Patent Publication Number: US-11657595-B1

Title: Detecting and locating actors in scenes based on degraded or supersaturated depth data

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/220,461, filed Dec. 14, 2018, now U.S. Pat. No. 10,915,783, the contents of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     A “time-of-flight” camera is a depth-sensing or range-sensing system that operates by illuminating a scene using a light source (e.g., a modulated light source) and capturing light that is reflected from various points of the scene following the illumination. Time-of-flight cameras are typically equipped with an illuminator for illuminating a scene, and a sensor for capturing reflected light from the scene. The reflected light that is captured by a time-of-flight camera sensor may be interpreted to generate a depth image or profile of portions of the scene within a field of view of the time-of-flight camera. Some time-of-flight cameras may capture and interpret reflected light, and generate depth images or profiles of portions of scenes from such reflected light, several dozen times per second. Depth images or profiles generated by time-of-flight cameras can be very accurate. 
     In some applications, the effectiveness of a time-of-flight camera may be diminished where pixels of a depth image are degraded for any reasons, such as when one or more retroreflective materials are present within a field of view of the time-of-flight camera. A retroreflector is an object or surface that reflects light with minimal scattering, typically with rays having vectors that are parallel to and opposite in direction from a source of the light. Retroreflective materials are commonly used in applications where enhanced visibility is desired, such as by applying retroreflective materials to surfaces of objects of importance in areas or environments having varying weather or lighting conditions, e.g., safety cones, street signs, highway barriers, or road stripes. Retroreflective materials may also be worn by workers who operate in such areas. For example, protective or working clothing such as jackets, vests, pants, shirts, hats, gloves or the like may be formed from fabrics having one or more strips, bands, layers, panels or other sectors of retroreflective material. Alternatively, some such clothing may be formed from retroreflective material in their entirety. 
     In particular, retroreflectivity may cause pixel sensors of a time-of-flight camera to be saturated, which distorts or corrupts a set of depth data generated by the time-of-flight camera, and results in one or more distortions or corruptions within a depth image generated from the set of depth data. Retroreflective materials may therefore disrupt the processes by which depth images are generated for a scene using time-of-flight cameras, and result in the generation of depth images having substantially limited value. Pixels of the depth data may also be degraded where one or more aspects of the time-of-flight data are not operating properly for any other reason, such as where an illuminator or one or more pixel sensors are malfunctioning. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A through  1 I  are views of aspects of one system for detecting or locating actors in accordance with implementations of the present disclosure. 
         FIGS.  2 A and  2 B  are block diagrams of one system for detecting or locating actors in accordance with implementations of the present disclosure. 
         FIG.  3    is a flow chart of one process for detecting or locating actors in accordance with implementations of the present disclosure. 
         FIGS.  4 A through  4 G  are views of aspects of one system for detecting or locating actors in accordance with implementations of the present disclosure. 
         FIGS.  5 A and  5 B  are a flow chart of one process for detecting or locating actors in accordance with implementations of the present disclosure. 
         FIGS.  6 A through  6 E  are views of aspects of one system for detecting or locating actors in accordance with implementations of the present disclosure. 
         FIGS.  7 A and  7 B  are a flow chart of one process for detecting or locating actors in accordance with implementations of the present disclosure. 
         FIGS.  8 A through  8 G  are views of aspects of one system for detecting or locating actors in accordance with implementations of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As is set forth in greater detail below, the present disclosure is directed to detecting or locating actors in scenes based on depth imaging data that is subject to varying degrees of degradation, such as saturation due to the presence of one or more retroreflective objects or surfaces on the scene, or for any other reason. More specifically, the systems and methods disclosed herein are directed to capturing depth imaging data from a scene (e.g., at a materials handling facility) using time-of-flight cameras, and determining whether the depth imaging data is of sufficiently high quality to generate or update a model of the scene, such as a location map that includes positions of one or more actors thereon, or to detect or locate one or more actors on the scene. Whether depth imaging data is of sufficiently high quality may be determined based on a ratio of saturated or otherwise degraded pixels within a frame of the depth imaging data to a total number of relevant pixels within the frame of the depth imaging data. If a depth image is determined to be of sufficient quality, the depth image may be processed to detect one or more actors or other objects thereon, and a location map of positions of the actors or the other objects may be determined based on the depth image. 
     If a depth image is determined to be of insufficient quality, however, a location map of positions of actors or other objects on the scene may be generated or updated based on one or more visual images. For example, where ratios of degraded pixels in depth images are determined to exceed a predetermined threshold, one or more visual images captured simultaneously or substantially simultaneously with the depth image may be provided as inputs to a machine learning system that operates on digital cameras that captured the visual image or on one or more other computer devices. Outputs received from the machine learning system may be processed to not only detect one or more body parts (e.g., heads) depicted therein but also to predict distances (or depths or ranges) to such body parts. A model of a scene may be updated based on depth images captured by one or more of the respective digital cameras or, alternatively, based on the detections and the distances predicted based on visual images captured by one or more of the respective digital cameras, in lieu of depth images of sufficient quality. Where an imaging device generates or updates a model of the scene based on depth images or visual images, such models may be combined into a composite location map that is representative of positions of actors or other objects on the scene. Moreover, where an actor has been located on a scene, and the actor is determined to be a source of saturation or other degradation of pixels within frames of depth imaging data, the actor may be tracked over time by searching for or identifying overlapping fields of view of time-of-flight cameras or other depth cameras having high ratios of saturated or otherwise degraded pixels. 
     Referring to  FIGS.  1 A through  1 I , views of aspects of one system for detecting or locating actors in accordance with the present disclosure are shown. As is shown in  FIGS.  1 A through  1 C , a plurality of imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  and a server  182  are connected to one another over a network  190 , which may include the Internet in whole or in part. The imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  are “RGBD cameras,” or other imaging devices that are outfitted with both depth sensors, e.g., time-of-flight sensors, and visual sensors. The imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  have fields of view  130 - 1 ,  130 - 2 ,  130 - 3  that include at least a portion of a materials handling facility, e.g., an inventory storage unit  195  (e.g., at least one of a shelf, a table, a bin, a rack, a bar or a hook) with a plurality of items stored thereon. The imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  include illuminators for illuminating the field of view with modulated light pulses, and sensors for capturing and interpreting light reflected from the storage unit  195 , light reflected from the items on the storage unit  195 , and/or light reflected from one or more actors  192 - 1 ,  192 - 2 ,  192 - 3 . In some implementations, the sensors of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  may include a plurality of photoreceptors or other pixel sensors, and such photoreceptors or pixel sensors may be arranged in an array, a grid or any other shape or arrangement. Additionally, the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  may be calibrated, such that mappings between coordinates of imaging data captured by such cameras and directions relative to their respective image sensors are known. 
     As is shown in  FIGS.  1 A and  1 B , the actors  192 - 1 ,  192 - 2  are each wearing retroreflective vests (or other retroreflective clothing)  194 - 1 ,  194 - 2  having one or more strips, bands, sections or other portions of retroreflective material  196 - 1 ,  196 - 2 . As is also shown in  FIGS.  1 A and  1 B , the actor  192 - 3  is not wearing any retroreflective clothing, however. 
     Where retroreflective materials are present within fields of view of time-of-flight cameras, pixel sensors that detect light reflected from such materials are often saturated, and the effects of such saturation may impact pixels corresponding to the retroreflective materials and also neighboring or surrounding pixels within raw sensor data or depth images generated therefrom, e.g., by obscuring distances to the retroreflective materials. 
     As is shown in  FIG.  1 C , the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  capture depth images  150 - 1 ,  150 - 2 ,  150 - 3  and visual images  152 - 1 ,  152 - 2 ,  152 - 3  based on light reflected from surfaces within their fields of view  130 - 1 ,  130 - 2 ,  130 - 3 . In some implementations of the present disclosure, each of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  may process the depth images  150 - 1 ,  150 - 2 ,  150 - 3  and visual images  152 - 1 ,  152 - 2 ,  152 - 3  respectively captured thereby, e.g., according to one or more machine learning tools or techniques. For example, one or more processors of each of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  may be programmed or configured to execute one or more machine learning tools or techniques to detect one or more body parts or other aspects of human actors or other objects, or to segment portions of imaging data that include human actors or other objects. In some implementations, such processors may receive one or more of the depth images  150 - 1 ,  150 - 2 ,  150 - 3  or one or more of the visual images  152 - 1 ,  152 - 2 ,  152 - 3  as inputs to such machine learning tools or techniques, and one or more body parts of an actor, e.g., one or more of the actors  192 - 1 ,  192 - 2 ,  192 - 3 , or distances to such actors  192 - 1 ,  192 - 2 ,  192 - 3 , may be determined or predicted based on outputs received from the one or more machine learning tools or techniques. 
     The server  182  may be configured to generate one or more location maps  185  or one or more other representations of the scene, including but not limited to positions or orientations of one or more aspects of the actors  192 - 1 ,  192 - 2 ,  192 - 3  or the storage unit  195 , based on information or data generated or received from the respective imaging devices  110 - 1 ,  110 - 2 ,  110 - 3 . The location maps  185  may be point clouds or other three-dimensional representations of features on the scene, e.g., the storage unit  195 , including coordinates of points on surfaces of the storage unit  195 , or any other objects that may be present on the scene, including but not limited to the actors  192 - 1 ,  192 - 2 ,  192 - 3 . For example, in some implementations, a location map  185  of a scene may be generated based on a background model that initially describes the scene, including positions and/or dimensions of background features thereon, such as the storage unit  195 , and may be subsequently updated to reflect changes in the composition of the scene. A background model may be generated in any manner, such as based on depth images captured using one or more of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3 , or in any other manner, where such depth images are not subject to excessive saturation or otherwise degraded. Alternatively, each of the depth images  150 - 1 ,  150 - 2 ,  150 - 3  and visual images  152 - 1 ,  152 - 2 ,  152 - 3  may be provided to the server  182  over the network  190 , and the server  182  may process such images  150 - 1 ,  150 - 2 ,  150 - 3 ,  152 - 1 ,  152 - 2 ,  152 - 3 , e.g., according to one or machine learning tools or techniques operating on the server  182 , to generate the location maps  185  of the scene based on the images  150 - 1 ,  150 - 2 ,  150 - 3 ,  152 - 1 ,  152 - 2 ,  152 - 3 . 
     The depth images  150 - 1 ,  150 - 2 ,  150 - 3  are generated from raw sensor data captured by the respective imaging devices  110 - 1 ,  110 - 2 ,  110 - 3 . In some implementations, the depth images  150 - 1 ,  150 - 2 ,  150 - 3 , which may be images or maps representing distances (or depths or ranges) to objects from the sensor of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3 , are generated based on the capture of light reflected from the scene as a result of the illumination at selected energies or powers. As is shown in  FIG.  1 D , the depth image  150 - 1  shows a region  155 - 1  corresponding to the actor  192 - 1  that is corrupted by supersaturation due to reflections from the retroreflective vest  194 - 1 , as shown in  FIGS.  1 A and  1 B , as well as a region  155 - 3  corresponding to distances to the actor  192 - 3  and a region  155 - 4  corresponding to distances to the storage unit  195  that are not so corrupted. The reflection of light from the retroreflective materials  196 - 1  worn by the actor  192 - 1  and the resulting saturation of pixel sensors cause the region  155 - 1  to have distortions or corruptions that encompass substantial portions of the depth image  150 - 1  which are larger than the size of the retroreflective vest  194 - 1  within the plane of the depth image  150 - 1 . The reflection of light from aspects of the actor  192 - 3  or the storage unit  195  are not so corrupted. A ratio of the number of degraded (e.g., supersaturated) pixels within the depth image  150 - 1 , e.g., within the region  155 - 1 , to the total number of pixels in the depth image  150 - 1  may be calculated by one or more processors aboard the imaging device  110 - 1  or, alternatively, the server  182 . For example, where the depth image  150 - 1  has dimensions of four thousand pixels by three thousand pixels, and the number of degraded (e.g., supersaturated) pixels in the region  155 - 1  shown in  FIG.  1 D  is 381,244, the ratio of degradation (e.g., supersaturation) within the depth image  150 - 1  is approximately 3.18 percent. 
     Similarly, as is shown in  FIG.  1 E , the depth image  150 - 2  shows the region  155 - 1  corresponding to the actor  192 - 1 , as corrupted by supersaturation due to reflections from the retroreflective vest  194 - 1 , along with another region  155 - 2  that is also corrupted by supersaturation due to reflections from the retroreflective vest  194 - 2  of the actor  192 - 2 , as well as the regions  155 - 3 ,  155 - 4 , which correspond to distances (or depths or ranges) to the actor  193 - 3  and the storage unit  195 , and are not so corrupted. Where the depth image  150 - 2  has dimensions of four thousand pixels by three thousand pixels, and the number of degraded pixels in the depth image  150 - 2  (viz., the regions  155 - 1 ,  155 - 2 ) shown in  FIG.  1 E  is 526,424, the ratio of degradation within the depth image  150 - 2  is approximately 4.38 percent. 
     Likewise, as is shown in  FIG.  1 F , the depth image  150 - 3  also shows the region  155 - 2  as corrupted by supersaturation due to reflections from the retroreflective vest  194 - 2 , along with the region  155 - 3  and the region  155 - 4  corresponding to distances to the actor  192 - 3  and the storage unit  195 , which are not so corrupted. Where the depth image  150 - 3  has dimensions of four thousand pixels by three thousand pixels, and the number of supersaturated pixels in the depth image  150 - 3  (viz., the region  155 - 3 ) shown in  FIG.  1 F  is 326,117, the ratio of supersaturation within the depth image  150 - 3  is approximately 2.72 percent. 
     The extent to which pixel sensors of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  are saturated or otherwise degraded by reflections from the retroreflective vests  194 - 1 ,  194 - 2  or any other retroreflective materials on the scene may depend on any factors. In some implementations, a size of a corrupted region, or a number of saturated pixel sensors, may depend on a visible contrast between a retroreflective material and neighboring portions of the scene, e.g., between the retroreflective vests  194 - 1 ,  194 - 2  and the retroreflective materials  196 - 1 ,  196 - 2 , or a relative difference in signals generated by the corresponding pixel sensors. For example, where a retroreflective material is applied against a bright background, e.g., a yellow or orange background, a relative difference in signals generated by the pixel sensors at or near an interface between the retroreflective material and the background may be comparatively low, and a corrupted region associated with the retroreflective materials will likewise be substantially small. Where a retroreflective material is applied against a dark background, however, e.g., a black or a brown background, a relative difference in signals generated by the pixel sensors at or near an interface between the retroreflective material and the background may be comparatively high, and a corrupted region associated with the retroreflective materials may be substantially large. 
     The regions  155 - 1 ,  155 - 2  may be identified as corrupted within the depth images  150 - 1 ,  150 - 2 ,  150 - 3 , or within the raw sensor data from which the depth images  150 - 1 ,  150 - 2 ,  150 - 3  were generated, on any basis. For example, where a clipping value (e.g., a distortion limit), e.g., 0 or 4095 for a twelve-bit pixel sensor, is reached for a predetermined threshold number of pixel sensors (e.g., for ten, one hundred, one thousand, or another number of the pixel sensors) for a given set of raw sensor data, the raw sensor data may be determined to be distorted or corrupted at locations corresponding to the saturated pixel sensors, and the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  may be presumed to have one or more retroreflective materials within the field of view. Where the depth images  150 - 1 ,  150 - 2 ,  150 - 3  are generated from a predetermined number of sets of raw sensor data (e.g., eight) captured at various frequencies or phase differences between the illumination pulsing and the pixel exposure, the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  may be presumed to have one or more retroreflective materials within the fields of view  130 - 1 ,  130 - 2 ,  130 - 3  when a threshold number of the sets of raw sensor data (e.g., six or seven) captured by the respective imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  are identified as having the predetermined threshold number of pixel sensors at or above the clipping value. 
     In accordance with some implementations of the present disclosure, where ratios of degradation (e.g., supersaturation) within one or more of the depth images  150 - 1 ,  150 - 2 ,  150 - 3  exceed predetermined thresholds, e.g., one-tenth of one percent, one-half of one percent, one percent, or another threshold, the depth images  150 - 1 ,  150 - 2 ,  150 - 3  may not be used to generate or update the location map  185 . Instead, the location map  185  depicting positions of one or more aspects of the actors  192 - 1 ,  192 - 2 ,  192 - 3  or the shelving unit  195  may be generated or updated based on detected objects such as one or more body parts of the actors  192 - 1 ,  192 - 2 ,  192 - 3  or the shelving unit  195  within visual images captured substantially simultaneously with the depth images  150 - 1 ,  150 - 2 ,  150 - 3 , and predicted distances (or depths or ranges) to such objects. In some implementations, the actors  192 - 1 ,  192 - 2 ,  192 - 3  and the shelving unit  195  are detected based on outputs from one or more machine learning tools or techniques. 
     For example, as is shown in  FIG.  1 G , the visual images  152 - 1 ,  152 - 2 ,  152 - 3  are provided to the server  182  over the network  190 , and positions of the one or more body parts  154 - 1 ,  154 - 2 ,  154 - 3  of the actors  192 - 1 ,  192 - 2 ,  192 - 3 , as well as predicted distances to such body parts  154 - 1 ,  154 - 2 ,  154 - 3 , are generated by the server  182 . For example, the visual images  152 - 1 ,  152 - 2 ,  152 - 3  may be provided to one or more machine learning tools or techniques operating on the server  182 , e.g., artificial neural networks, which may be trained to detect one or more body parts within visual imaging data, or to predict distances to such body parts, to varying degrees of confidence. Similarly, positions of one or more aspects of the shelving unit  195  or any other objects on the scene and within fields of view of one or more of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  may also be generated by the server  182  based on outputs received from one or more machine learning tools or techniques, such as where the machine learning tools or techniques are trained to detect such objects, e.g., the shelving unit  195 , within imaging data or to predict distances (or depths or ranges) to aspects of such objects. Alternatively, the positions of the body parts  154 - 1 ,  154 - 2 ,  154 - 3  and the predicted distances to such body parts  154 - 1 ,  154 - 2 ,  154 - 3  may be generated by each of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  based on the visual images  152 - 1 ,  152 - 2 ,  152 - 3  captured thereby, rather than by the server  182 . For example, the positions and the predicted distances may be generated based on outputs received from machine learning tools or techniques operated by processors on each of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3 . Once generated by each of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3 , the positions and the predicted distances may be provided to the server  182  over the network  190 . Similarly, positions of aspects of the shelving unit a  195  and predicted distances to such aspects may also be generated by each of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  based on the visual images  152 - 1 ,  152 - 2 ,  152 - 3  captured thereby, and provided to the server  182  over the network  190 . 
     As is shown in  FIG.  1 H , where the actors  192 - 1 ,  192 - 2 ,  192 - 3  appear within fields of view of two or more of the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3 , the server  182  may further refine positions of the actors  192 - 1 ,  192 - 2 ,  192 - 3  by triangulating lines (e.g., rays) extending from visual sensors of the respective imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  through the respective body parts  154 - 1 ,  154 - 2 ,  154 - 3  within the image planes of the visual images  152 - 1 ,  152 - 2 ,  152 - 3 . Where such lines extending through the common body parts  154 - 1 ,  154 - 2 ,  154 - 3  intersect, positions P 1 (x, y, z), P 2 (x, y, z), P 3 (x, y, z) in three-dimensional space of the body parts that the imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  and/or the server  182  was configured to recognize within visual imaging data are determined. Alternatively, the positions P 1 (x, y, z), P 2 (x, y, z), P 3 (x, y, z) may be determined for each of a plurality of points on surfaces of the actors  192 - 1 ,  192 - 2 ,  192 - 3  that appear in common within two or more of the visual images  152 - 1 ,  152 - 2 ,  152 - 3 , e.g., by stereo triangulation. Similarly, positions to one or more aspects of the shelving unit  195  may also be determined by triangulating rays extending from the visual sensors of the respective imaging devices  110 - 1 ,  110 - 2 ,  110 - 3  through pixels corresponding to such aspects. 
     Alternatively, where an object (e.g., one or more of the actors  192 - 1 ,  192 - 2 ,  192 - 3  or one or more aspects of the shelving unit  195 ) appears within imaging data captured using a single imaging device, positions of the object and predicted distances (or depths or ranges) to the object, e.g., as determined based on outputs received from one or more machine learning systems, may be utilized without triangulation. 
     Therefore, in accordance with the present disclosure, the server  182  may be configured to generate or update the model  185  of a scene to include positions of one or more actors or other objects on the scene, e.g., one or more of the actors  192 - 1 ,  192 - 2 ,  192 - 3  or the shelving unit  195 , based on a plurality of depth images  150 - 1 ,  150 - 2 ,  150 - 3  where proportions of the depth images  150 - 1 ,  150 - 2 ,  150 - 3  that are supersaturated or otherwise degraded are sufficiently low, e.g., where ratios of degraded pixels to total pixels are below a predetermined threshold. Alternatively, however, where ratios of degraded pixels to total pixels are above the predetermined threshold, such as is shown in  FIG.  1 I , the location map  185  may be generated based on positions in three-dimensional space of the body parts detected within the visual images  152 - 1 ,  152 - 2 ,  152 - 3  and predicted distances to such body parts as determined from the visual images  152 - 1 ,  152 - 2 ,  152 - 3  may be used to generate or update the location map  185 , e.g., to include the actors  192 - 1 ,  192 - 2 ,  192 - 3  and/or the storage unit  195  therein. 
     Additionally, in some implementations, where levels of excessive supersaturation or other forms of degradation are recognized within coverage areas defined in imaging data captured by one or more imaging devices including time-of-flight cameras, numbers of coverage areas of imaging devices may be determined, and a detection of an actor that has been “lost” due to supersaturation or other degradation may be maintained for a predetermined period of time or for a predetermined number of frames in order to accommodate for the effects of the supersaturation or degradation, which may be temporary in nature. 
     Accordingly, the systems and methods of the present disclosure may be used to generate or update models of scenes, or to detect or locate actors that are present on such scenes, even where one or more retroreflective objects are located within fields of view of time-of-flight cameras, or where depth imaging data captured by such cameras is degraded for any other reason. Retroreflective material may be detected within raw sensor data obtained by a time-of-flight camera that illuminates a scene in a plurality of phases and captures light that is reflected from the scene, where a predetermined number of photoreceptors or other pixel sensors are exceedingly saturated (e.g., supersaturated) by the reflected light. Where retroreflective material covering an object is present within a field of view of an imaging device that includes a time-of-flight sensor, the presence of the retroreflective material within the field of view creates additional pixels of noise or other adverse effects around the object while also obscuring depth pixels corresponding to the object or causing such depth pixels to disappear or otherwise be degraded. The systems and methods of the present disclosure may be used to generate or update models of scenes, or detect and locate actors or other objects that are present on such scenes, despite such adverse effects, by processing visual images captured simultaneously or substantially simultaneously by such imaging devices to detect one or more objects (e.g., body parts of an actor) and to predict distances (or depths or ranges to such objects) accordingly, and updating a map of locations of objects based on the visual images, rather than the depth images that are plagued by saturation or other forms of degradation. 
     A time-of-flight camera is an imaging device having an illuminator and a sensor that determines distances to each of a plurality of pixels corresponding to a portion of a scene within a field of view by illuminating the scene using the illuminator, e.g., a modulated light source, and capturing light reflected from the scene by the sensor. The illuminator typically emits light (e.g., infrared or near-infrared light having a wavelength between eight hundred and nine hundred nanometers, or 800-900 nm) by way of a laser, a light-emitting diode (or “LED”), or another light source. The sensor may be an array of photodetectors, photosensitive components or other pixel sensors such as a charge coupled device (“CCD”), a complementary metal-oxide sensor (“CMOS”), photodiodes, or the like. The sensor may capture light reflected from objects on the scene, and assign one or more quantitative values (e.g., distances or ranges to such objects) to aspects of the reflected light. In some implementations, a time-of-flight camera may be a discrete, standalone unit. In some other implementations, a time-of-flight camera may be integrally or functionally joined with a visual camera, e.g., a color camera, a grayscale camera or a black-and-white camera. 
     In some implementations, a time-of-flight camera operates by illuminating a scene and determining a time required for light to travel from the illuminator to an individual photodetector of the sensor. A distance from the time-of-flight camera to each of the respective portions of a scene corresponding to one of the photodetectors may be determined as a function of the speed of light, or approximately 300 million meters per second (or 3.0×10 8  m/s). Because a time-of-flight camera may illuminate an entire field of view simultaneously, a time-of-flight camera may be used to generate depth images or profiles of the field of view quickly and efficiently from each illumination. 
     A time-of-flight camera may operate to illuminate a scene, or to capture light reflected from the scene, in any number of ways. For example, in some implementations, an illuminator of a time-of-flight camera may operate using one or more pulsed light sources. Pulsed modulation may be achieved by integrating photoelectrons from reflected light, or by counting light contact at a first detection of a reflection, e.g., by a single photon avalanche diode, or another type of sensor having any number of photoreceptors (or other pixel sensors) thereon. Electrical charges may be accumulated and measured by each of the photoreceptors, for each of the pulses, during windows that are in-phase and/or out-of-phase with the illumination. The measured electrical charges may be used to calculate distances to portions of the scene appearing within the field of view that correspond to such photoreceptors accordingly. Where the electrical charge accumulated by a photoreceptor exceeds a predetermined threshold, the photoreceptor is determined to be saturated, and a corresponding pixel (e.g., a voxel) of a depth image generated based on the electrical charge will be distorted or corrupted. A number of the photoreceptors that are determined to be saturated to a predetermined extent may be counted based on the imaging data, e.g., a number of pixels of the depth image that are distorted or corrupted. 
     In some implementations, an illuminator of a time-of-flight camera may operate using a continuous wave light source which captures a number of samples at different phases with respect to the illumination. For example, a continuous wave time-of-flight camera may capture four samples, e.g., at four phases, each phase-stepped by ninety degrees (viz., zero, ninety, one hundred eighty or two hundred seventy degrees). In some implementations, a phase-shift of a signal, a signal strength and/or an offset (or a gray-scale value) may be calculated for each of the photoreceptors of a sensor, or pixels of a depth image, as a function of the electrical charge accumulated in each of the samples. A distance associated with each of the photoreceptors of the sensor, or pixels of the depth image (e.g., voxels) may be calculated as a function of the speed of light, the phase-shift and a modulation frequency, which may be selected based on the distances or ranges to be measured, a desired level of accuracy, or any other basis. 
     In some implementations, raw values associated with each of the photoreceptors of a sensor of the time-of-flight camera may be determined, and whether such photoreceptors are saturated may be determined by comparing values of pixels (e.g., voxels) of a depth image generated thereby to a selected threshold. For example, in some implementations, where photoreceptors of a time-of-flight camera are twelve-bit sensors, a given photoreceptor may be determined to be saturated where a raw value exceeds 3500, or another threshold, between clipping values of 0 to 4095, for any of the phases at which the time-of-flight camera captures samples. In some implementations, the photoreceptor may be determined to be supersaturated where the raw value exceeds a higher threshold, e.g., 4000, or equals a clipping value (e.g., 0 or 4095), for each of the phases. A number of the photoreceptors that are determined to be saturated to a predetermined extent may be counted. Values may be expressed in an absolute sense, e.g., within a range of 0 to 4095 for a twelve-bit sensor, or as ratios or fractions of clipping values, e.g., within a range of 0 to 1. 
     Continuous waves may be illuminated at different modulation frequencies, thereby increasing a level of accuracy of depth images captured by the time-of-flight camera, or distances embodied in such depth images, accordingly. In some implementations, a continuous wave time-of-flight camera may operate at a pair of modulation frequencies, and may capture four sets of sensor data at each of the modulation frequencies, for a total of eight sets of sensor data. Each of the sets of sensor data may be evaluated to determine a number of the photoreceptors that is saturated to a predetermined extent. 
     Reflected light may be captured or detected by an imaging device (e.g., a time-of-flight camera) if the reflected light is within the imaging device&#39;s field of view, which is defined as a function of a distance between a sensor and a lens within the imaging device, viz., a focal length, as well as a location of the imaging device and an angular orientation of the imaging device&#39;s lens. Accordingly, where an object appears within a depth of field, or a distance within the field of view where the clarity and focus is sufficiently sharp, an imaging device may capture light that is reflected off objects of any kind to a sufficiently high degree of resolution using one or more sensors thereof, and store information regarding the reflected light in one or more data files. 
     Many imaging devices also include manual or automatic features for modifying their respective fields of view or orientations. For example, an imaging device may be configured in a fixed position, or with a fixed focal length (e.g., fixed-focus lenses) or angular orientation. Alternatively, an imaging device may include one or more actuated or motorized features for adjusting a position of the imaging device, or for adjusting either the focal length (e.g., zooming the imaging device) or the angular orientation (e.g., the roll angle, the pitch angle or the yaw angle) of the imaging device, by causing a change in a distance between the sensor and the lens (e.g., optical zoom lenses or digital zoom lenses), a change in a location of the imaging device, or a change in one or more of the angles defining an angular orientation. 
     For example, an imaging device may be hard-mounted to a support or mounting that maintains the imaging device in a fixed configuration or angle with respect to one, two or three axes. Alternatively, however, an imaging device may be provided with one or more motors and/or controllers for manually or automatically operating one or more of the components, or for reorienting the axis or direction of the imaging device, i.e., by panning or tilting the imaging device. Panning an imaging device may cause a rotation within a horizontal plane or about a vertical axis (e.g., a yaw), while tilting a imaging device may cause a rotation within a vertical plane or about a horizontal axis (e.g., a pitch). Additionally, an imaging device may be rolled, or rotated about its axis of rotation, and within a plane that is perpendicular to the axis of rotation and substantially parallel to a field of view of the imaging device. 
     The systems and methods of the present disclosure may be utilized in any number of applications in which depth imaging operations are desired, including but not limited to applications associated with operations occurring within a materials handling facility. As used herein, the term “materials handling facility” may include, but is not limited to, warehouses, distribution centers, cross-docking facilities, order fulfillment facilities, packaging facilities, shipping facilities, rental facilities, libraries, retail stores or establishments, wholesale stores, museums, or other facilities or combinations of facilities for performing one or more functions of material or inventory handling for any purpose. For example, in some implementations, one or more of the systems and methods disclosed herein may be used to detect and distinguish between actors (e.g., customers) and recognize their respective interactions within a materials handling facility, where one or more actors is wearing retroreflective materials within the materials handling facility, or where one or more retroreflective objects are expected to appear within a field of view of a time-of-flight camera at the materials handling facility. Such systems and methods may also be utilized to detect and locate actors and their interactions within transportation centers, financial institutions or like structures in which diverse collections of people, objects or machines enter and exit from such environments at regular or irregular times or on predictable or unpredictable schedules, and where one or more of the people, objects or machines is wearing or covered at least in part by retroreflective materials, or where external surfaces of one or more of the people, the objects or the machines embodies retroreflective properties. 
     Retroflective materials may cause photoreceptors of a time-of-flight camera sensor corresponding to such materials, and photoreceptors corresponding to nearby portions, to be saturated when sensor data is captured at selected illumination energies and for selected exposure times. Sensor data determined from photoreceptors of the time-of-flight camera sensor that are not saturated, however, may remain unaffected, and may therefore be used to construct one or more depth images therefrom. 
     A time-of-flight camera may be configured to illuminate a scene, and to expose a sensor, at any energies or power levels or for any selected exposure times, respectively, in accordance with the present disclosure. For example, in some implementations, one energy level (or power level) at which a time-of-flight camera may illuminate a scene may be approximately five to ten watts (5-10 W), and one exposure time may be approximately five hundred to two thousand microseconds (500-2000 μs). 
     Referring to  FIGS.  2 A and  2 B , a block diagram of one system  200  for detecting or locating actors in accordance with implementations of the present disclosure is shown. The system  200  of  FIGS.  2 A and  2 B  includes a plurality of imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  and a data processing system  270  that are connected to one another over a network  290 , which may include the Internet, in whole or in part. Except where otherwise noted, reference numerals preceded by the number “2” shown in  FIGS.  2 A and  2 B  indicate components or features that are similar to components or features having reference numerals preceded by the number “1” shown in  FIGS.  1 A through  1 I . 
     The imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may be any form of optical recording devices that may be used to photograph or otherwise record imaging data of structures, facilities, terrain or any other elements appearing within their respective fields of view. The imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may include one or more sensors, memory or storage components and processors, and such sensors, memory components or processors may further include one or more photosensitive surfaces, filters, chips, electrodes, clocks, boards, timers, power sources, connectors or any other relevant features (not shown). Such imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may capture imaging data in the form of one or more still or moving images of any kind or form, as well as any relevant audio signals or other information. The imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may also be adapted or otherwise configured to communicate with the data processing system  270  by way of the network  290 . 
     Each of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  shown in  FIG.  2 A  may include one or more sensors, memory or storage components and processors, and such sensors, memory components or processors may further include one or more photosensitive surfaces, filters, chips, electrodes, clocks, boards, timers or any other relevant features (not shown). As is shown in  FIG.  2 B , a representative imaging device  210 - i  includes one or more optical sensors, including one or more depth sensors  220 - i , one or more illuminators  222 - i  and one or more RGB sensors  224 - i  (e.g., color sensors, or grayscale sensors, black-and-white sensors, or other visual sensors) that are configured to capture visual imaging data (e.g., textures) or depth imaging data (e.g., ranges) to objects within one or more fields of view of the imaging device  210 - i.    
     The depth sensors  220 - i  and the RGB sensors  224 - i  may be any sensors having single elements or a plurality of photoreceptors or photosensitive components (e.g., a CCD sensor, a CMOS sensor, or another sensor), which may be typically arranged in an array. Light reflected from objects within a field of view of the imaging device  210 - i  may be captured by the depth sensors  220 - i  and the RGB sensors  224 - i , and quantitative values, e.g., pixels, may be assigned to one or more aspects of the reflected light. The illuminators  222 - i  may be any system such as a laser system or a light-emitting diode (or “LED”) for illuminating a portion of a scene appearing within a field of view of the imaging device  210 - i , e.g., by infrared or near-infrared light, such as light with wavelengths ranging from approximately seven hundred to approximately one thousand nanometers (700-1000 nm), or light having any other wavelengths. 
     The imaging device  210 - i  further includes one or more processors  212 - i  and one or more memory components  214 - i , and any other components (not shown) that may be required in order to capture, analyze and/or store imaging data. For example, the imaging device  210 - i  may capture one or more still or moving images (e.g., streams of visual and/or depth image frames), along with any relevant audio signals or other information (e.g., position data), and may also connect to or otherwise communicate with the server  272 , or any other computer devices over the network  290 , through the sending and receiving of digital data. In some implementations, the imaging device  210 - i  may be configured to communicate through one or more wired or wireless means, e.g., wired technologies such as Universal Serial Bus (or “USB”) or fiber optic cable, or standard wireless protocols such as Bluetooth® or any Wireless Fidelity (or “Wi-Fi”) protocol, either by way of the servers  272  or over the network  290  directly. The processors  212 - i  may be configured to process imaging data captured by one or more of the depth sensors  220 - i  or the RGB sensors  224 - i . For example, in some implementations, the processors  212 - i  may be configured to execute any type or form of machine learning tools or technique, e.g., an artificial neural network. 
     The imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may be homogenous or heterogeneous in nature, in accordance with the present disclosure. For example, one or more of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may have both the depth sensor  220 - i  and the RGB sensor  224 - i . Alternatively, one or more of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may have just a depth sensor  220 - i  or just a RGB sensor  224 - i  (or grayscale sensor or black-and-white sensor). For example, one or more of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may be an RGB color camera, a still camera, a motion capture/video camera or any other type or form of camera. In other implementations, one or more of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may be depth-sensing cameras, such as an RGBD or RGBz camera. In still other implementations, one or more of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may be a thermographic or infrared (IR) camera. Additionally, in some implementations, the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may simply be camera modules that include a lens and an image sensor configured to convert an optical image obtained using the lens of the camera into a digital signal or digital representation of the image (generally referred to herein as imaging data). 
     The imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may also include manual or automatic features for modifying their respective fields of view or orientations. For example, one or more of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may be configured in a fixed position, or with a fixed focal length (e.g., fixed-focus lenses) or angular orientation. Alternatively, one or more of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may include one or more motorized features for adjusting a position of the imaging device, or for adjusting either the focal length (e.g., zooming the imaging device) or the angular orientation (e.g., the roll angle, the pitch angle or the yaw angle), by causing changes in the distance between the sensor and the lens (e.g., optical zoom lenses or digital zoom lenses), changes in the location of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n , or changes in one or more of the angles defining the angular orientation. 
     For example, one or more of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may be hard-mounted to a support or mounting that maintains the device in a fixed configuration or angle with respect to one, two or three axes. Alternatively, however, one or more of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may be provided with one or more motors and/or controllers for manually or automatically operating one or more of the components, or for reorienting the axis or direction of the device, i.e., by panning or tilting the device. Panning an imaging device may cause a rotation within a horizontal axis or about a vertical axis (e.g., a yaw), while tilting an imaging device may cause a rotation within a vertical plane or about a horizontal axis (e.g., a pitch). Additionally, an imaging device may be rolled, or rotated about its axis of rotation, and within a plane that is perpendicular to the axis of rotation and substantially parallel to a field of view of the device. 
     Some of the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may digitally or electronically adjust an image captured from a field of view, subject to one or more physical and operational constraints. For example, a digital camera may virtually stretch or condense the pixels of an image in order to focus or broaden a field of view of the digital camera, and also translate one or more portions of images within the field of view. Imaging devices having optically adjustable focal lengths or axes of orientation are commonly referred to as pan-tilt-zoom (or “PTZ”) imaging devices, while imaging devices having digitally or electronically adjustable zooming or translating features are commonly referred to as electronic PTZ (or “ePTZ”) imaging devices. 
     Once the characteristics of stationary or moving objects or portions thereof have been recognized in one or more digital images, such characteristics of the objects or portions thereof may be matched against information regarding contours, outlines, colors, textures, silhouettes, shapes or other characteristics of known objects, which may be stored in one or more data stores. In this regard, stationary or moving objects may be classified based at least in part on the extent to which the characteristics identified in one or more digital images correspond to one or more of the characteristics of the known objects. 
     Although the system  200  of  FIG.  2 A  includes boxes corresponding to three imaging devices  210 - 1 ,  210 - 2  . . .  210 - n , those of ordinary skill in the pertinent arts will recognize that any number or type of imaging devices may be operated in accordance with the present disclosure, including but not limited to digital cameras, depth sensors or range cameras, infrared cameras, radiographic cameras or other devices having one or more optical sensors. In some implementations, the system  200  may include dozens or even hundreds of imaging devices of any type or form. 
     The data processing system  270  includes one or more physical computer servers  272  having one or more computer processors  274  and any number of data stores  276  (e.g., databases) associated therewith, as well as provided for any specific or general purpose. For example, the data processing system  270  of  FIG.  2 A  may be independently provided for the exclusive purpose of receiving, analyzing or storing imaging data or other information or data received from the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  or, alternatively, provided in connection with one or more physical or virtual services that are configured to receive, analyze or store such imaging data or other information or data, as well as one or more other functions. In some implementations, the data processing system  270  may be associated with a materials handling facility, or any other physical or virtual facility. 
     The servers  272  may be connected to or otherwise communicate with the processors  274  and the data stores  276 , which may store any type of information or data, including but not limited to acoustic signals, information or data relating to imaging data, or information or data regarding environmental conditions, operational characteristics, or positions, for any purpose. The servers  272 , the computer processors  274  and/or the data stores  276  may also connect to or otherwise communicate with the network  290 , as indicated by line  278 , through the sending and receiving of digital data. For example, the data processing system  270  may include any facilities, stations or locations having the ability or capacity to receive and store information or data, such as media files, in one or more data stores, e.g., media files received from the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n , or from one or more other external computer systems (not shown) via the network  290 . In some implementations, the data processing system  270  may be provided in a physical location. In other such implementations, the data processing system  270  may be provided in one or more alternate or virtual locations, e.g., in a “cloud”-based environment. In still other implementations, the data processing system  270  may be provided onboard one or more vehicles, e.g., an unmanned aerial vehicle. 
     The network  290  may be any wired network, wireless network, or combination thereof, and may comprise the Internet in whole or in part. In addition, the network  290  may be a personal area network, local area network, wide area network, cable network, satellite network, cellular telephone network, or combination thereof. The network  290  may also be a publicly accessible network of linked networks, possibly operated by various distinct parties, such as the Internet. In some implementations, the network  290  may be a private or semi-private network, such as a corporate or university intranet. The network  290  may include one or more wireless networks, such as a Global System for Mobile Communications (GSM) network, a Code Division Multiple Access (CDMA) network, a Long-Term Evolution (LTE) network, or some other type of wireless network. Protocols and components for communicating via the Internet or any of the other aforementioned types of communication networks are well known to those skilled in the art of computer communications and thus, need not be described in more detail herein. 
     The computers, servers, devices and the like described herein have the necessary electronics, software, memory, storage, databases, firmware, logic/state machines, microprocessors, communication links, displays or other visual or audio user interfaces, printing devices, and any other input/output interfaces to provide any of the functions or services described herein and/or achieve the results described herein. Also, those of ordinary skill in the pertinent art will recognize that users of such computers, servers, devices and the like may operate a keyboard, keypad, mouse, stylus, touch screen, or other device (not shown) or method to interact with the computers, servers, devices and the like, or to “select” an item, link, node, hub or any other aspect of the present disclosure. 
     The imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  and/or the data processing system  270  may use any web-enabled or Internet applications or features, or any other client-server applications or features including E-mail or other messaging techniques, to connect to the network  290 , or to communicate with one another, such as through short or multimedia messaging service (SMS or MMS) text messages. For example, the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  may be adapted to transmit information or data in the form of synchronous or asynchronous messages to the data processing system  270  or to any other computer device in real time or in near-real time, or in one or more offline processes, via the network  290 . Those of ordinary skill in the pertinent art would recognize that the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  or the data processing system  270  may operate or be operated by any of a number of computing devices that are capable of communicating over the network, including but not limited to set-top boxes, personal digital assistants, digital media players, web pads, laptop computers, desktop computers, electronic book readers, and the like. The protocols and components for providing communication between such devices are well known to those skilled in the art of computer communications and need not be described in more detail herein. 
     The data and/or computer executable instructions, programs, firmware, software and the like (also referred to herein as “computer executable” components) described herein may be stored on a computer-readable medium that is within or accessible by computers or computer components such as the processors  212 - i  or the processor  274 , or any other computers or control systems utilized by the imaging devices  210 - 1 ,  210 - 2  . . .  210 - n  or the data processing system  270 , and having sequences of instructions which, when executed by a processor (e.g., a central processing unit, or “CPU”), cause the processor to perform all or a portion of the functions, services and/or methods described herein. Such computer executable instructions, programs, software, and the like may be loaded into the memory of one or more computers using a drive mechanism associated with the computer readable medium, such as a floppy drive, CD-ROM drive, DVD-ROM drive, network interface, or the like, or via external connections. 
     Some implementations of the systems and methods of the present disclosure may also be provided as a computer-executable program product including a non-transitory machine-readable storage medium having stored thereon instructions (in compressed or uncompressed form) that may be used to program a computer (or other electronic device) to perform processes or methods described herein. The machine-readable storage media of the present disclosure may include, but is not limited to, hard drives, floppy diskettes, optical disks, CD-ROMs, DVDs, ROMs, RAMs, erasable programmable ROMs (“EPROM”), electrically erasable programmable ROMs (“EEPROM”), flash memory, magnetic or optical cards, solid-state memory devices, or other types of media/machine-readable medium that may be suitable for storing electronic instructions. Further, implementations may also be provided as a computer executable program product that includes a transitory machine-readable signal (in compressed or uncompressed form). Examples of machine-readable signals, whether modulated using a carrier or not, may include, but are not limited to, signals that a computer system or machine hosting or running a computer program can be configured to access, or including signals that may be downloaded through the Internet or other networks. 
     In some implementations, the systems and methods of the present disclosure may be configured to generate a background model of a scene, or update the background model, based on depth imaging data that is not saturated or otherwise degraded. Where the depth imaging data is saturated or otherwise degraded, however, the background model may not be updated based on the depth imaging data. Referring to  FIG.  3   , a flow chart  300  of one process for detecting or locating actors in accordance with implementations of the present disclosure is shown. At box  310 , a value of a step variable i is set to one, or i=1. At box  315 , a depth image i is captured using a depth camera. The depth camera may be one of a plurality of depth cameras aligned with fields of view that are aligned with vertically downward or substantially downward orientations, toward floors or traveling surfaces on a scene, and overlap with one another. In some implementations, the depth camera may be a part of a camera having both depth sensors and visual sensors, e.g., an RGBD camera, and the depth image i may be captured along with one or more other visual images, such as black-and-white, grayscale or color visual images. 
     At box  320 , a background model of the scene is generated by one or more processors associated with the depth camera based at least in part on the depth image i. For example, the background model may be generated based on the depth image i captured by a single depth camera exclusively, or based on a plurality of depth images, including the depth image i, that were captured by a plurality of depth cameras. Depth data included within such depth images may be fused with one another to define the background model. 
     At box  325 , the value of the step variable i is incremented by one, or i=i+1. At box  330 , a depth image i is captured using the depth camera, and at box  335 , a number of degraded pixels within one or more designated regions of the depth image i is determined. In some implementations, a number of degraded pixels within regions of the depth image i may correspond to a subset of the photoreceptors of the sensor that are supersaturated by the reflected light. For example, the degraded pixels may be supersaturated pixels that are identified as degraded by comparing raw values of such pixels to one or more thresholds. In some implementations, a pixel may be identified as degraded where a raw value for the pixel equals a clipping value for a photoreceptor, e.g., 0 or 4,095 for a twelve-bit sensor, for one or more sampled phases with respect to an illumination. For example, a continuous wave time-of-flight camera may capture four samples, e.g., at four phases, each phase-stepped by ninety degrees (viz., zero, ninety, one hundred eighty or two hundred seventy degrees). Alternatively, a pixel may be identified as degraded where a raw value for the pixel meets or exceeds a predetermined threshold identified with respect to the clipping value for a corresponding photoreceptor, in one or more sampled phases. Alternatively, pixels of the depth image i may be identified as degraded on any other basis. For example, a pixel may be identified as degraded where a photoreceptor has failed, where circuitry or logic for determining values of such pixels are not functioning properly, or on any other basis. Moreover, such pixels may be identified for an image as a whole, e.g., where the designated region of the depth image i consists of the image in its entirety, or for one or more subsets of the depth image i, e.g., portions of the depth image i corresponding to floors or travelling surfaces. 
     At box  340 , a ratio of the number of degraded pixels to a total number of pixels within the designated region. At box  350 , whether the ratio exceeds a predetermined threshold is determined. The predetermined threshold may be set on any basis, such as one-tenth of one percent, one-half of one percent, or one percent (0.1%, 0.5%, 1.0%), or any other value. 
     If the ratio exceeds the predetermined threshold, then the process advances to box  355 , where the background model is not updated based on the depth image i. For example, where the depth image i is saturated or otherwise degraded beyond a predetermined threshold, updating a background model of a scene based on the depth image i implies that the composition of the background features on the scene have changed, e.g., at locations in three-dimensional space corresponding to the saturated or otherwise degraded pixels within the depth image i, when, in reality, the background features on the scene have not changed. 
     If the ratio does not exceed the predetermined threshold, however, then the process advances to box  360 , where the background model is updated based on the depth image i. In some implementations, where a background model of a scene is generated or updated by one or more processors associated with an imaging device, the background model or other representation may be provided to a server or other computer device or system in communication with the imaging device and a plurality of other imaging devices. At box  370 , whether the continued monitoring of the scene by depth images is desired is determined. If the continued monitoring of the scene is desired, then the process returns to box  325 , where the value of the step variable i is again incremented by one, or i=i+1, and to box  330 , where another depth image i is captured using the depth camera. If the continued monitoring of the scene is no longer desired, however, then the process ends. 
     Referring to  FIGS.  4 A through  4 G , views of aspects of one system for detecting or locating actors in accordance with implementations of the present disclosure are shown. Except where otherwise noted, reference numerals preceded by the number “4” shown in  FIGS.  4 A through  4 G  indicate components or features that are similar to components or features having reference numerals preceded by the number “2” shown in  FIG.  2 A or  2 B , or by the number “1” shown in  FIGS.  1 A through  1 I . 
     As is shown in  FIGS.  4 A and  4 B , a system  400  includes a depth camera  410  (e.g., a time-of-flight camera) mounted above a shelving unit  495 , which includes a plurality of sets of shelves having items thereon. The depth camera  410  is configured to capture imaging data (e.g., depth images) from a field of view  430  that includes the shelving unit  495  as well as one or more floors or traveling surfaces within a scene. For example, as is shown in  FIGS.  4 A and  4 B , the depth camera  410  is mounted above the shelving unit  495  and configured to capture depth images regarding motion of one or more actors on the scene. In some implementations, the depth camera  410  may be an RGBD camera or a component thereof that is configured to capture both visual images and depth images from the scene. 
     As is shown in  FIG.  4 C , a background model  485  of the scene may be generated based on depth images captured using the depth camera  410 . The background model  485  may represent positions of a plurality of points in three-dimensional space on the scene, as determined based on the depth images. Alternatively, in some implementations, the background model  485  of the scene may be generated based on depth images captured using the depth camera  410  and any number of other depth cameras (not shown). In such implementations, depth data captured by each of such depth cameras, including the depth camera  410 , may be fused to generate the background model  485  accordingly. 
     As is shown in  FIGS.  4 D and  4 E , the depth camera  410  may capture one or more depth images, viz., a depth image  450 - 1 , as an actor  492 - 1  appears on the scene and executes one or more interactions with the storage unit  495 , or executes one or more other poses or gestures. As is shown in  FIG.  4 D , the actor  492 - 1  is not wearing, carrying or otherwise associated with (e.g., near) any retroreflective materials. As is shown in  FIG.  4 E , the depth image  450 - 1  includes a region  455 - 1  of pixels corresponding to the actor  492 - 1  and a region  455 - 2  of pixels corresponding to the storage unit  495 . Because the actor  492 - 1  is not wearing, carrying or otherwise associated with any retroreflective materials, the region  455 - 1  includes varying values of distances corresponding to aspects of the actor  492 - 1 , including a head, shoulders, or other body parts of the actor  492 - 1 . 
     The depth image  450 - 1  may be processed to calculate a ratio of saturated or otherwise degraded pixels within the depth image  450 - 1  in order to determine whether the background model  485  should be updated based on the data included in the depth image  450 - 1 . For example, as is shown in  FIG.  4 E , because the actor  492 - 1  is not wearing, carrying or otherwise associated with any retroreflective material, a number of supersaturated or otherwise degraded pixels N D  on the scene is approximately zero. Thus, a ratio of the number of degraded pixels N D  to a total number of pixels N 1  in the depth image  450 - 1 , or a number of pixels N 2  in a working area (e.g., an area where actors such as the actor  492 - 1  may congregate or execute one or more interactions with the shelving unit  495  or other poses or gestures) is likewise zero. Where the ratios N D /N 1  and/or N D /N 2  are zero or otherwise below a selected threshold, depth values of the depth image  450 - 1  may thus be used to update the background model  485 . 
     As is shown in  FIGS.  4 F and  4 G , the depth camera  410  may also capture one or more depth images, viz., a depth image  450 - 2 , as an actor  492 - 2  appears on the scene and executes one or more interactions with the storage unit  495 . As is shown in  FIG.  4 F , the actor  492 - 2  is wearing a retroreflective hat  494 . As is shown in  FIG.  4 G , the depth image  450 - 2  includes the region  455 - 2  corresponding to the storage unit  495  and a region  455 - 3  of supersaturated pixels corresponding to the retroreflective hat  494  worn by the actor  492 - 2 . 
     The depth image  450 - 2  may be processed to calculate a ratio of saturated or otherwise degraded pixels within the depth image  450 - 2  in order to determine whether the background model  485  may be updated based on the data included in the depth image  450 - 2 . For example, as is shown in  FIG.  4 G , because the actor  492 - 2  is wearing the retroreflective hat  494 , a substantial, non-zero number of supersaturated or otherwise degraded pixels N D  on the scene is present within the depth image  450 - 2 . Thus, ratios of the number of degraded pixels N D  to the total number of pixels N 1  in the depth image  450 - 2 , or to the number of pixels N 2  in the working area (e.g., an area where actors such as the actor  492 - 2  may congregate or execute one or more interactions with the shelving unit  495 ) are also substantial and not zero. Therefore, due to the presence and number of the supersaturated pixels within the depth image  450 - 2 , viz., the region  455 - 3  of supersaturated pixels, the values of the depth image  450 - 2  may not be used to update the background model  485  where the ratios N D /N 1  and/or N D /N 2  exceed the predetermined threshold. 
     Referring to  FIGS.  5 A and  5 B , a flow chart of one process for detecting or locating actors in accordance with implementations of the present disclosure is shown. At box  510 , a plurality of depth images and visual images of a scene are captured using cameras having fields of view that overlap on a scene. For example, in some implementations, the cameras may have both depth sensors and visual sensors, viz., RGBD cameras, and may be configured to capture both depth images and visual images. In some other implementations, however, one or more of the cameras may have only depth sensors, or only visual sensors, and may be configured to capture either depth images or visual images, but not both. 
     At box  520 , a number of degraded pixels within one or more of the depth images is determined. For example, a number of depth pixels that are identified as supersaturated, or to otherwise have raw values above a saturation threshold, or other threshold, may be determined for each of the depth images. For example, in some implementations, for each of the one or more depth images, a number of sensor values that are equal to a clipping value in one or more of a plurality of phases may be counted. Alternatively, depth pixels that are determined to be degraded on any basis may be identified and counted. At box  530 , a ratio of the number of degraded pixels determined at box  520  for each of the depth images to a total number of pixels within each of the depth images is calculated. In some implementations, the ratio may be calculated with respect to a total number of the pixels within the depth images. In some other implementations, the ratio may be calculated with respect to a number of the pixels within designated regions of the depth images, e.g., on floors or traveling surfaces. 
     At box  540 , whether the ratio for a given depth image is in excess of a predetermined threshold is determined. If the ratio is not in excess of the predetermined threshold, then the process advances to box  550 , where the depth images are provided to a machine learning system that is trained to identify one or more body parts within the depth imaging data. For example, the machine learning system may be a trained artificial neural network or any other machine learning tool or technique. The inputs to the machine learning system may include a single depth image captured using one of the cameras, e.g., where the machine learning system is operated by one or more processors of a camera that captured the depth image, or, alternatively, one or more depth images captured by one or more cameras, or another set of data generated based on one or more of the depth images. 
     At box  552 , one or more body parts are identified within the depth images based on outputs received from the trained machine learning system. The body parts may be identified as points in absolute three-dimensional space, e.g., by sets of three coordinates, or with respect to a position of an image sensor, or on any other basis. For example, the body parts may be identified as points within image planes of the depth images, e.g., by sets of two coordinates, or with respect to one or more angles and/or radii. Alternatively, the one or more body parts may be identified from the depth images directly, e.g., where a depth profile determined from the depth images indicates the presence of one or more body parts on the scene. 
     At box  554 , the body parts detected within depth images captured by all of the depth cameras are combined. For example, where a plurality of body parts are identified in two or more depth images captured using the various cameras, a record or set of information or data regarding the body parts, including but not limited to points in three-dimensional space of each of such body parts, may be generated and stored accordingly. 
     Alternatively, if a ratio for a given depth image exceeds the predetermined threshold, then the process advances to box advances to box  560 , where the visual images captured at box  510  are provided to a machine learning system that is trained to identify one or more body parts within visual imaging data as inputs and/or to predict distances (or depths or ranges) to such body parts based on the visual imaging data. For example, the machine learning system may be trained to detect one or more objects, such as body parts of humans or other animals, structures, or the like, as well as one or more aspects of a materials handling facility, such as shelving units, tables, bins, racks, bars, hooks or any other features, and to determine distances (or depths or ranges) to such objects. In some implementations, the machine learning system may be a convolutional neural network, a deep learning neural network, or other machine learning tool or technique that operates on one or more processors on the imaging devices that captured the visual images and includes one or more output layers for predicting whether one or more pixels in an image received as an input depicts a given body part or other object, and for determining a confidence score associated with such predictions. In some implementations, one or more processors or processor units operating on other computer devices or machines, e.g., a server in communication with one or more imaging devices, may be configured to operate the machine learning system to detect the body parts or other objects within the images captured by the imaging device, or to associate such body parts or other objects with locations within such images, as well as to determine one or more distances (or depths or ranges) to such body parts or other objects. In some implementations, such processors or processor units may be provided in the same physical location as the digital camera, or in close proximity to the digital camera. In other implementations, the processors or processor units may be provided in one or more computer devices or machines in other locations, e.g., alternate or virtual locations, such as in a “cloud”-based environment. At box  562 , one or more body parts depicted within the visual images are detected and distances (or depths or ranges) to such body parts are determined based on outputs received from the machine learning system. 
     At box  564 , a predicted location map or image is generated for each camera based on the body parts or other objects depicted within the visual images, and distances (or depths or ranges) to such objects by that camera. For example, in some implementations, the location map may be a two-dimensional representation of positions of one or more body parts of a human actor, or other objects, as circles, bounding boxes, or other shapes corresponding to uncertainties in their respective positions, e.g., with respect to coordinates of a field of view of a camera that captured the visual images. In some implementations, the location map may be a three-dimensional representation of the positions of the body parts or other objects, e.g., as the circles, boxes or other shapes, and with heights corresponding to the distances from the imaging devices to such body parts. In such implementations, the actors or other objects may be expressed within the location map as cylinders, prisms or other three-dimensional shapes with respect to their positions within the visual images and the predicted distances to the actors or the other objects. In some implementations, the location map may be a point cloud or other three-dimensional representation of the scene from the perspective of the camera, or globally, and may include coordinates of points of body parts detected by that camera, or a set of data including such coordinates. 
     At box  566 , the locations of body parts or other objects on the scene are further refined by triangulating the predicted location maps generated at box  564  across all of the cameras. For example, where the position maps generated at box  564  collectively indicate positions of detected body parts or other objects within close proximity of one another, triangulating the predicted location maps may resolve any discrepancies between the body parts as detected from each of the cameras, and may provide a more accurate account of the positions of the body parts based on visual images captured from a single imaging device alone. Alternatively, in some implementations, the predicted location maps need not be triangulated. 
     At box  570 , after the detections of body parts within depth images have been combined across all cameras at box  554 , or after the location maps have been generated or triangulated at box  564  or  566 , the process advances to box  570 , where a location map (or other model) of the scene is generated based on the visual images or the depth images. For example, where the ratios of saturation or other forms of degradation calculated at box  530  for the depth images captured at box  510  are sufficiently low, the location map (or model) may be generated based on the depth images exclusively or, alternatively, based on both the depth images and the visual images captured at box  510 . Where the ratios are sufficiently high, however, the location map (or model) may be generated or updated based on the visual images alone, and the depth images need not be considered. 
     After the location map is generated or updated at box  570 , the process advances to box  580 , where whether the continued monitoring of the scene based on imaging data is desired is determined. If the continued monitoring of the scene based on imaging data is desired, then the process returns to box  510 , where depth images and visual images of the scene are again captured by the cameras. If the continued monitoring of the scene based on imaging data is no longer desired, however, then the process ends. 
     In some implementations, each of a plurality of depth images and each of a plurality of visual images captured by an imaging device, e.g., an RGBD camera, at box  510  may be processed independently, such as according to box  550 , box  552  and box  554  for depth images and according to box  560 , box  562 , box  564  and box  566  for visual images. Whether a location map of a scene is to be generated based on one or more depth images captured at a given time, or on one or more visual images captured at the given time, or both the depth images and the visual images, may be determined based on the extent to which pixels of one or more of the depth images are saturated or otherwise degraded, as may be determined based on a ratio of degraded pixels to total pixels, or in any other manner. 
     Referring to  FIGS.  6 A through  6 E , views of aspects of one system for detecting or locating actors in accordance with implementations of the present disclosure are shown. Except where otherwise noted, reference numerals preceded by the number “6” shown in  FIGS.  6 A through  6 E  indicate components or features that are similar to components or features having reference numerals preceded by the number “4” shown in  FIGS.  4 A through  4 G , by the number “2” shown in  FIG.  2 A or  2 B , or by the number “1” shown in  FIGS.  1 A through  1 I . 
     As is shown in  FIG.  6 A , a system  600  includes three RGBD cameras  610 - 1 ,  610 - 2 ,  610 - 3  and a server  682  that are connected to one another over a network  690 , which may include the Internet in whole or in part. The first RGBD camera  610 - 1  has a first field of view  630 - 1 . The second RGBD camera  610 - 2  has a second field of view  630 - 2 . The third RGBD camera  610 - 3  has a third field of view  630 - 3 . Each of the first field of view  630 - 1 , the second field of view  630 - 2  and the third field of view  630 - 3  overlap with at least one other of the first field of view  630 - 1 , the second field of view  630 - 2  and the third field of view  630 - 3 , at least in part. 
     The RGBD cameras  610 - 1 ,  610 - 2 ,  610 - 3  may be programmed to capture depth images and visual images from their respective fields of view  630 - 1 ,  630 - 2 ,  630 - 3  as one or more actors enter thereon or travel therethrough. As is shown in  FIG.  6 B , an actor  692  wearing one or more retroreflective articles of clothing  694  passes within the fields of view  630 - 1 ,  630 - 2 ,  630 - 3 . As is shown in  FIG.  6 C , a plurality of depth images  650 - 1 ,  650 - 2 ,  650 - 3  captured using each of the RGBD cameras  610 - 1 ,  610 - 2 ,  610 - 3  are determined to have ratios of numbers of degraded (e.g., supersaturated) pixels N D  to total numbers of pixels N TOT , or N D /N TOT , in excess of one or more thresholds. In particular, each of the depth images  650 - 1 ,  650 - 2 ,  650 - 3  includes a region  655  of pixels that are degraded by reflections from the retroreflective hat  694  worn by the actor  692 . 
     Because the ratios N D /N TOT  for the depth images  650 - 1 ,  650 - 2 ,  650 - 3  are in excess of predetermined thresholds, the depth images  650 - 1 ,  650 - 2 ,  650 - 3  are of insufficient quality to generate or update a location map (or model) of the scene to determine locations of the actor  692  or any other actors or objects thereon. Therefore, a location map (or model) of the scene may be generated based on positions of actors or objects, viz., the actor  692 , determined based on visual images captured from the scene. As is shown in  FIG.  6 D , a plurality of visual images  652 - 1 ,  652 - 2 ,  652 - 3  captured by the RGBD cameras  610 - 1 ,  610 - 2 ,  610 - 3  each include body parts of the actor  692  at points P and predicted distances (or depths or ranges) to the actor  692  within the visual images  652 - 1 ,  652 - 2 ,  652 - 3 . The body parts of the actor  692  at the points P may be identified by providing the visual images  652 - 1 ,  652 - 2 ,  652 - 3  to a trained machine learning system, e.g., a convolutional neural network, a deep learning neural network, or other machine learning tool or technique operating on the imaging devices  610 - 1 ,  610 - 2 ,  610 - 3  or, alternatively, by one or more external computer device or machines, as inputs, and by interpreting outputs received from the trained machine learning system. Alternatively, the body parts of the actor  692  at the points P may be determined in any other manner. 
     As is shown in  FIG.  6 E , a position of the point P in three-dimensional space may be further refined where the RGBD cameras  610 - 1 ,  610 - 2 ,  610 - 3  are calibrated, e.g., by extending rays from visual sensors of the respective RGBD cameras  610 - 1 ,  610 - 2 ,  610 - 3  through the points P within the image planes of the visual images  652 - 1 ,  652 - 2 ,  652 - 3  at angles θ 1 ,  02 ,  03 , and determining where such rays intersect, or by selecting one or more points based on such rays. The processes shown in  FIG.  6 E  may be repeated for any body parts of any actors within the visual images captured using the RGBD cameras  610 - 1 ,  610 - 2 ,  610 - 3 , or for any number of points corresponding to the actor  692 , as detected within the respective visual images  652 - 1 ,  652 - 2 ,  652 - 3 . Thus, as is shown in  FIG.  6 E , positions of the actor  692  may be determined based on depth images that are not degraded or, where depth images are degraded beyond a predetermined extent, based on visual images, including based on one or more body parts of the actor  692 , or one or more predicted distances (or depths or ranges) to the actor  692 , as determined based on the visual images  652 - 1 ,  652 - 2 ,  652 - 3 . 
     Referring to  FIGS.  7 A and  7 B , a flow chart of one process for detecting or locating actors in accordance with implementations of the present disclosure is shown. At box  710 , fields of view of depth cameras that are aligned to capture depth images of a scene are determined. For example, attributes of the fields of view may be determined based on a position of a depth sensor of a depth camera, as well as an angle of orientation of a lens of the depth camera, which may be used to determine points in space on the scene that are within the fields of view or are not within the fields of view. At box  715 , a plane at a level above a ground surface of the scene is selected. For example, the plane may be identified based on a fixed distance above an actual ground surface that is, or is assumed to be, flat. Alternatively, the plane may be identified based on a fixed distance above an average elevation of a ground surface of the scene. In some implementations, the plane may be identified based on a distance from a ceiling, a roof or another structure. At box  720 , coverage areas are generated for each of the depth cameras within the plane. In some implementations, the coverage areas may be defined based on intersections between the plane identified at box  715  and the fields of view determined at box  710 . For example, assuming that a field of view of a depth camera determined at box  710  has a pyramidal shape, a coverage area may be defined as an intersection between the field of view and the plane identified at box  715 . Alternatively, in some implementations, a coverage area may be defined for each depth camera in any manner. 
     At box  725 , an actor is detected on a scene. The actor may be detected in any manner, including but not limited to differences between a location map (or model) of the scene defined and updated by depth images successively captured from the scene, or based on one or more visual images captured from the scene. At box  730 , a tracklet of the actor on the scene is determined, e.g., based on two or more positions of the actor on the scene, as well as times at which the actor was present in such positions (e.g., time-stamps). 
     In some implementations, the actor may be detected, and the tracklet may be determined, on any basis. For example, the tracklet may be a trajectory of any length or duration that is representative of motion of the actor on the scene, and includes an identifier of the actor. Tracklets may be generated for each of a plurality of actors that are present on the scene. The tracklets may be representative of locations or motion of one or more body parts of the actor on the scene and may be generated based on the presence of such body parts within images captured by a single imaging device, e.g., from a common field of view, or within images captured by multiple imaging devices. The tracklets may be generated over a predetermined number or series of frames (e.g., tens of frames or more), subject to any compatibility or incompatibility parameters or constraints. At box  735 , an indication of the actor on the scene is stored in a data store. 
     At box  740 , depth images are captured from the scene using the depth cameras, and at box  745 , ratios of degraded pixels to a total number of pixels within the depth images, e.g., each of the pixels within the depth images, or a number of the pixels within designated regions of the depth images, such as on one or more floors or traveling surfaces, are calculated. 
     At box  750 , coverage areas of the cameras that have ratios of degraded pixels in excess of a predetermined threshold are assigned to a buffer. The coverage areas may be subsets of the plane at the level above the ground surface of the scene and may have any relative size or shape. 
     At box  755 , whether the number of coverage areas in the buffer exceeds a predetermined threshold is determined. If the number of coverage areas in the buffer exceeds the predetermined threshold, the process advances to box  760 , where the actor is designated as a source of the degraded pixels, such as a wearer of retroreflective material, e.g., vests, hats or other articles of clothing, or as otherwise being responsible for the degraded pixels. 
     After the number of coverage areas in the buffer is determined to not exceed the predetermined threshold at box  755 , or after the actor has been designated as a source of the degraded pixels at box  760 , the process advances to box  765 , where whether the actor is detected on the scene is determined. 
     If the actor remains on the scene, then the process returns to box  740 , where depth images are captured from the scene using the depth cameras, and to box  745 , where ratios of degraded pixels to the total numbers of pixels within the depth images are calculated. Whether the actor remains on the scene may be determined on any basis, such as based on one or more visual images, or in any other manner. Alternatively, in some implementations, whether the actor remains on the scene need not be considered, and in such implementations, the process may advance from box  755  to box  770 . 
     If the actor is no longer detected on the scene, then the process advances to box  770 , where whether the actor had been designated as a source of degraded pixels at box  760  is determined. If the actor has not been designated as a source of the degraded pixels at box  760 , then the process advances to box  780 , where an indication of the actor as being present on the scene is removed from the data store, and the process ends. For example, where the actor is not determined to be a source of degraded pixels, or where the actor is determined to be not wearing one or more articles of clothing having retroreflective material thereon, the fact that the actor is not detected on the scene indicates that the actor is not likely on the scene. 
     If the actor has been designated as a source of degraded pixels, however, then the actor may in fact be present on the scene, but may be undetectable on account of one or more articles of clothing having retroreflective materials thereon. The process then advances to box  775 , where whether the actor has not been detected within a predetermined number of image frames is determined. For example, where one or more imaging devices are configured to capture imaging data at rates of fifteen frames per second, thirty frames per second, sixty frames per second, or at any other frame rate, an actor who has been designated as a source of degraded pixels may continue to be tracked for a predetermined number of depth image frames, e.g., five, ten, fifteen, twenty, or any other number, as if the actor remains on the scene despite the fact that the actor is not detected within any of the depth image frames. Alternatively, in some implementations, the actor may continue to be tracked for a corresponding period of time, rather than a predetermined number of frames. If the actor has not been detected within a predetermined number of image frames, the process advances to box  780 , where an indication of the actor as being present on the scene is removed from the data store, and the process ends. 
     If the actor has been detected within the predetermined number of image frames, however, then the process returns to box  740 , where depth images are captured from the scene using the depth cameras, and to box  745 , where ratios of degraded pixels to the total numbers of pixels within the depth images are calculated. 
     Referring to  FIGS.  8 A through  8 G , views of aspects of one system for detecting or locating actors in accordance with implementations of the present disclosure are shown. Except where otherwise noted, reference numerals preceded by the number “8” shown in  FIGS.  8 A through  8 G  indicate components or features that are similar to components or features having reference numerals preceded by the number “6” shown in  FIGS.  6 A through  6 E , by the number “4” shown in  FIGS.  4 A through  4 G , by the number “2” shown in  FIG.  2 A or  2 B , or by the number “1” shown in  FIGS.  1 A through  1 I . 
     As is shown in  FIGS.  8 A and  8 B , a system  800  includes a plurality of time-of-flight cameras  810 - 1 ,  810 - 2 ,  810 - 3 ,  810 - 4 ,  810 - 5 ,  810 - 6 ,  810 - 7 ,  810 - 8 ,  810 - 9  that are mounted in association with a facility (e.g., a materials handling facility). The time-of-flight cameras  810 - 1 ,  810 - 2 ,  810 - 3 ,  810 - 4 ,  810 - 5 ,  810 - 6 ,  810 - 7 ,  810 - 8 ,  810 - 9  have fields of view  830 - 1 ,  830 - 2 ,  830 - 3 ,  830 - 4 ,  830 - 5 ,  830 - 6 ,  830 - 7 ,  830 - 8 ,  830 - 9  that overlap at least in part. 
     Two-dimensional coverage areas of the respective fields of view  830 - 1 ,  830 - 2 ,  830 - 3 ,  830 - 4 ,  830 - 5 ,  830 - 6 ,  830 - 7 ,  830 - 8 ,  830 - 9  may be defined with respect to a distance z above a working surface on the scene. The distance z may be a nominal or average distance with respect to the working surface and may have any value. For example, in some implementations, the distance z may have a value of three hundred millimeters (300 mm) above the working surface, or any other value. The coverage areas for the fields of view  830 - 1 ,  830 - 2 ,  830 - 3 ,  830 - 4 ,  830 - 5 ,  830 - 6 ,  830 - 7 ,  830 - 8 ,  830 - 9  may be defined where a plane at the distance z from the working surface intersects frustrums of the fields of view  830 - 1 ,  830 - 2 ,  830 - 3 ,  830 - 4 ,  830 - 5 ,  830 - 6 ,  830 - 7 ,  830 - 8 ,  830 - 9 . The coverage areas may be used to determine whether an actor is a source of supersaturated or otherwise degraded pixels. For example, because the coverage areas of the respective fields of view  830 - 1 ,  830 - 2 ,  830 - 3 ,  830 - 4 ,  830 - 5 ,  830 - 6 ,  830 - 7 ,  830 - 8 ,  830 - 9  overlap, retroreflective material in any location is likely to result in supersaturation of pixels in more than one of the coverage areas. 
     Thus, where multiple coverage areas include ratios of supersaturated or otherwise degraded pixels in excess of a predetermined threshold, the cause of the excessive ratios may be presumed to be the presence of retroreflective material, which may not only obscure depth pixels surrounding an object having the retroreflective material thereon but also cause depth pixels corresponding to the object to disappear or otherwise be degraded, rather than any one fault with any specific imaging device. Where the object is an actor on the scene, the retroreflective material may cause the actor to effectively disappear from depth imaging data even where the actor remains on the scene. An actor may be identified as a source of supersaturation or other degradation within depth imaging data where a number of coverage areas including supersaturated or otherwise degraded pixels exceeds a predetermined threshold. A detection or a tracklet of the actor may be presumed to remain on the scene for at least a predetermined number of depth image frames even where the depth image frames do not appear to depict the actor. 
     As is shown in  FIGS.  8 C and  8 D , an actor  892  wearing retroreflective clothing  894  enters the scene and passes within the fields of view  830 - 1 ,  830 - 2 ,  830 - 3 ,  830 - 4 ,  830 - 5 ,  830 - 6 ,  830 - 7 ,  830 - 8 ,  830 - 9  of the time-of-flight cameras  810 - 1 ,  810 - 2 ,  810 - 3 ,  810 - 4 ,  810 - 5 ,  810 - 6 ,  810 - 7 ,  810 - 8 ,  810 - 9 . The fields of view  830 - 1 ,  830 - 2 ,  830 - 3 ,  830 - 4 ,  830 - 5 ,  830 - 6 ,  830 - 7 ,  830 - 8 ,  830 - 9  are illuminated by illuminators within the time-of-flight cameras  810 - 1 ,  810 - 2 ,  810 - 3 ,  810 - 4 ,  810 - 5 ,  810 - 6 ,  810 - 7 ,  810 - 8 ,  810 - 9 , and reflections of the light from surfaces within the scene, including but not limited to reflections of light from the retroreflective materials  894 , are captured by sensors of the time-of-flight cameras  810 - 1 ,  810 - 2 ,  810 - 3 ,  810 - 4 ,  810 - 5 ,  810 - 6 ,  810 - 7 ,  810 - 8 ,  810 - 9 . 
     As is shown in  FIGS.  8 E through  8 G , a location of the actor  892  may be tracked where the number of coverage areas having excessive ratios of supersaturated or otherwise degraded pixels is greater than a predetermined threshold, such as two, three, four, five or six of such coverage areas. For example, as is shown in  FIG.  8 E , where coverage areas  835 - 4 ,  835 - 5 ,  835 - 7 ,  835 - 8  (viz., four of the coverage areas) defined with respect to fields of view  830 - 4 ,  830 - 5 ,  830 - 7 ,  830 - 8  are determined to include supersaturated or otherwise degraded pixels in excess of a predetermined threshold at time to, the actor  892  may be determined to be a source of the supersaturated or otherwise degraded pixels, and one or more indications that the actor  892  is the source of the supersaturated or otherwise degraded pixels may be stored in one or more data stores. 
     Subsequently, as is shown in  FIG.  8 F , where coverage areas  835 - 4 ,  835 - 5 ,  835 - 6 ,  835 - 7 ,  835 - 8 ,  835 - 9  (viz., six of the coverage areas) defined with respect to the fields of view  830 - 4 ,  830 - 5 ,  830 - 6 ,  830 - 7 ,  830 - 8 ,  830 - 9  are determined to include supersaturated or otherwise degraded pixels in excess of a predetermined threshold at time t 1 , the actor  892  may be determined to be located on the scene at a location where the coverage areas  835 - 4 ,  835 - 5 ,  835 - 6 ,  835 - 7 ,  835 - 8 ,  835 - 9  overlap at least in part, even if the actor  892  is not expressly identified within depth imaging data or visual imaging data captured using one or more of the time-of-flight cameras  810 - 1 ,  810 - 2 ,  810 - 3 ,  810 - 4 ,  810 - 5 ,  810 - 6 ,  810 - 7 ,  810 - 8 ,  810 - 9 . As is shown in  FIG.  8 G , where coverage  835 - 8 ,  835 - 9  (viz., six of the coverage areas) defined with respect to the fields of view  830 - 8 ,  830 - 9  are determined to include supersaturated or otherwise degraded pixels in excess of a predetermined threshold at time t 2 , the actor  892  may be determined to be located on the scene at a location where the coverage areas  835 - 8 ,  835 - 9  overlap at least in part, even if the actor  892  is not expressly identified within depth imaging data or visual imaging data captured using one or more of the time-of-flight cameras  810 - 8 ,  810 - 9 . 
     Although some of the implementations disclosed herein reference the use of time-of-flight cameras in materials handling facilities or like environments, those of ordinary skill in the pertinent arts will recognize that the systems and methods disclosed herein are not so limited, and may be utilized in connection with time-of-flight cameras that are provided for any intended industrial, commercial, recreational or other use. 
     It should be understood that, unless otherwise explicitly or implicitly indicated herein, any of the features, characteristics, alternatives or modifications described regarding a particular implementation herein may also be applied, used, or incorporated with any other implementation described herein, and that the drawings and detailed description of the present disclosure are intended to cover all modifications, equivalents and alternatives to the various implementations as defined by the appended claims. Moreover, with respect to the one or more methods or processes of the present disclosure described herein, including but not limited to the processes represented in the flow chart of  FIGS.  3 ,  5 A and  5 B, and  7 A and  7 B , orders in which such methods or processes are presented are not intended to be construed as any limitation on the claimed inventions, and any number of the method or process steps or boxes described herein can be combined in any order and/or in parallel to implement the methods or processes described herein. Also, the drawings herein are not drawn to scale. 
     Conditional language, such as, among others, “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey in a permissive manner that certain implementations could include, or have the potential to include, but do not mandate or require, certain features, elements and/or steps. In a similar manner, terms such as “include,” “including” and “includes” are generally intended to mean “including, but not limited to.” Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more implementations or that one or more implementations necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular implementation. 
     Disjunctive language such as the phrase “at least one of X, Y, or Z,” or “at least one of X, Y and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to present that an item, term, etc., may be either X, Y, or Z, or any combination thereof (e.g., X, Y, and/or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain implementations require at least one of X, at least one of Y, or at least one of Z to each be present. 
     Unless otherwise explicitly stated, articles such as “a” or “an” should generally be interpreted to include one or more described items. Accordingly, phrases such as “a device configured to” are intended to include one or more recited devices. Such one or more recited devices can also be collectively configured to carry out the stated recitations. For example, “a processor configured to carry out recitations A, B and C” can include a first processor configured to carry out recitation A working in conjunction with a second processor configured to carry out recitations B and C. 
     Language of degree used herein, such as the terms “about,” “approximately,” “generally,” “nearly” or “substantially” as used herein, represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “about,” “approximately,” “generally,” “nearly” or “substantially” may refer to an amount that is within less than 10% of, within less than 5% of, within less than 1% of, within less than 0.1% of, and within less than 0.01% of the stated amount. 
     Although the invention has been described and illustrated with respect to illustrative implementations thereof, the foregoing and various other additions and omissions may be made therein and thereto without departing from the spirit and scope of the present disclosure.