Patent Publication Number: US-2023133026-A1

Title: Sparse and/or dense depth estimation from stereoscopic imaging

Description:
BACKGROUND 
     Stereoscopic cameras leverage multiple lenses (or images captured from multiple cameras from different perspectives) in order to create dense depth maps from two (or more) stereo images (also referred to herein as “stereoscopic images”). In the computer vision context, this dense depth estimation can involve a considerable amount of computation, such as pixel-wise comparisons for visual feature matching and disparity calculation. This computation may require considerable computing resources such as processor cycles and memory, not to mention time. The more repetitive and homogeneous the visual features depicted in the two stereo images are, the more error-prone the process becomes e.g., due to mismatched visual features. This is not normally an issue in domains where visual features and/or noise are relatively sparse and/or heterogeneous but becomes problematic when visual features are abundant, densely-packed, and/or homogeneous. For example, conventional dense depth estimation generally performs poorly on fine structures and at the edges of objects. 
     In the agricultural domain, for example, increasingly large amounts of vision data are being captured, e.g., by agricultural robots or rovers, unmanned aerial vehicles (UAVs), and sensor packages transported through fields by agricultural vehicles. These often high-resolution vision data include close-up depictions of plants with enormous numbers of relatively homogenous and densely-packed visual features, such as leaves, blades, stems, branches, and/or other constituent plant parts. Performing conventional depth estimation on such vision data, e.g., on a pixel-by-pixel basis, may constitute an unacceptable bottleneck for farmers who rely on near real-time agricultural inferences to remain competitive. 
     SUMMARY 
     Implementations are described herein for performing sparse depth estimation in domains where visual features and/or noise are abundant. More particularly, but not exclusively, techniques are described herein for leveraging visual annotations of plant trait instances in stereo images to perform sparse depth estimation in the agricultural domain. In various implementations, instances of plant traits are visually annotated in pairs of stereo images. Rather than performing depth estimation using all pixels of the stereo images, sparse depth estimation is performed using attributes of the visual annotations themselves, including spatial attributes such as location, size, shape, height, width, distance(s) to other visual annotations, etc. Visual features outside of these annotated plant trait instances may not be relevant or useful to agricultural personnel. Accordingly, computationally-expensive and/or error-prone dense depth estimation can be reduced or avoided altogether. 
     In various implementations, a method for performing sparse depth estimation may be implemented using one or more processors and may include: comparing one or more spatial aspects of a first visual annotation of a first candidate plant trait instance depicted in a first stereo image to one or more spatial aspects of a second visual annotation of a second candidate plant trait instance depicted in a second stereo image; based on the comparing, determining that the first and second candidate plant trait instances are a match for the same plant trait instance across the first and second stereo images; determining a disparity between the first and second candidate plant trait instances; based on the disparity, estimating a depth associated with the same plant trait instance; and causing output to be provided at a computing device, wherein the output is generated based on the depth associated with the same plant trait instance. 
     In various implementations, the method may further include: comparing one or more spatial aspects of a third visual annotation of a third candidate plant trait instance depicted in the first stereo image to one or more spatial aspects of a fourth visual annotation of a fourth candidate plant trait instance depicted in the second stereo image; based on the comparing, determining that the third and fourth candidate plant trait instances are a match for an additional same plant trait instance across the first and second stereo images; and determining an additional depth associated with the additional same plant trait instance. The output may be further generated based on the additional depth. 
     In various implementations, the method may further include comparing the depth with the additional depth to determine a spatial dimension of a plant-part-of-interest, wherein the output conveys the spatial dimension of the plant-part-of-interest. In various implementations, the same plant trait instance may include one end of a head of wheat and the additional same plant trait instance comprises an opposite end of the head of wheat. In various implementations, the same plant trait instance may include one end of a pod and the additional same plant trait instance comprises an opposite end of the pod. In various implementations, the same plant trait instance may include one end of an ear or tassel of corn and the additional same plant trait instance comprises an opposite end of the ear or tassel of corn. 
     In various implementations, the method may include matching one or more additional phenotypic traits of a plant depicted in the first stereo image having the first candidate plant trait instance to one or more additional phenotypic traits of a plant depicted in the second stereo image having the second candidate plant trait instance, wherein the determining that the first and second candidate plant instances are a match is further based on the matching. In various implementations, the first and second visual annotations may include bounding boxes or key points. In various implementations, the same plant trait instance may be a flower, nut, fruit, or berry. 
     In another aspect, a method for generating a pair of synthetic stereoscopic training images may include: generating one or more three-dimensional synthetic plants in a three-dimensional space, wherein the one or more three-dimensional synthetic plants include homogenous and densely-distributed synthetic plant parts; projecting the plurality of three-dimensional synthetic plants onto a two-dimensional plane from a first perspective in the three-dimensional space to form a first synthetic stereoscopic image of the pair; projecting the plurality of three-dimensional synthetic plants onto another two-dimensional plane from a second perspective in the three-dimensional space to form a second synthetic stereoscopic image of the pair; annotating the first and second synthetic stereoscopic images to create a mapping between the individual synthetic plant parts across the first synthetic stereoscopic images; and training a feature matching machine learning model based on the mapping. 
     In various implementations, the homogenous and densely-distributed synthetic plant parts may include, for instance, individual blades of grass, individual leaves of the one or more three-dimensional synthetic plants, individual stems or culms of the one or more three-dimensional synthetic plants, individual flowers or pedicles of the one or more three-dimensional synthetic plants, and/or individual peduncles of the one or more three-dimensional synthetic plants. 
     In another aspect, a method for performing depth estimation may include: comparing one or more spatial aspects of a first visual annotation of a first candidate plant trait instance depicted in a first stereo image to one or more spatial aspects of a second visual annotation of a second candidate plant trait instance depicted in a second stereo image; based on the comparing, calculating an annotation-based similarity score between the first and second candidate plant trait instances across the first and second stereo images; processing the first and second stereo images based on a feature matching machine learning model to determine a feature-based similarity score between the first and second candidate plant trait instances across the first and second stereo images; based on the annotation based similarity score and the feature-based similarity score, determining that the first and second candidate plant trait instances are a match for the same plant trait instance across the first and second stereo images; determining a disparity between the first and second candidate plant trait instances; based on the disparity, estimating a depth associated with the same plant trait instance; and causing output to be provided at a computing device, wherein the output is generated based on the depth associated with the same plant trait instance. 
     In various implementations, determining that the first and second candidate plant trait instances are a match for the same plant trait instance across the first and second stereo images may include performing bipartite graph matching. In various implementations, the method may further include matching one or more additional phenotypic traits of a plant depicted in the first stereo image having the first candidate plant trait instance to one or more additional phenotypic traits of a plant depicted in the second stereo image having the second candidate plant trait instance, wherein the determining that the first and second candidate plant instances are a match is further based on the matching. 
     In various implementations, the first and second visual annotations may include bounding boxes or key points. In various implementations, the method may include calculating a composite similarity score based on the annotation based similarity score and the feature-based similarity score, wherein the determination that the first and second candidate plant trait instances are a match for the same plant trait instance across the first and second stereo images is based on the composite similarity score. In various implementations, the annotation based similarity score and the feature-based similarity score may be weighted differently when calculating the composite similarity score. 
     In various implementations, the annotation based similarity score and the feature-based similarity score may be used to corroborate and/or refute each other. In various implementations, the feature matching machine learning model may include a convolutional neural network. 
     In addition, some implementations include one or more processors (e.g., central processing unit(s) (CPU(s)), graphics processing unit(s) (GPU(s), and/or tensor processing unit(s) (TPU(s)) of one or more computing devices, where the one or more processors are operable to execute instructions stored in associated memory, and where the instructions are configured to cause performance of any of the aforementioned methods. Some implementations also include one or more non-transitory computer readable storage media storing computer instructions executable by one or more processors to perform any of the aforementioned methods. 
     It should be appreciated that all combinations of the foregoing concepts and additional concepts described in greater detail herein are contemplated as being part of the subject matter disclosed herein. For example, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the subject matter disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    schematically depicts an example environment in which selected aspects of the present disclosure may be employed in accordance with various implementations. 
         FIG.  2    schematically depicts components and a process for practicing selected aspects of the present disclosure, in accordance with various implementations. 
         FIG.  3    depicts an example of how various visual annotations may be used to perform sparse depth estimation, in accordance with various implementations. 
         FIG.  4    is a flowchart of an example method in accordance with various implementations described herein. 
         FIG.  5    is a flowchart of another example method in accordance with various implementations described herein. 
         FIG.  6 A  and  FIG.  6 B  demonstrate one example of how stereo imagery captured by a stereoscopic camera deployed in a field can be processed to perform sparse depth estimation, which in turn can be used to estimate plant height. 
         FIG.  7    schematically depicts an example architecture of a computer system. 
         FIG.  8    is a flowchart of another example method in accordance with various implementations described herein. 
         FIG.  9    schematically depicts an example process flow, in accordance with various implementations. 
         FIG.  10    is a flowchart of another example method in accordance with various implementations described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Implementations are described herein for performing sparse depth estimation in domains where visual features and/or noise are abundant. More particularly, but not exclusively, techniques are described herein for leveraging visual annotation of plant trait instances in stereo images to perform sparse depth estimation in the agricultural domain. In various implementations, instances of plant traits are visually annotated in pairs of stereo images. Rather than performing depth estimation using all pixels of the stereo images, sparse depth estimation is performed using attributes of the visual annotations themselves, including spatial attributes such as location, size, shape, height, width, distance(s) to other visual annotations, etc. Visual features outside of these annotated plant trait instances may not be relevant or useful to agricultural personnel. Accordingly, computationally-expensive and/or error-prone dense depth estimation can be reduced or avoided altogether. 
     Various plant traits may be leveraged to perform sparse depth estimation, depending on the task at hand. These plant traits may include, for instance, location and/or attributes of plant-parts-of economic and/or nutritional interest, locations of particular plant “landmarks” such as the top/bottom of a head of wheat or locations of laterally-extending branches, location of disease or pest infestation, particular plant types (e.g., weeds), and so forth. Plant traits may be identified using various object recognition techniques, such as scale-invariant feature transform (SIFT), speeded up robot feature (SURF), histogram of oriented gradients (HOG), single shot detectors (SSD), spatial pyramid pooling (SPP), you only look once (YOLO), etc. In some implementations, a machine learning model such as a convolutional neural network (CNN) may be used to process pairs of stereo images to identify objects. The output of such object recognition processing may be visual annotations of those images. These visual annotations may include, for instance, key points or bounding shapes, to name a few. 
     In various implementations, spatial aspect(s) of a visual annotation may be compared across a pair of stereo images to identify matching pairs of visual annotations that annotate the same plant trait instance. For example, locations of key points, by themselves and relative to other visual annotation(s), may be compared to identify matching key points across pairs of stereo images. With bounding shapes such as bounding boxes, height, width, and/or area may also be considered. With pixel-wise annotations, attributes of clusters of annotated pixels may be considered, such as cluster shapes, cluster diameters, cluster skews, etc. Once matching pairs of visual annotations are identified, pixel disparities between those matching pairs of visual annotations may be determined across the pair of stereo images. 
     A matching pair of visual annotations and the disparit(ies) between them across a pair of stereo images may be used to estimate a depth associated with the plant trait instance that is annotated by the visual annotations. This depth may be coupled with other data points, such as horizontal (x) and vertical (y) coordinates of the stereo imagery, focal length(s), etc., to represent a coordinate in 3D space. Such a 3D coordinate may then be used for various purposes. As one example, the depth associated with the plant trait instance may be compared to a distance to ground to determine a height of the plant trait instance, which may be predictive of, for instance, crop yield, plant health, etc. In some implementations, a sparse depth map may be generated that includes depths for plant traits of potential interest, as opposed to a dense depth map that includes pixel-wise depths. 
     As another example, a 3D coordinate (including the estimated depth) associated with one plant trait instance may be compared with another 3D coordinate associated with another plant trait instance, e.g., to determine a spatial dimension of one or more plant-parts-of-interest. In some implementations where each plant trait instance is an instance of a plant-part-of interest (e.g., a nut, berry, flower, etc.), distances between each instance may be determined, e.g., as a distribution of distances between plant-parts-of-interest, to estimate a density or other quantitative measure of the plant-part-of-interest, a general health of the plant, etc. 
     In other implementations, each plant trait may correspond to a landmark of a plant that can be used for various purposes. For example, one plant trait may be the top of a head of wheat and another plant trait may be a bottom end (or opposite end) of the head of wheat. Instances of these two plant traits may be annotated, e.g., with key points. Techniques described herein may be used to determine 3D coordinates of these plant trait instances based on those annotations. The 3D coordinates may be used to determine a distance between these plant trait instances. That distance may correspond to the size of the plant trait. Thus, it is possible to quickly estimate the size of heads of wheat, or other similar plant traits such as ears or tassels of corn, pods carrying various contents (e.g., beans, peas), and so forth. 
       FIG.  1    schematically illustrates one example environment in which one or more selected aspects of the present disclosure may be implemented, in accordance with various implementations. The example environment depicted in  FIG.  1    relates to the agriculture domain, which as noted previously is a beneficial domain for implementing selected aspects of the present disclosure. For example, techniques described herein may be useful where there are abundant, densely-packed, and/or homogeneous clusters of visual features, but actual plant-traits-of-interest are less abundant (e.g., detecting heads of wheat among a background of leaves). Techniques described herein are also useful where traits of interest are very small, thin, or otherwise fine. Conventional dense depth estimation (e.g., pixel-wise) generally performs poorly on fine structures and at the edges of objects. However, it may be desirable to detect the height or distances at precisely these points (e.g., the bottom of a wheat head where the stem is very fine and thin, or the distance between one edge of a crop and the other edge to get its length). However, this is not meant to be limiting. Techniques described here may be useful in any domain where visual features are abundant, densely-packed, and/or homogeneous. 
     The environment of  FIG.  1    includes a plurality of edge sites  102   1-N  (e.g., farms, fields, plots, or other areas in which crops are grown) and a central agricultural knowledge system  104 A. Additionally, one or more of the edge sites  102 , including at least edge site  102   1 , includes an edge agricultural knowledge system  104 B, a plurality of client devices  106   1-X , human-controlled and/or autonomous farm equipment  108   1-M , and one or more fields  112  that are used to grow one or more crops. Field(s)  112  may be used to grow various types of crops that may produce plant parts of economic and/or nutritional interest. These crops may include but are not limited to everbearing crops such as strawberries, tomato plants, or any other everbearing or non-everbearing crops, such as soybeans, corn, lettuce, spinach, beans, cherries, nuts, cereal grains, wheat, berries, grapes, sugar beets, and so forth. 
     One edge site  102   1  is depicted in detail in  FIG.  1    for illustrative purposes. However, as demonstrated by additional edge sites  102   2-N , there may be any number of edge sites  102  corresponding to any number of farms, fields, or other areas in which crops are grown, and in which large-scale agricultural tasks such as harvesting, weed remediation, fertilizer application, herbicide application, planting, tilling, etc. are performed. Each edge site  102  may include the same or similar components as those depicted in  FIG.  1    as part of edge site  102   1 . 
     In various implementations, components of edge sites  102   1-N  and central agricultural knowledge system  104 A collectively form a distributed computing network in which edge nodes (e.g., client device  106 , edge agricultural knowledge system  104 B, farm equipment  108 ) are in network communication with central agricultural knowledge system  104 A via one or more networks, such as one or more wide area networks (“WANs”)  110 A. Components within edge site  102   1 , by contrast, may be relatively close to each other (e.g., part of the same farm or plurality of fields in a general area), and may be in communication with each other via one or more local area networks (“LANs”, e.g., Wi-Fi, Ethernet, various mesh networks) and/or personal area networks (“PANs”, e.g., Bluetooth), indicated generally at  110 B. 
     An individual (which in the current context may also be referred to as a “user”) may operate a client device  106  to interact with other components depicted in  FIG.  1   . Each client device  106  may be, for example, a desktop computing device, a laptop computing device, a tablet computing device, a mobile phone computing device, a computing device of a vehicle of the participant (e.g., an in-vehicle communications system, an in-vehicle entertainment system, an in-vehicle navigation system), a standalone interactive speaker (with or without a display), or a wearable apparatus that includes a computing device, such as a head-mounted display (“HMD”) that provides an AR or VR immersive computing experience, a “smart” watch, and so forth. Additional and/or alternative client devices may be provided. 
     Central agricultural knowledge system  104 A and edge agricultural knowledge system  104 B (collectively referred to herein as “agricultural knowledge system  104 ”) comprise an example of a distributed computing network for which techniques described herein may be particularly beneficial. Each of client devices  106 , agricultural knowledge system  104 , and/or farm equipment  108  may include one or more memories for storage of data and software applications, one or more processors for accessing data and executing applications, and other components that facilitate communication over a network. The computational operations performed by client device  106 , farm equipment  108 , and/or agricultural knowledge system  104  may be distributed across multiple computer systems. 
     Each client device  106  and some farm equipment  108  may operate a variety of different applications that may be used, for instance, to perform sparse depth estimation using techniques described herein. For example, a first client device  106   1  operates an agricultural (AG) client  107  (e.g., which may be standalone or part of another application, such as part of a web browser) that may allow the user to, among other things, view depth maps generated using techniques described herein. Another client device  106   X  may take the form of a HMD that is configured to render 2D and/or 3D data to a wearer as part of a VR immersive computing experience. For example, the wearer of client device  106   X  may be presented with 3D point clouds representing various aspects of objects of interest, such as fruit/vegetables of crops, weeds, crop yield predictions, etc. The wearer may interact with the presented data, e.g., using HMD input techniques such as gaze directions, blinks, etc. 
     Individual pieces of farm equipment  108   1-M  may take various forms. Some farm equipment  108  may be operated at least partially autonomously, and may include, for instance, an unmanned aerial vehicle  108   1  that captures sensor data such as digital images from overhead field(s)  112 . Other autonomous farm equipment (e.g., robots) may include a robot (not depicted) that is propelled along a wire, track, rail or other similar component that passes over and/or between crops, a wheeled robot  108   M , or any other form of robot capable of being propelled or propelling itself past crops of interest. In some implementations, different autonomous farm equipment may have different roles, e.g., depending on their capabilities. For example, in some implementations, one or more robots may be designed to capture data, other robots may be designed to manipulate plants or perform physical agricultural tasks, and/or other robots may do both. Other farm equipment, such as a tractor  108   2 , may be autonomous, semi-autonomous, and/or human-driven. Any of farm equipment  108  may include various types of sensors, such as vision sensors (e.g., 2D digital cameras, 3D cameras, 2.5D cameras, infrared cameras), inertial measurement unit (“IMU”) sensors, Global Positioning System (“GPS”) sensors, X-ray sensors, moisture sensors, barometers (for local weather information), photodiodes (e.g., for sunlight), thermometers, etc. 
     In some implementations, farm equipment  108  may take the form of one or more modular edge computing nodes  108   3 . An edge computing node  108   3  may be a modular and/or portable data processing device and/or sensor package that may be carried through an agricultural field  112 , e.g., by being mounted on another piece of farm equipment (e.g., on a boom affixed to tractor  108   2  or to a truck) that is driven through field  112  and/or by being carried by agricultural personnel. Edge computing node  108   3  may include logic such as processor(s), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGA), etc., configured with selected aspects of the present disclosure to capture and/or process various types of sensor data to make agricultural inferences, and/or to perform sparse depth estimation. 
     In some examples, one or more of the components depicted as part of edge agricultural knowledge system  104 B may be implemented in whole or in part on a single edge computing node  108   3 , across multiple edge computing nodes  108   3 , and/or across other computing devices, such as client device(s)  106 . Thus, when operations are described herein as being performed by/at edge agricultural knowledge system  104 B, or as being performed “in situ,” it should be understood that those operations may be performed by one or more edge computing nodes  108   3 , and/or may be performed by one or more other computing devices at the edge  102 , such as on client device(s)  106 . 
     In various implementations, edge agricultural knowledge system  104 B may include a vision data module  114 B, an edge inference module  116 B, a matching module  118 , and a depth module  119 . Edge agricultural knowledge system  104 B may also include one or more edge databases  120 B for storing various data used by and/or generated by modules  114 B,  116 B,  118 , and  119  such as vision and/or other sensor data gathered by farm equipment  108   1-M , agricultural inferences, machine learning models that are applied and/or trained using techniques described herein to generate agricultural inferences, depth estimations and/or depth maps calculated by depth module  119 , and so forth. In some implementations one or more of modules  114 B,  116 B,  118 , and/or  119  may be omitted, combined, and/or implemented in a component that is separate from edge agricultural knowledge system  104 B. 
     In various implementations, central agricultural knowledge system  104 A may be implemented across one or more computing systems that may be referred to as the “cloud.” Central agricultural knowledge system  104 A may receive massive sensor data generated by farm equipment  108   1-M  (and/or farm equipment at other edge sites  102   2-N ) and process it using various techniques, including but not limited to application of machine learning state machines generated using techniques described herein, to make agricultural inferences. 
     However, the agricultural inferences generated by central agricultural knowledge system  104 A may be delayed, e.g., by the time required to physically transport portable data devices (e.g., hard drives) from edge sites  102   1-N  to central agricultural knowledge system  104 A, and/or by the time required by central agricultural knowledge system  104 A to computationally process this massive data. Agricultural personnel (e.g., farmers) at edge sites  102  may desire agricultural information, such as depth estimations and/or maps generated using techniques described herein, much more quickly than this. Moreover, farmers may value the privacy of their data and may prefer that their data not be sent to the cloud for processing. Accordingly, in various implementations, techniques described herein for sparse depth estimation may be implemented in situ at edge agricultural knowledge system  104 B if possible/feasible, and at central agricultural knowledge system  104 A if necessary/beneficial. Although not depicted identical to edge agricultural knowledge system  104 B, central agricultural knowledge system  104 A may include the same or similar components as edge agricultural knowledge system  104 B. 
     In some implementations, vision data module  114 B may be configured to provide sensor data to edge inference module  116 B. In some implementations, the vision sensor data may be applied, e.g., continuously and/or periodically by edge inference module  116 B, as input across one or more machine learning models stored in edge database  120 B to generate inferences detected in/on one or more plants in the agricultural field  112 . Inference module  116 B may process the inference data in situ at the edge using one or more of the machine learning models stored in database  120 B. In some cases, one or more of these machine learning model(s) may be stored and/or applied directly on farm equipment  108 , such as edge computing node  108   3 , to make inferences about plants of the agricultural field  112 . 
     As noted previously, various types of machine learning models may be applied by inference modules  116 A/B to generate various types of agricultural inferences (e.g., object recognition, classification, disease detection, pest detection, etc.). Additionally, various types of machine learning models may be used to generate image embeddings that are applied as input across the various machine learning models. These various models may include, but are not limited to, various types of recurrent neural networks (RNNs) such as long short-term memory (LSTM) or gated recurrent unit (GRU) networks, transformer networks, feed-forward neural networks, CNNs, support vector machines, random forests, decision trees, etc. 
     Additionally, other data  124  may be applied as input across these models besides sensor data or embeddings generated therefrom. Other data  124  may include, but is not limited to, historical data, weather data (obtained from sources other than local weather sensors), data about chemicals and/or nutrients applied to crops and/or soil, pest data, crop cycle data, previous crop yields, farming techniques employed, cover crop history, and so forth. Weather data may be obtained from various sources other than sensor(s) of farm equipment  108 , such as regional/county weather stations, etc. In implementations in which local weather and/or local weather sensors are not available, weather data may be extrapolated from other areas for which weather data is available, and which are known to experience similar weather patterns (e.g., from the next county, neighboring farms, neighboring fields, etc.). 
     Matching module  118  may be configured to compare one or more spatial aspects of a first visual annotation of a first candidate plant trait instance depicted in a first stereo image, e.g., provided by vision data module  114 B, to one or more spatial aspects of a second visual annotation of a second candidate plant trait instance depicted in a second stereo image, e.g., provided by vision data module  114 B. In various implementations, the first and second stereo images may be captured by multiple lenses of a single stereoscopic camera, or by the same camera from two different perspectives. Based on this comparison, matching module  118  may determine whether the first and second candidate plant trait instances are a match for the same plant trait instance across the first and second stereo images. 
     Matching module  118  may compare and match visual annotations in various ways, depending on factors such as the type of annotations used. For example, key points, which each may comprise a single point (e.g., a pixel of a digital image) may be compared on the basis of their relative locations across the stereo images, locations within clusters of key points, etc. Bounding shapes such as bounding boxes, by contrast, may be compared not only based on their relative locations across stereo images, but also based on their relative sizes, heights, widths, etc. In some implementations where each stereo image contains multiple candidate visual annotations, techniques such as bipartite graph matching (e.g., using the Hungarian algorithm) may be employed to match visual annotations across stereo images. 
     Depth module  119  may be configured to generate depth information about the matched plant traits. This depth information may include, for instance, updated visual annotations that include depths, a depth distribution (e.g., in numeric or graphical form), statistics, and/or a depth map. To generate this depth information, in various implementations, depth module  119  may be configured to determine a disparity (sometimes referred to as a “binocular disparity”) between the matched first and second candidate plant trait instances. As used herein, disparity may refer to a shift or difference in apparent position (e.g., a parallax) between matching features across pairs of stereo images. This shift or difference may be horizontal if the lenses/cameras are offset from each other horizontally, or it may be vertical if the lenses/cameras are offset from each other vertically. Disparity may be measured and/or expressed in various ways, such as in units such as pixels, inches, centimeters, millimeters, etc. 
     Based on the disparity, depth module  119  may estimate a depth associated with the same plant trait instance, e.g., using techniques such as triangulation. In some implementations, for a given visual feature, depth module  119  may estimate its depth z using an equation such as the following: 
     
       
         
           
             
               
                 
                   z 
                   = 
                   
                     
                       f 
                       × 
                       B 
                     
                     d 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where f is the focal length of the stereoscopic vision sensor, B is the interaxial distance between the lenses/cameras, and d is the disparity in pixels. 
     In this specification, the term “database” and “index” will be used broadly to refer to any collection of data. The data of the database and/or the index does not need to be structured in any particular way and it can be stored on storage devices in one or more geographic locations. Thus, for example, database(s)  120 A and  120 B may include multiple collections of data, each of which may be organized and accessed differently. 
       FIG.  2    schematically depicts how various elements depicted in  FIG.  1    may process data to practice selected aspects of the present disclosure. Starting at top, a left stereo image  226  and a right stereo image  228  may be captured using a stereoscopic vision sensor. Left stereo image  226  may capture crops in field  112  from a first perspective. Right stereo image  228  may capture crops in field  112  from a second perspective that is offset from the first perspective, e.g., by an interaxial distance. 
     Edge inference module  116 B may separately process stereo images  226 ,  228  (e.g., in parallel, one after the other) using one or more machine learning models stored in database  120 B to generate an annotated left stereo image  226 ′ and an annotated right stereo image  228 ′. In particular, these images  226 ′,  228 ′ may be annotated with visual annotations such as key points, bounding boxes, pixel-wise annotations, etc. of various plant traits. Inference module  116 B may use various types of machine learning models for this purpose, such as various types of CNNs trained to detect/annotate various plant traits. In other implementations, other object detection techniques may be employed, such as SIFT, SURF, HOG, SSD, SPP, YOLO, etc. 
     Matching module  118  may be configured to compare the visual annotations contained in annotation stereo images  226 ′,  228 ′ to identify matching visual annotations, and hence, matched underlying plant trait(s)  230 . As noted previously, matching module  118  may employ various techniques to perform this matching, such as bipartite graph matching, brute force matching, fast library for approximate nearest neighbors (FLANN) matching, etc. In some implementations, matching module  118  may encode visual annotations (and in some cases the pixels they envelop) into embeddings or feature vectors. Matching module  118  may then compare these embeddings in embedding space to determine similarity measures, e.g., using techniques such as Euclidean distance, cosine similarity, dot product, etc. 
     The matched plant trait(s)  230  may be used by depth module  119  to determine disparities (e.g., a disparity map) between various visual features. As discussed previously, depth module  119  may calculate depth(s) of the matched plant trait(s)  230  using various techniques such as triangulation, equation (1) above, etc. In some implementations, depth module  119  may generate sparse depth map(s)  232  of the various matched plant trait(s)  230 . These depth map(s) may be provided to AG client  107 , which may output them to a user in various ways. 
     In some implementations, AG client  107  may render a composite or 3D view generated from both stereo images  226 ,  228 . The composite view may include annotations of matched plant traits, e.g., denoted with respective depths. In some implementations, AG client  107  may provide a graphical user interface (GUI) that allows a user to select which plant traits and corresponding depths they wish to view, e.g., akin to activating or deactivating layers. For example, the user may interact with one or more graphical elements to select plant traits such as flowers, buds, nuts, landmarks (e.g., tops/bottoms of pods, heads of wheat, ears of corn) that the user wishes to see visualized. In some such implementations, statistics associated with each type of plant trait may be displayed as well. These statistics may vary widely depending on the application, and may include, for instance, mean/median heights of plant traits, density of plant traits, mean/median sizes of plant traits, etc. 
       FIG.  3    depicts an example of how annotations of stereo images of crops may be used to perform sparse depth estimation. In this example, the crops at issue are wheat plants, but this is not meant to be limiting. Similar techniques may be applied to other types of crops, such as corn (maize), various types of peas or beans, legumes, etc. A stereoscopic vision sensor  336  includes two lenses,  337 A and  337 B, that are separated by an interaxial distance  339 . Lenses  337 A,  337 B capture respective fields of view  340 A,  340 B that are slightly offset from each other due to interaxial distance  339 . 
     Stereoscopic vision sensor  336  is overhead the wheat plants so stereo images it generates would depict the wheat plants from overhead. For example, stereoscopic vision sensor  336  may be integrated with a computing node  108   3  that is mounted to a boom of a tractor or to a center pivot deployed in field  112 . However, for ease of explanation and understanding, the wheat plants are shown from the side in  FIG.  3   . To this end, the wheat plants are shown with annotations included at 3D locations that correspond to landmarks of those plants. For example, tops of the heads of wheat are annotated with five-pointed stars at  340   1-6 . In the stereo images generated by stereoscopic vision sensor  336 , these annotations would more likely be key points than actual stars; stars are used here for demonstration purposes. Likewise, bottoms of the heads of wheat are annotated with four-point stars at  342   1-6 . 
     In various implementations, these landmarks may be used for a variety of purposes. As explained previously, they may be presented to a user of AG client  107 , e.g., as selectable layers that the user can toggle on/off. Statistics about these landmarks may be provided as well. For example, a distribution of the wheat head tops  340   1-6  may be presented and/or used to present statistics such as an average wheat head height, a standard deviation, etc. 
     In some implementations, multiple visual annotations of different plant traits may be used in tandem to make additional agricultural estimations about combined or aggregated plant traits. For example, each of annotations  340   1-6  and  340   1-6  represents a point in 3D space that is some measured depth from stereoscopic vision sensor  336 . Distances between these points may be representative of the spatial dimension(s) of the heads of wheat. For example, the distance D 1  between annotations  340   1  and  342   1  may represent a length of the left-most head of wheat. The distance D 2  between annotations  340   2  and  342   2  may represent a length of the second-from-left head of wheat. And so on. Once distances D 1-6  are computed, they may be used, for instance, to provide statistics about spatial dimensions of wheat heads, such as average length, median length, average mass, etc. 
     In some implementations, other phenotypic traits of a plant and/or plant-part-of-interest may be used in conjunction with visual annotation(s) for various purposes. For example, to match key points at the top and bottom of a particular head of wheat, another phenotypic trait about the head of wheat, such as its maturity rating (the stem just below the head of wheat begins turning brown once it reaches full maturity), color, moisture content, thickness, etc., may be leveraged. In particular, two key points may be determined to be associated with different heads of wheat if one key point is associated with a first head of wheat having a first maturity rating, and the other key point is associated with a second head of wheat having a different maturity rating. 
       FIG.  4    illustrates a flowchart of an example method  400  of sparse depth estimation. The operations of  FIG.  4    can be performed by one or more processors, such as one or more processors of the various computing devices/systems described herein, such as by agricultural knowledge system  104 , particularly edge agricultural knowledge system  104 B. For convenience, operations of method  400  will be described as being performed by a system configured with selected aspects of the present disclosure. Other implementations may include additional operations than those illustrated in  FIG.  4   , may perform step(s) of  FIG.  4    in a different order and/or in parallel, and/or may omit one or more of the operations of  FIG.  4   . 
     At block  402 , the system, e.g., by way of matching module  118 , may compare one or more spatial aspects of a first visual annotation of a first candidate plant trait instance depicted in a first stereo image (e.g.,  226 ) to one or more spatial aspects of a second visual annotation of a second candidate plant trait instance depicted in a second stereo image (e.g.,  228 ). For example, matching module  118  may compare a width, height, area, and/or location of a bounding box in the first stereo image to corresponding spatial aspects of one or more candidate bounding boxes in the second stereo image. 
     In some implementations where each stereo image includes multiple visual annotations, matching module  118  may use bipartite graph matching to identify matching visual annotations. In some implementations, the edges of such a graph may correspond to similarity measures. In some implementations, those similarity measures (or similarity measures used outside of bipartite matching) may correspond to Euclidean distances or cosine similarities between embeddings generated from the visual annotations and/or the pixels they annotate. 
     Back to  FIG.  4   , at block  404 , the system may match one or more additional phenotypic traits of a plant depicted in the first stereo image having the first candidate plant trait instance to one or more additional phenotypic traits of a plant depicted in the second stereo image having the second candidate plant trait instance. This additional matching may be used as an additional matching signal to increase the confidence that the plant trait instances are, in fact, matching. For example, two different fruits may be at similar locations and may have similar sizes, which would lead to their bounding shape annotations having similar spatial aspects, and hence, being difficult to disambiguate. However, one of the fruits may be ripe (or mature, moist, etc.) whereas the other may be unripe. In such a case, annotations associated with the ripe fruit may be matched to each other and annotations associated with the unripe fruit may be matched to each other. At block  406 , based on the comparing of block  402  and the matching of block  404 , matching module  118  may determine that the first and second candidate plant trait instances are a match for the same plant trait instance across the first and second stereo images. 
     At block  408 , the system, e.g., by way of matching module  118  or depth module  119 , may determine a disparity between the first and second candidate plant trait instances. For example, depth module  119  may identify a pixel displacement or shift between the matched visual annotation in the first stereo image and the matched visual annotation in the second stereo image. Based on the disparity, at block  410 , the system may estimate a depth associated with the same plant trait instance, e.g., using triangulation, equation (1) above, etc. 
     At block  412 , the system may cause output to be provided at a computing device, such as client device  106   1 . In various implementations, wherein the output may be generated based on the depth associated with the same plant trait instance. For example, the depth may be presented to the user as an addendum to the existing visual annotation. Additionally or alternatively, the depth may be presented to the user as part of a whole depth map that is presented to the user. 
     While not depicted in  FIG.  4   , in some implementations, the depth determined at block  410  may be used for other purposes. In various implementations, a depth map generated using techniques described herein may be used to dynamically adjust various hardware and/or software parameters of a vision sensor or another sensor affixed to an agricultural vehicle. For example, as a tractor carries computing node  108   3  (with an integral stereoscopic vision sensor such as  336 ) through a field, a depth map/distribution of particular plant traits may be repeatedly and/or continuously generated and monitored. If that depth distribution becomes too skewed one way or another, edge agricultural knowledge system  104  may generate and issue command(s) to various sensors/devices to adjust parameters such as focal length or zoom. Similarly, if that depth distribution becomes noisy or the individual depths are associated with low confidence measures, other parameters may be adjusted as well, such as aperture speed, use of flash, framerate, resolution, etc. 
       FIG.  5    illustrates a flowchart of another example method  500  of sparse depth estimation. The operations of  FIG.  5    can be performed by one or more processors, such as one or more processors of the various computing devices/systems described herein, such as by agricultural knowledge system  104 , particularly edge agricultural knowledge system  104 B. For convenience, operations of method  500  will be described as being performed by a system configured with selected aspects of the present disclosure. Other implementations may include additional operations than those illustrated in  FIG.  5   , may perform step(s) of  FIG.  5    in a different order and/or in parallel, and/or may omit one or more of the operations of  FIG.  5   . 
     Many operations of method  500  are similar to those of method  400 . For example, blocks  502 - 504  of method  500  may correspond to blocks  402  and  406  of method  400  (the matching of block  404  could be included in method  500  but is omitted in  FIG.  5    for the sake of brevity). The difference is that in method  500 , multiple plant traits are being considered in tandem to determine spatial attributes of plant-part-of-interest. For example, at block  506 , the system, e.g., by way of matching module  118 , may compare one or more spatial aspects of a third visual annotation (e.g., any of  340   1-6  in  FIG.  3   ) of a third candidate plant trait instance depicted in the first stereo image (e.g.,  226 ) to one or more spatial aspects of a fourth visual annotation (e.g., any of  342   1-6 ) of a fourth candidate plant trait instance depicted in a second stereo image (e.g.,  228 ). 
     Similar to block  406  and  504 , at block  508 , the system, e.g., by way of matching module  118 , may determine that the third and fourth candidate plant trait instances are a match for an additional same plant trait instance across the stereo images. And similar to block  408  of  FIG.  4   , at block  510 , the system, e.g., by way of matching module  118  or depth module  119 , may determine a disparity between matched plant trait instances. 
     Based on the disparity, at block  512 , the system, e.g., by way of depth module  119 , may estimate depths associated with the same plant trait instance and the additional same plant trait instance, similar to block  410  of  FIG.  4   . At block  514 , the system, e.g., by way of depth module  119 , may compare the depths to determine a spatial dimension of a plant-part-of-interest. In  FIG.  3   , for instance, the depth (or 3D coordinate) of the top  340  of a head of wheat may be compared to a depth (or 3D coordinate) of the bottom  342  of the head of wheat to determine a length of the head of wheat. At block  516 , the system may cause output to be provided, wherein the output is generated based on the spatial dimension determined at block  514 . 
       FIGS.  6 A and  6 B  demonstrate one example of how stereo imagery captured by a stereoscopic camera  636  deployed in a field can be processed to perform sparse depth estimation, which in turn can be used to estimate plant height. In this example, stereoscopic camera  636  has a field of view that is tilted relative to the ground, such that a z-axis represented by dashed line  664  is at an oblique angle to the ground. A y-axis that is perpendicular to the z-axis  664  is represented by dashed line  668 . 
     In this example, it can be assumed that the height of the plants are measured based on the exclusion of the wheat head (or tassel as the case may be). Accordingly, imagery captured by stereoscopic camera  636  is processed using a machine learning model such as a CNN to detect and annotate the bottoms of the wheat heads with key points  642   1-3 . Key points  642   1-2  are associated with two plants that are at roughly similar distances along the z-axis  664  from stereoscopic camera  636 , e.g., by virtue of their being in the same row of a field. The third plant associated with the key point  642   3  is further away from stereoscopic camera  636  along the z-axis, e.g., because it belongs to a different row. A dashed arrow  670  is shown passing from one of the lenses of stereoscopic camera  636  through second key point  642   2  to y-axis  668 . Dashed arrow  670  illustrates how a y-coordinate of second key point  642   2  corresponds to the double-headed arrow  672 . 
     To determine a height of the second plant that corresponds to second key point  642   2 , and for illustrative purposes, the coordinate system ( 664 ,  668 ) depicted in  FIG.  6 A  may be rotated such that z-axis  664  is parallel to the ground, and y-axis  668  is perpendicular to the ground, as indicated by the white arrow. The result is shown in  FIG.  6 B . In  FIG.  6 B , it becomes clear that the height of the second plant (excluding the wheat head) can be determined (using addition or subtraction depending on signs employed for the y-axis) based on y-coordinate  662  and a known height H of stereoscopic camera  636 . 
     In addition, as noted above, agricultural imagery tends to be noisy and dense. To ease computational burden and/or to prevent a user from being inundated with too much information, in various implementations, a z-axis filter may be applied that excludes annotations outside of some range. For example, in  FIGS.  6 A-B , third key point (and other key points detected on plants in the same row) may be excluded from being presented to a user (or otherwise used) by virtue of its z-coordinates falling outside of some range of z-coordinates. This range of permissible z-coordinates may otherwise capture plants in the first row that contains the plants with key points  642   1-2 . 
       FIG.  7    is a block diagram of an example computing device  710  that may optionally be utilized to perform one or more aspects of techniques described herein. Computing device  710  typically includes at least one processor  714  which communicates with a number of peripheral devices via bus subsystem  712 . These peripheral devices may include a storage subsystem  724 , including, for example, a memory subsystem  725  and a file storage subsystem  726 , user interface output devices  720 , user interface input devices  722 , and a network interface subsystem  716 . The input and output devices allow user interaction with computing device  710 . Network interface subsystem  716  provides an interface to outside networks and is coupled to corresponding interface devices in other computing devices. 
     User interface input devices  722  may include a keyboard, pointing devices such as a mouse, trackball, touchpad, or graphics tablet, a scanner, a touch screen incorporated into the display, audio input devices such as voice recognition systems, microphones, and/or other types of input devices. In some implementations in which computing device  710  takes the form of a HMD or smart glasses, a pose of a user&#39;s eyes may be tracked for use, e.g., alone or in combination with other stimuli (e.g., blinking, pressing a button, etc.), as user input. In general, use of the term “input device” is intended to include all possible types of devices and ways to input information into computing device  710  or onto a communication network. 
     User interface output devices  720  may include a display subsystem, a printer, a fax machine, or non-visual displays such as audio output devices. The display subsystem may include a cathode ray tube (CRT), a flat-panel device such as a liquid crystal display (LCD), a projection device, one or more displays forming part of a HMD, or some other mechanism for creating a visible image. The display subsystem may also provide non-visual display such as via audio output devices. In general, use of the term “output device” is intended to include all possible types of devices and ways to output information from computing device  710  to the user or to another machine or computing device. 
     Storage subsystem  724  stores programming and data constructs that provide the functionality of some or all of the modules described herein. For example, the storage subsystem  724  may include the logic to perform selected aspects of methods  400  and  500  described herein, as well as to implement various components depicted in  FIGS.  1  and  2   . 
     These software modules are generally executed by processor  714  alone or in combination with other processors. Memory  725  used in the storage subsystem  724  can include a number of memories including a main random access memory (RAM)  730  for storage of instructions and data during program execution and a read only memory (ROM)  732  in which fixed instructions are stored. A file storage subsystem  726  can provide persistent storage for program and data files, and may include a hard disk drive, a floppy disk drive along with associated removable media, a CD-ROM drive, an optical drive, or removable media cartridges. The modules implementing the functionality of certain implementations may be stored by file storage subsystem  726  in the storage subsystem  724 , or in other machines accessible by the processor(s)  714 . 
     Bus subsystem  712  provides a mechanism for letting the various components and subsystems of computing device  710  communicate with each other as intended. Although bus subsystem  712  is shown schematically as a single bus, alternative implementations of the bus subsystem may use multiple busses. 
     Computing device  710  can be of varying types including a workstation, server, computing cluster, blade server, server farm, or any other data processing system or computing device. Due to the ever-changing nature of computers and networks, the description of computing device  710  depicted in  FIG.  7    is intended only as a specific example for purposes of illustrating some implementations. Many other configurations of computing device  710  are possible having more or fewer components than the computing device depicted in  FIG.  7   . 
     Various implementations described thus far have related to sparse depth estimation using spatial aspects of visual annotations. Sparse depth estimation significantly reduces the search space for finding matching features across stereoscopic images, especially where those images depict dense feature spaces such as plants with myriad leaves or blades (e.g., grass, wheat, etc.). 
     Dense depth estimation using similarly dense feature spaces, on the other hand, can be difficult for a variety of reasons, not the least of which is the lack of training data. Manually labeling matching features across stereoscopic images may not be feasible when dealing with images of grass or other densely distributed and homogenous visual features. Accordingly, implementations are described herein for generating synthetic training data that includes synthetic pairs of stereoscopic images with visual features that are matched across the images based on a priori knowledge. 
       FIG.  8    illustrates a flowchart of an example method  800  for generating synthetic stereoscopic training images that are labeled automatically. The operations of  FIG.  8    can be performed by one or more processors, such as one or more processors of the various computing devices/systems described herein, such as edge agricultural knowledge system  104 B or central agricultural knowledge system  104 A. For convenience, operations of method  800  will be described as being performed by a system configured with selected aspects of the present disclosure. Other implementations may include additional operations than those illustrated in  FIG.  8   , may perform step(s) of  FIG.  8    in a different order and/or in parallel, and/or may omit one or more of the operations of  FIG.  8   . 
     At bock  802 , the system may generate one or more three-dimensional synthetic plants in a three-dimensional space. In various implementations, the one or more three-dimensional synthetic plants may include homogenous and densely-distributed synthetic plant parts. These homogenous and densely-distributed synthetic plant parts may include, for instance, individual blades of grass, individual leaves of the one or more three-dimensional synthetic plants, individual stems or culms of the one or more three-dimensional synthetic plants, individual flowers or pedicles of the one or more three-dimensional synthetic plants, individual peduncles of the one or more three-dimensional synthetic plants, and so forth. One example of how three-dimensional synthetic plants may be generated is depicted in  FIG.  9   . 
     At block  804 , the system may project the plurality of three-dimensional synthetic plants onto a two-dimensional plane from a first perspective in the three-dimensional space to form a first synthetic stereoscopic image of the pair. Similarly, at block  806 , the system may project the plurality of three-dimensional synthetic plants onto another two-dimensional plane from a second perspective in the three-dimensional space to form a second synthetic stereoscopic image of the pair. The second perspective may be a location that is selected based on, for instance, an interaxial distance  339  between lenses  337 A,  337 B of a stereoscopic vision sensor  336 . For example, the first perspective may be assumed to be the first lens  337 A, and the second perspective may be separated from the first perspective by the interaxial distance  339 . 
     At block  808 , the system may annotate the first and second synthetic stereoscopic images to create an a priori mapping between the individual synthetic plant parts across the first synthetic stereoscopic images. In some implementations, this may include annotating each pixel, or groups of pixels, with identifying information that is duplicated across the images in order to match features. Notably, these visual features are known to be matches because they were generated synthetically as part of the aforementioned three-dimensional plants. When generating each three-dimensional plant, each visual feature, no matter how small or homogeneous, can be tracked using a priori knowledge across the pair of two-dimensional projections. 
     At block  810 , the system may train a feature matching machine learning model, such as a CNN, based on the mapping. For example, the pair of synthetic stereoscopic images may be processed using the CNN to generate candidate predicted mappings between visual features. These candidate predicted mappings may then be compared to the a priori mapping created at block  808  to determine an error. This error may be used to train the feature matching machine learning model, e.g., using techniques such as gradient descent, back propagation, etc. Once sufficiently trained, the feature matching machine learning model may be used, alone or in combination with the sparse feature matching techniques described herein, to perform depth estimation. 
       FIG.  9    depicts an example process flow for generating synthetic stereoscopic training data in accordance with various implementations described herein. Other process flows that differ from that depicted in  FIG.  9    are contemplated. Starting at top left, plant model resources  930  may include data, parameters, and/or any other information about plants that may be usable to generate realistic synthetic plant models in three dimensions (3D). While the following description will assume the use of a 3D simulator  944 , this is not meant to be limiting and techniques described can be employed with 2D plant simulation. 
     Plant model resources  930  may include, for instance, textures  932  (for leaves, stems, roots, etc.), various functions  934 , contours  936  for leafs, stems, roots, and/or other aspects of plants, materials  938 , and miscellaneous parameters  940  such as leaf sizes, scale ratios, desired resolution, etc. Functions  934  may include, for instance, programmatic routines, computer-readable instructions, scripts, pseudo-code, algorithms, heuristics, statistics, etc., that are used, e.g., by an object generator  942  or 3D simulator  944 , to generate 3D plant models. Materials  938  may include, for instance, data similar to texture data that is used to simulate different types of plant structures, such as stems, leaves, pedicles, peduncles, etc. 
     A series of arrows A-M is depicted in  FIG.  9    to demonstrate one example of how data may be processed in order to generate a pair of synthetic stereoscopic training images  954 , and how those synthetic stereoscopic training images  954  may then be used to train one or more machine learning models stored in machine learning model database  120 . At arrow A, object generator  942  (which may be implemented using any combination of hardware and computer-readable instructions) may randomly select various assets from 3D model resources  930 . 
     This selection may be random or pseudo random in order that each 3D model plant that is generated for inclusion in training data is different from each other, as would be the case in real life. In some implementations, the random or pseudo random selection may be based at least in part on other data  952 , which may include environmental features (e.g., time-series data) associated with the agricultural area under consideration. For example, assets that are more consistent with the environmental features of the agricultural area may be weighted more heavily (and hence, more likely to be selected) than other assets that are less consistent with the environmental conditions. These randomly-selected assets may be returned to object generator  942  at arrow B. 
     At arrow C, the randomly-selected assets and/or an object representative thereof (e.g., an XML file that spells out how many leaves, their size, a size of the plant, a color of the plant, parameters of a targeted plant disease, etc.) may be provided to 3D simulator  944 , which may be implemented using any combination of hardware or computer-readable instructions. Based on these objects and/or the assets represented/conveyed by these objects, 3D simulator  944  may generate synthetic plant model(s) that simulate targeted plants, including plants having densely-distributed and homogenous visual features. For example, to train a machine learning model to match blades of grass across pairs of stereoscopic images, 3D simulator  944  may generate a plurality of 3D model grass plants. 3D simulator  944  may return the generated 3D models to object generator  942  at arrow D or may provide them directly to another downstream component. 
     In some implementations, the other data  952  may be used, e.g., by object generator  942  and/or 3D simulator  944 , to generate the objects/3D models. For example, as described previously, the other data  952  may affect probabilities of constituent assets of resources  930  being selected stochastically. The agricultural area of interest may be, for instance, a field or plot of which one or more digital images have been captured, e.g., by one or more robots  108   1-M . These digital images may effectively be used as canvases on which 3D models can be graphically incorporated (e.g., projected) to generate the pair of synthetic stereoscopic digital images  954 . In fact, these digital images, prior to being augmented with synthetic plants, may be referred to herein as “canvas” digital images. In some implementations, synthetic plants may be graphically incorporated into fully synthetic/simulated scenes, in addition to or instead of digital images capturing “real” agricultural areas. 
     In some implementations, other data  952  may include, for instance, an agricultural history of the agricultural area  112  during a time interval, e.g., a time interval that precedes capture of image data that depicts the agricultural area. This agricultural history may include a wide variety of data about the agricultural area being depicted, such as precipitation data, stewardship data, soil data, weather data, fertilizer data, herbicide data, pest data, plant disease data, etc. In some implementations, agricultural history and/or other data points within other data  952  may be simulated entirely, simulated based on ground truth data, simulated randomly, etc. 
     In some implementations, the other data  952  may include time-series data corresponding to one or more environmental conditions of the agricultural area, such as daily precipitation, daily sun exposure, periodic fertilizer application, periodic temperature readings, periodic soil composition measurements, periodic soil pH levels, and/or pesticide/fungicide applications, to name a few. In some such implementations, object generator  942  and/or 3D simulator  944  may employ a time-series machine learning model such as a recurrent neural network (“RNN”), a long short-term memory (“LSTM”), a gated recurrent unit (“GRU”) network, etc., to process this time-series data and generate 3D synthetic plants at various stages of growth. For example, a new 3D version of a synthetic plant at a given stage of growth may be generated at each iteration of an RNN or LSTM based on the previous iteration&#39;s output and the current iteration&#39;s corresponding time series data. 
     A time-series machine learning model such as an RNN, LSTM, or GRU-based network may be trained to generate 3D plant models in various ways. For example, a temporal sequence of ground truth digital images depicting examples of a plant to be modeled may be captured, e.g., from a controlled test garden. Other data  952 , such as time-series data indicative of environmental conditions of the controlled test garden when the images were captured may also be obtained. The environmental conditions time-series data may then be iteratively applied as inputs across the time-series machine learning model to generate, at each iteration, output indicative of expected growth progression. In some implementations, the output of a previous iteration may also be applied as input across the time-series model, in effect providing the time-series model with memory. 
     In some implementations, the output generated at each iteration may include 3D plant models of the plant(s) at that point in time. In other implementations the output may include predicted features of plant(s) at that point in time, such as leaf size, number of leaves per plant, plant height, leaf color, fruit density, etc. In either case, the output may be compared to ground truth attribute(s) of plants depicted in the corresponding ground truth digital image of the temporal sequence. The difference or error between the output and the ground truth may be used to train the time-series machine learning model, e.g., using techniques such as back propagation, gradient descent, etc. 
     As a consequence of utilizing other data  952 , especially with time-series machine learning models, the resultant 3D plant models may be highly realistic, particularly in the context of an agricultural area  112  depicted in a digital image onto which those 3D plant models are to be projected. For example, if the environmental conditions in agricultural area  112  have been favorable to a type of plant being modeled, then the 3D plant models may correspond to healthy plants, e.g., with robust color (e.g., deep and uniform green), numerous and/or dense leaves/foliage, strong/numerous branches, etc. By contrast, if the environmental conditions in the agricultural area have been unfavorable for the type of plant being modeled, the resultant 3D plant models may correspond to less healthy plants, e.g., that are smaller and/or include less leaves, and/or that have a less healthy color (e.g., brown spots or grayish color). 
     In some implementations, object generator  942  and/or 3D simulator  944  may take into account conditions at the time a digital image of the agricultural area  112  of interest—and onto which the 3D plant models are to be projected—was captured. For example, the digital image may depict the agricultural area  112  under a lighting condition that existed in the agricultural area when a camera captured the digital image. The sun may be out or occluded by clouds. Depending on the time of day and/or time of year, the sun may be at different positions in the sky. Consequently, the agricultural area may be illuminated in different ways with different amounts of light. Moreover, the sun&#39;s position in the sky at the time the image was captured may cause objects such as plants to cast shadows of various shapes and sizes. Therefore, in some implementations, other data  952  may include attribute(s) of the lighting condition at the time the digital image of the agricultural area was captured. This lighting condition data may be conveyed in the image itself or extracted and included, for instance, as metadata that accompanies the image. 
     In some implementations, object generator  942  and/or 3D simulator  944  may render 3D plant models to include visual features that are dictated based on the lighting condition. For example, leaves may exhibit one hue under a cloudy sky, another under a noon sun, another at dawn/dusk, and/or cast various types of shadows at various times of day and/or in various lighting conditions, etc. And depending on the textures of the 3D plant models, their leaves and/or other structure may reflect various amounts of sunlight. Additionally or alternatively, in some implementations, an observed wetness of the area may be accounted for. For example, if the digital image was captured during or soon after a rain storm, plants in the area may have a shimmer due to their wetness. The presence of such a shimmer may be determined, e.g., from the image itself or from environmental data associated with the agricultural area, and used to create a similar shimmer on 3D plant models. 
     Referring back to  FIG.  9   , in some implementations, at arrow E, object generator  942  may provide the 3D plant model(s) to a model convertor  946 . As with other components in  FIG.  9   , model convertor  946  may be implemented using any combination of hardware and/or computer-readable instructions. At arrow F, model converter  946  may be configured to convert the 3D models into various formats of 3D models  948 , some of which may be standardized formats for representing 3D objects, such as *.obj, *.mtl, *.urdf, and so forth. In other implementations, model convertor  946  may be omitted. 
     At arrow G, the 3D plant models  948  may be provided (from model convertor  946 , object generator  942 , or even 3D simulator  944 ) to a synthetic training image generator  950 . Synthetic training image generator  950 , which again may be implemented using any combination of hardware and/or computer-readable instructions, may be configured to carry out selected aspects of the present disclosure to generate synthetic training data, such as a pair of synthetic stereoscopic training images  954 . For example, at arrow H, one or more images may be retrieved from an imagery database  947 . These images may depict agricultural area(s) (e.g.,  112 ) under a variety of conditions. For example, multiple images may capture the same agricultural area under a cloudy sky, during a rain storm, on a sunny day, on a foggy day, at different times of day (at which points the sun will be at different positions in the sky), and so forth. 
     In addition to the imagery retrieved from database  947 , in some implementations, at least some of the other data  952  mentioned previously may also be retrieved and used by synthetic training image generator  950  for use in generating synthetic stereoscopic training images  954 , particularly for graphically incorporating (e.g., projecting) the 3D plant models into the canvas digital images that depict the agricultural area  112 . This may include, for example, the other data  952  that coincides with the agricultural area(s) depicted in the imagery, such as environmental conditions leading up to and including the moment at which the imagery was captured. This may also include the lighting condition mentioned previously. For example, synthetic training image generator  950  may render adjacent 3D plant models on the two-dimensional canvas image and shadows that simulate real life shadows that would be present if the 3D plant models were actually present in the agricultural area  112 . 
     As shown in  FIG.  9    and mentioned previously, synthetic training image generator  950  may be configured to obtain a digital image that captures an area from database  947 . Based on this lighting condition at the time the image was captured and/or based on an agricultural history of the area, synthetic training image generator  950  may generate a pair of synthetic stereoscopic training images  954  to depict the plurality of 3D synthetic plants in the agricultural area from two different perspectives. In some implementations, the generating may include graphically incorporating the plurality of 3D plant models into the digital image capturing the agricultural area  112 , taking into account the lighting condition at the time the underlying digital image was captured. 
     In some implementations, and as shown in the dashed exploded view, synthetic training image generator  950  may employ a generator model  951  that is trained as part of a larger generative adversarial network (“GAN”)  949  to generate the pair of synthetic stereoscopic training images  954  that are as realistic as possible. Such a GAN  949  may also include a discriminator model  953  that is trained to generate output that predicts whether an input image is, for instance, “synthetic” or “real.” The generator model  951  and discriminator model  953  may be trained in tandem. The discriminator model  953  may be used to process both real images (e.g., captured using a camera) and synthetic images generated by the generator model  951 . The fact that the discriminator model  953  correctly classifies an image as synthetic may be used as a negative training example that causes the generator model  951  to improve its own performance. When the discriminator model  953  incorrectly classifies a “real” image as synthetic, the discriminator model  953  may be trained to improve its own performance. The generator model  951  may be considered to be sufficiently trained when it is able to “fool” the discriminator model  953 , e.g., more than some threshold number of times. 
     At arrow I, synthetic training image generator  950  may output the pair of synthetic stereoscopic training images  954 . At arrow J, an inference module  116  (e.g., the same as or sharing characteristic(s) with inference modules  116 A-B of  FIG.  1   ) may apply the synthetic training images  954  as input across one or more machine learning models (e.g., CNN) from machine learning model database  120 . As shown by the arrow, in some implementations, especially where the plant disease detection machine learning model is a time-series model, at least some of the other data  952  may also be applied across the machine learning model. 
     Meanwhile, at arrow K, the pair of synthetic stereoscopic training images  954  (and in some cases, other data  952 ) may also be provided to a training module  956 . Because the pair of synthetic stereoscopic training images  954  are generated using 3D synthetic plants that are graphically incorporated at known pixel locations, they may comprise annotated or labeled training data that can compared by training module  956  with the output generated by inference module  116  at arrow L (e.g., pixel-wise annotations of visual features that are predicted to match). The error between the output of inference module  116  and the annotations/labels can be used by training module  956  to train the machine learning model, which is updated in database at arrow M. 
       FIG.  10    illustrates a flowchart of an example method  1000  of performing depth estimation. The operations of  FIG.  10    can be performed by one or more processors, such as one or more processors of the various computing devices/systems described herein, such as by agricultural knowledge system  104 , particularly edge agricultural knowledge system  104 B. For convenience, operations of method  1000  will be described as being performed by a system configured with selected aspects of the present disclosure. Other implementations may include additional operations than those illustrated in  FIG.  10   , may perform step(s) of  FIG.  10    in a different order and/or in parallel, and/or may omit one or more of the operations of  FIG.  10   . 
     At block  1002 , and similar to block  402  of  FIG.  4   , the system may compare one or more spatial aspects of a first visual annotation of a first candidate plant trait instance depicted in a first stereo image to one or more spatial aspects of a second visual annotation of a second candidate plant trait instance depicted in a second stereo image. Based on the comparing of block  1002 , at block  1004 , the system may calculate an annotation-based similarity score between the first and second candidate plant trait instances across the first and second stereo images. 
     This annotation-based similarity score may be a metric that indicates how likely the first and second candidate plant trait instances are to be a match across the pair stereo images. It is not calculated directly based on the underlying visual features. Rather, the annotation-based similarity score is calculated by comparing annotations of visual features. The annotation-based similarity score may be used, for instance, by matching module  118  when processing annotated left and right images  226 ′ and  228 ′ as depicted in  FIG.  2    to identify matched plant traits  230 . In that context, if two plant traits have an annotation score that satisfies a threshold, they may be considered a match; otherwise they may be considered not a match, or an inconclusive match. 
     At block  1006 , the system may process the first and second stereo images based on a feature matching machine learning model (e.g., trained previously using the technique depicted in  FIG.  8   ) to determine a feature-based similarity score between the first and second candidate plant trait instances across the first and second stereo images. 
     Based on the annotation based similarity score and the feature-based similarity score, at block  1008 , the system may determine that the first and second candidate plant trait instances are a match for the same plant trait instance across the first and second stereo images. In some implementations, the operations of block  1008  may include performing bipartite graph matching, e.g., using the Hungarian algorithm, to match visual features across the pair of stereoscopic images. In some such implementations, each edge of the bipartite graph may represent a composite similarity score that is calculated (e.g., as a weighted or non-weighted average) based on both the annotation-based similarly score calculated at block  1004  and the feature-based similarity score calculated at block  1006 . 
     Unlike the annotation-based similarity score, this feature-based similarity score is not based on annotation similarity across images, but instead is based directly on visual feature similarity across images. For example, a feature similarity score may be calculated between two candidate blades of grass across stereo images. Calculation of this feature-based similarity score may be facilitated by the training techniques described in  FIGS.  8 - 9   , where densely-distributed and homogeneous features such as blades of grass can be labeled automatically, as having humans label such features may be impractical. 
     In other implementations, the annotation-based similarly score and the feature-based similarity score may be weighted based on factors such as confidence measures associated with the annotation(s), quality metrics associated with the synthetic stereoscopic images, etc. In some implementations, the annotation-based similarly score and the feature-based similarity score may be used to corroborate and/or refute each other. For instance, if an annotation-based similarly score strongly suggests that two visual features match across images, but the feature-based similarity score suggests the opposite, then those two visual features may not be matched, or may be matched with a flag or warning that the match is inconclusive. As another example, suppose one stereo image has multiple candidate features that potentially match a feature of another stereo image, e.g., because the multiple candidate features all have relatively high annotation-based similarity scores. In such a scenario, the feature-based similarity score may be used to select from the multiple candidate features (e.g., break a tie). 
     Referring back to  FIG.  10   , at block  1010 , the system may determine a disparity between the first and second candidate plant trait instances, similar to block  408  of  FIG.  4   . At block  1012 , based on the disparity, the system may estimate a depth associated with the same plant trait instance, similar to block  410 . At block  1014 , the system may cause output to be provided at a computing device, wherein the output is generated based on the depth associated with the same plant trait instance (similar to block  412 ). 
     While several implementations have been described and illustrated herein, a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein may be utilized, and each of such variations and/or modifications is deemed to be within the scope of the implementations described herein. More generally, all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific implementations described herein. It is, therefore, to be understood that the foregoing implementations are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, implementations may be practiced otherwise than as specifically described and claimed. Implementations of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein.