102 , in accordance with some embodiments. The method  900  is performed at an electronic device, such as the movable platform  102 , the imaging device  216 , the remote control  108 , and/or the computing device  110 . In some other embodiments, the method  900  is performed by other electronic device(s), such as a mobile device or a computing device paired with the remote control  108  for operating the movable platform  102 . Operations performed in  FIGS. 9A-9C  correspond to instructions stored in computer memories or other computer-readable storage mediums of the corresponding device(s). 
     The electronic device identifies ( 902 ), within a disparity map (e.g., disparity map  620 ,  FIG. 6B ), an object (e.g., object  626  or portion  625  of the object  626 ,  FIG. 6B ) for tracking by the movable platform  102 . In some embodiments, the electronic device obtains ( 910 ) the disparity map based on stereoscopic image frames captured by stereoscopic cameras borne on the movable platform  102 . In some embodiments, the electronic device determines ( 912 ) a continuous region including one or more elements having disparity values within a predefined range as the object, the disparity values of the one or more elements being higher than that of the other elements within the continuous region. In some embodiments, the electronic device selects ( 914 ), from a raw disparity map obtained directly from the stereoscopic image frames, a subset of disparity values for the disparity map between threshold disparity values of corresponding projection points on a 2-dimensional mask (e.g., 2-dimensional mask  350 ,  FIGS. 3B and 3D ). The 2-dimensional mask includes a plurality of projection points defining a predefined 3-dimensional volume (e.g., 3-dimensional volume  300 ,  FIGS. 3A and 3C ) adjacent the movable platform. Each projection point has a threshold disparity value for objects within the predefined 3-dimensional volume. The subset of disparity values represents actual objects within the predefined 3-dimensional volume. 
     The electronic device determines ( 904 ) a location of an element representing the object in a first image frame (e.g., image frame  650 ) captured by the imaging device  216  borne on the movable platform  102 . In some embodiments, the location of the element on the first image frame is ( 918 ) determined based on relative spatial information between the imaging device  216  and the movable platform  102 . For example, the object is projected to the image frame based on IMU/GPS/gimbal data. 
     The electronic device selects ( 906 ) one or more characteristic points (e.g., points  664 ,  666 ,  668 ,  FIG. 6C ) of the element representing the object as tracking points of the object on the first image frame. In some embodiments, the one or more characteristic points are ( 916 ) selected to be closer to a navigation path (e.g., navigation path  652 ,  FIG. 6C ) of the movable platform  102  than other parts of the object. In some embodiments, the characteristic points are selected using corner detection algorithm, such as FAST, HARRIS algorithm. In some embodiments, the characteristic points are selected using a machine learning model trained by historical data related to characteristic point selections. In some embodiments, the navigation path is determined based on speed and attitude data of the movable platform  102 . 
     The electronic device updates ( 908 ) the locations of the characteristic points (e.g., tracking points) of the element on a second image frame captured by the imaging device  216  in accordance with an updated disparity map and a current location of the movable platform  102 . In some embodiments, the current location of the movable platform  102  is ( 920 ) determined based on data from a plurality of sensors associated with the movable platform  102 , such as spatial data from IMU, GPS, and disparity data from stereoscopic cameras. In some embodiments, updating the locations of the tracking points on the second image frame further comprises ( 922 ) tracking motion trails of the tracking points using optical flow generated from the tracking points from the first image frame to the second image frame. 
     In some embodiments, the electronic device determines ( 924 ) a distance between the object and the movable platform  102  based on information obtained from the updated disparity map. For example, the electronic device integrates data from stereoscopic cameras (disparity map) and IMU/GPS (speed, flying distance) of the movable platform  102 . The electronic device assigns different weights to the disparity data and to the IMU/GPS data. For example, when the movable platform  102  moves closer to the object, a greater weight is assigned to the disparity data. When the movable platform  102  is farther from the object, a greater weight is assigned to the IMU/GPS data. 
     In some embodiments, in accordance with a determination that the distance between the object and the movable platform is within a predetermined threshold value, the electronic device generates ( 926 ) a notification to be sent to a controlling device of the movable platform  102 . 
     Many features of the present disclosure can be performed in, using, or with the assistance of hardware, software, firmware, or combinations thereof. Consequently, features of the present disclosure may be implemented using a processing system. Exemplary processing systems (e.g., processor(s)  202 , processors of the remote control  108 , processors of the computing device  110 , and/or processors of the imaging device  216 ) include, without limitation, one or more general purpose microprocessors (for example, single or multi-core processors), application-specific integrated circuits, application-specific instruction-set processors, field-programmable gate arrays, graphics processors, physics processors, digital signal processors, coprocessors, network processors, audio processors, encryption processors, and the like. 
     Features of the present disclosure can be implemented in, using, or with the assistance of a computer program product, such as a storage medium (media) or computer readable storage medium (media) having instructions stored thereon/in which can be used to program a processing system to perform any of the features presented herein. The storage medium (e.g., the memory  204 ) can include, but is not limited to, any type of disk including floppy disks, optical discs, DVD, CD-ROMs, microdrive, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, DRAMs, VRAMs, DDR RAMs, flash memory devices, magnetic or optical cards, nanosystems (including molecular memory ICs), or any type of media or device suitable for storing instructions and/or data. 
     Stored on any one of the machine readable medium (media), features of the present disclosure can be incorporated in software and/or firmware for controlling the hardware of a processing system, and for enabling a processing system to interact with other mechanism utilizing the results of the present disclosure. Such software or firmware may include, but is not limited to, application code, device drivers, operating systems, and execution environments/containers. 
     Communication systems as referred to herein (e.g., the communication system  206 ) optionally communicate via wired and/or wireless communication connections. For example, communication systems optionally receive and send RF signals, also called electromagnetic signals. RF circuitry of the communication systems convert electrical signals to/from electromagnetic signals and communicate with communications networks and other communications devices via the electromagnetic signals. RF circuitry optionally includes well-known circuitry for performing these functions, including but not limited to an antenna system, an RF transceiver, one or more amplifiers, a tuner, one or more oscillators, a digital signal processor, a CODEC chipset, a subscriber identity module (SIM) card, memory, and so forth. Communication systems optionally communicate with networks, such as the Internet, also referred to as the World Wide Web (WWW), an intranet and/or a wireless network, such as a cellular telephone network, a wireless local area network (LAN) and/or a metropolitan area network (MAN), and other devices by wireless communication. Wireless communication connections optionally use any of a plurality of communications standards, protocols and technologies, including but not limited to Global System for Mobile Communications (GSM), Enhanced Data GSM Environment (EDGE), high-speed downlink packet access (HSDPA), high-speed uplink packet access (HSDPA), Evolution, Data-Only (EV-DO), HSPA, HSPA+, Dual-Cell HSPA (DC-HSPDA), long term evolution (LTE), near field communication (NFC), wideband code division multiple access (W-CDMA), code division multiple access (CDMA), time division multiple access (TDMA), Bluetooth, Wireless Fidelity (Wi-Fi) (e.g., IEEE 102.11a, IEEE 102.11ac, IEEE 102.11ax, IEEE 102.11b, IEEE 102.11g and/or IEEE 102.11n), voice over Internet Protocol (VoIP), Wi-MAX, a protocol for e-mail (e.g., Internet message access protocol (IMAP) and/or post office protocol (POP)), instant messaging (e.g., extensible messaging and presence protocol (XMPP), Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions (SIMPLE), Instant Messaging and Presence Service (IMPS)), and/or Short Message Service (SMS), spread spectrum technology such as FASST or DESST, or any other suitable communication protocol, including communication protocols not yet developed as of the filing date of this document. 
     While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the disclosure. 
     The present disclosure has been described above with the aid of functional building blocks illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks have often been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the disclosure. 
     The terminology used in the description of the various described embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various described embodiments and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “includes,” “including,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     As used herein, the term “if” may be construed to mean “when” or “upon” or “in response to determining” or “in accordance with a determination” or “in response to detecting,” that a stated condition precedent is true, depending on the context. Similarly, the phrase “if it is determined [that a stated condition precedent is true]” or “if [a stated condition precedent is true]” or “when [a stated condition precedent is true]” may be construed to mean “upon determining” or “in response to determining” or “in accordance with a determination” or “upon detecting” or “in response to detecting” that the stated condition precedent is true, depending on the context. 
     The foregoing description of the present disclosure has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments. Many modifications and variations will be apparent to the practitioner skilled in the art. The modifications and variations include any relevant combination of the disclosed features. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical application, thereby enabling others skilled in the art to understand the disclosure for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalence.