Patent Publication Number: US-2023150127-A1

Title: Optimizing policy controllers for robotic agents using image embeddings

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is a continuation (and claims the benefit of priority under 35 USC 120) of U.S. patent application Ser. No. 16/649,596, filed Mar. 20, 2020, which is a U.S. National Phase Application under U.S.C. § 371 of International Application No. PCT/US2018/052078, filed Sep.20, 2018, which claims priority to U.S. Provisional Patent Application Ser. No. 62/561,133, filed Sep. 20, 2017, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     This specification relates to reinforcement learning. 
     In a reinforcement learning system, an agent interacts with an environment by performing actions that are selected by the reinforcement learning system in response to receiving observations that characterize the current state of the environment. 
     Some reinforcement learning systems select the action to be performed by the agent in response to receiving a given observation in accordance with an output of a neural network. Neural networks are machine learning models that employ one or more layers of nonlinear units to predict an output for a received input. Some neural networks include one or more hidden layers in addition to an output layer. The output of each hidden layer is used as input to the next layer in the network, i.e., the next hidden layer or the output layer. Each layer of the network generates an output from a received input in accordance with current values of a respective set of parameters. 
     SUMMARY 
     This specification describes a system implemented as computer programs on one or more computers in one or more locations that optimizes a policy controller that is used to select actions to be performed by a robotic agent interacting with an environment. In particular, the policy controller can be used to select actions so that the robotic agent can successfully perform a robotic task, e.g., an object grasping task, an object moving task, a navigation task, or another task that requires the agent to interact with the real-world environment for some specific purpose. In some cases, the policy controller is a trajectory-centric controller, e.g., a time-varying Gaussian controller. In other cases, the policy controller is a deep neural network. 
     The subject matter described in this specification can be implemented in particular embodiments so as to realize one or more of the following advantages. The system as described in this specification can make use of a neural network that has been trained to generate numeric embeddings that are invariant to certain transformations, e.g., invariant to transformations such as viewpoint, occlusions, motion-blur, lighting, background or object instances, to effectively optimize a policy controller to select actions to be performed by a robotic agent. Accordingly, the system can optimize the controller using only raw video demonstrations for supervision, i.e., without any explicit joint-level correspondence or other labeled data. In particular, the system can both train the time contrastive neural network that generates the embeddings and optimize the policy controller using only raw video data. Learned invariance to factors such as view transformation can improve robotic performance in imitating motion by another agent, for example. In particular, the described system can optimize a policy controller to control the agent to perform a task only from third-person images of another agent performing the task, even though only first-person images taken by the robotic agent are available while the robotic agent performs the task. That is, the system can effectively optimize the policy controller even when the viewpoint of the demonstration images is different from the viewpoint of the images captured by the robotic agent while the agent performs the task. 
     The details of one or more embodiments of the subject matter of this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows an example reinforcement learning system. 
         FIG.  2    shows an example training system configured to train a time-contrastive neural network. 
         FIG.  3    shows an example triple of observations captured by two different modalities. 
         FIG.  4    shows another example triple of observations including an anchor observation, a positive observation, and a negative observation captured by a single modality. 
         FIG.  5    is a flow diagram of an example process for training a time-contrastive neural network. 
         FIG.  6    is a flow diagram of an example process for optimizing a policy controller. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG.  1    shows an example reinforcement learning system  100  that optimizes a policy controller  110  used to control a robotic agent  112 . The reinforcement learning system  100  is an example of a system implemented as computer programs on one or more computers in one or more locations in which the systems, components, and techniques described below are implemented. The robotic agent  112  may be, e.g., a robotic arm or a more complex robot. 
     The policy controller  110  is a controller that is used to select actions to be performed by the robotic agent  112  as the robotic agent  112  interacts with an environment. In particular, the policy controller  110  receives data characterizing the current state of the environment and generates an output that defines an action to be performed by the robotic agent  112 . The data can be features of the current state and the output can define a control input for the robotic agent, e.g., torques to be applied to one or more joints of the robotic agent. For example, the output can be, for each joint, the parameters of a distribution from which the torque can be sampled. 
     In some cases, the features of the current state are low-dimensional features, e.g., a low-dimensional feature vector or feature matrix, characterizing the state of the environment and the policy controller  110  is a trajectory-centric controller, e.g., a time-varying Gaussian controller. For example, the features can include for each joint of the robotic agent  112  a respective current joint angle and current joint velocity. In some cases, the features of the current state can also include features, e.g., a feature vector, characterizing the environment external to the agent. For example, the feature vector can be an embedding of an image of the environment captured by a camera of the agent  112 , e.g., an embedding generated by a time-contrastive neural network  130  as will be described in more detail below. 
     In some other cases, the features of the current state are high-dimensional features, e.g., images captured by the robotic agent  112  as the agent interacts with the environment, and the policy controller  110  is a deep neural network, e.g., a convolutional neural network or a recurrent neural network. 
     For the policy controller  110  to control the robotic agent  112  means that the system  100  or another system causes the robotic agent  112  to perform the actions that are defined by the outputs of the policy controller  110 , i.e., by providing the control inputs to the robotic agent  112  or to a control system for the robotic agent  112 . 
     The system  100  optimizes the policy controller  110  so that the policy controller  110  can be used to control the robotic agent  112  to cause the robotic agent  112  to successfully perform a robotic task, e.g., an object grasping task, an object moving task (e.g., pouring water from one container to another or moving a container from one location to another), a navigation task, or another task that requires the agent to interact with the real-world environment for some specific purpose. 
     In particular, the system  100  optimizes the policy controller  110  using a demonstration sequence of demonstration images  120  of another agent, e.g., another robotic agent or a human demonstrator, performing a version of the specified task. That is, each of the demonstration images  120  are images of the other agent captured while the other agent performs the version of the specific task. Generally, the demonstration sequence will include images starting from when the other agent first begins to perform a task and ending with the other agent successfully completing the task. For example, the demonstration images  120  in the sequence may be captured at regular intervals while the demonstrating agent performs the task. The version of the specified task may be the same as the specified task to be performed by the robotic agent  112  or may differ in certain ways from the task to be performed by the robotic agent  112 . For example, if the task for which the controller is to be optimized is to move an object to a destination location in the environment, the version of the task performed in the demonstration images may move a somewhat different object from the object that the robotic agent  112  will be tasked with moving, e.g., an object that is a different color, has a different shape, or has other different properties from the object that the agent  112  will be tasked with moving. 
     The system  100  processes each demonstration image  120  in the demonstration sequence using a time contrastive neural network  130  to generate a respective demonstration embedding  132  of each of the demonstration images. 
     The time contrastive neural network  120  is a neural network that has been trained to receive an input image of the environment and to process the input image to generate a numeric embedding, e.g., a vector or other ordered collection of numeric values, of the input image that characterizes a state of the environment as depicted in the input image. Because of the way that the time contrastive neural network  130  has been trained, the numeric embeddings generated by the neural network  130  characterize properties of the scene depicted in an input image while being invariant to certain transformations, e.g., transformations such as viewpoint, occlusions, motion-blur, lighting, background or object instances. Training the time-contrastive neural network  130  to achieve this is described in more detail below with reference to  FIGS.  2 - 5   . 
     The system  100  can then iteratively update the policy controller  110  using the demonstration embeddings  132 . 
     In particular, at each iteration of optimization process, the system  100  obtains a robot sequence of robot images  140  of the robotic agent performing the specified task by performing actions selected using the current policy controller, i.e. the policy controller as of the current iteration. Each robot image  140  in the robot sequence corresponds to a respective demonstration image in the demonstration sequence, i.e., is taken at approximately the same time during performance of the task as one of the demonstration images. The system  100  can obtain the robot sequence by causing (or by having another system cause) the robotic agent  112  to perform actions defined by outputs of the current policy controller. 
     The system  100  then processes each robot image  140  in the robot sequence using the time contrastive neural network  130  to generate a respective robot embedding  142  for each of the robot images. 
     An optimization engine  150  then updates the current policy controller using the demonstration embeddings  132  and the robot embeddings  142 . In particular, the optimization engine  150  updates the current policy controller by performing an iteration of a reinforcement learning technique to optimize a reward function that depends on, for each demonstration image  120 , a distance between the demonstration embedding  132  of the demonstration image  120  and the robot embedding  142  of the corresponding robot image  140 . Updating the policy controller based on these distances is described in more detail below with reference to  FIG.  6   . 
     By iteratively updating the policy controller  110  in this manner, the system  100  causes distances between corresponding demonstration and robotic, or robot, embeddings to be reduced and performance on the specified task by the robotic agent  112  to be improved. In other words, the policy controller  110  is updated so that actions performed by the robotic agent  112  accurately “imitate” the actions performed by the demonstrating agent in the demonstration images  120 . Because the demonstrating agent successfully performed the task, accurately imitating the actions performed by the demonstrating agent causes the robotic agent  112  to successfully perform the task. 
     The system  100  can terminate this iterative updating once the performance of the robotic agent  112  on the robotic task is satisfactory (for example, the distances between corresponding demonstration and robotic embeddings satisfy one or more predetermined criteria) or when some other termination criteria are reached, e.g., a specified amount of time has elapsed. 
     As will be described below, the time contrastive neural network  130  has been trained in an unsupervised manner, i.e., entirely from unlabeled image sequences. The trained time contrastive neural network  130  is then used to provide the supervision for the optimization of the policy controller  110  without any external supervision being required. Accordingly, the robotic agent  112  can achieve high quality performance on the robotic task only from video or other image sequence data of another agent performing the task and without any external supervision. Efficiency of the process may therefore be improved. 
     Because of the properties of the numeric embeddings described above, e.g., invariance to viewpoint, the system  100  can effectively update the policy controller  110  even when the robot images  140  are captured from a different view than the demonstration images  120 . Utility of the system may therefore be improved. In the example of  FIG.  1   , the robot images  140  are from a first person view, i.e., a view captured by a camera of the robotic agent  112  as the agent  112  performs the task. The demonstration images  120 , on the other hand, are captured from a third person view, i.e., a view that a second agent who is observing the agent depicted in the demonstration images  120 , would see. In some other cases, however, the robot images and the demonstration images can be captured from the same viewpoint. 
     Once the controller  110  has been optimized, the system can use the optimized controller  110  to control the agent  112  as the agent performs the robotic task or can provide data specifying the optimized controller  110 , e.g., data specifying the parameters or weights of the optimized controller  110 , to another system for use in controlling the agent  112 . 
       FIG.  2    shows an example training system  200  that trains the time-contrastive neural network  130  using a gradient of a time-contrastive loss. The training system  200  can be implemented as computer programs on one or more computers in one or more locations. 
     In particular, as described above, the time-contrastive neural network  130  is configured to receive an input observation characterizing a state of an environment and to process the input observation to generate a numeric embedding of the state of the environment. The numeric embedding is an ordered collection of numeric values, e.g., a vector of floating point values or of quantized floating point values. An observation characterizing the state of an environment can be an image of the environment, or a frame taken from a video of the environment. In some implementations, the observation can also include other kinds of data collected from the environment, e.g., tactile sensor data or audio data. Generally, the environment includes an agent (e.g., a robotic agent or human) that is moving in the environment or is interacting with other objects in the environment to perform a specific task, e.g., robotic manipulation task, liquid pouring task, or robotic imitation task. 
     The system  200  can train the neural network  130  on training data that is collected while an agent interacts with the environment. The agent can be the robotic agent  112 , the demonstrating agent depicted in the demonstration images, or yet another agent. 
     In general, one the time-contrastive neural network  130  has been trained, the system  200  or the system  100  can use the trained time-contrastive neural network  130  to optimize the policy controller  110  of  FIG.  1   . 
     The time-contrastive neural network  130  includes a deep neural network  204  followed by an embedding neural network layer  206 . The embedding neural network layer  206  is a fully connected neural network layer, e.g., a fully connected layer with a number of units equal to the number of numeric values in the embeddings. 
     In some implementations, the deep neural network  204  may include part of an Inception model followed by one or more convolutional neural network layers, a spatial softmax layer and a fully connected layer. The part of an Inception model included in the deep neural network  204  can be an Inception model up until the “Mixed_5d” layer, which is shown in the open-source code available at https://github.com/tensorflow/models/blob/master/research/slim/nets/inception_v3.py. An example of an Inception model is described in detail in C. Szegedy et al. “Rethinking the inception architecture for computer vision.” CoRR, abs/2522.00567, 2025, the contents of which in regard to at least the Inception model are hereby incorporated by reference. For each feature dimension in an activation map received from previous layers (e.g. in a 209×209×32 activation map, the image is 209×209 and there are 32 feature dimensions), the spatial softmax layer is configured to output the spatial coordinates where the feature is maximally activated. An example of a spatial softmax layer is described in detail in C. Finn et al. “Learning visual feature spaces for robotic manipulation with deep spatial autoencoders.” CoRR, abs/2509.06293, 2025, the contents of which in regard to at least the spatial softmax layer are hereby incorporated by reference. 
     In some cases, the system  200  can train time-contrastive neural network  130  from scratch. In some other cases, part of the time-contrastive neural network  130  can be pre-trained and used as a baseline for further training (e.g., the deep neural network  204  may have pre-trained weights from an Inception model trained on an object recognition task). 
     As part of training the time-contrastive neural network  130 , the training system  200  obtains training input data including multiple “triples” of training observations. In some cases, the training observations include observations captured from multiple different viewpoints by multiple modalities. In other cases, the training observations include observations captured from a single viewpoint by a single modality. Generally, a modality specifies an input capturing device that captures observations (e.g., images, audios, or videos) characterizing states of an environment from a specific viewpoint. An input capturing device can be, for example, a camera, audio capturing device, video recorder, or tactile input capturing device. 
     In particular, in some implementations (hereafter referred to as multi-view implementations), each triple of observations includes (i) an anchor observation  208  captured by a first modality, (ii) a positive observation  210  that is co-occurring (i.e. occurring at substantially the same time) with the anchor observation  208  and that is captured by a second, different modality, and (iii) a negative observation  212  captured by the first modality that is not co-occurring with the anchor observation. In some cases, the observations are images, with the first modality being a camera at a first viewpoint and the second modality being a camera at a second, different viewpoint. In general, however, the first and second modalities can be two different modalities of the same or of different types, e.g., cameras, audio capturing devices, tactile input capturing devices, and so on. An example of anchor, positive, and negative observations captured by two modalities at two different viewpoints is illustrated in  FIG.  3    (described below). 
     In some implementations, the pair of first and second modalities are the same for all of the triples of observations included in the training input data. 
     In some implementations, some triples of observations in the training input data can be captured by the pair of first and second modalities, while some other triples of observations in the training input data are captured by a different pair of first and second modalities. That is, in the example where the modalities are cameras at various locations, the relative locations of the cameras that captured the positive and negative observations can vary across different triples. 
     In some other implementations (hereafter referred to as single-view implementations), each triple of observations includes (i) an anchor observation  208  captured by the modality, (ii) a positive observation  210  captured by the modality and being within a margin temporal neighborhood (or margin range) of the anchor observation  208 , and (iii) a negative observation  212  captured by the modality and being outside of the margin temporal neighborhood of the first observation, i.e., the negative observation  212  is within a negative range of the first observation. The margin range of the anchor observation  208  defines a predetermined time window around the anchor observation  208 . For example, assuming the anchor observation  208  is captured at the timestep t, the margin range of the anchor observation  208  includes observations that are captured from time step t−n to time step t+n, where n is a predetermined number of time steps. n can be a small number of time steps such as one, two, five, or ten seconds. In other words, the (margin) temporal neighborhood, or margin range, represents a predetermined time window defining a window, or period, of time, around the time at which the anchor observation was captured (i.e. a window of time around time step t). The positive observation  210  is captured by the modality within this predetermined time window (i.e. in this example is captured at a time step between t−n and t+n). The negative observation  212  is captured by the modality outside of this predetermined time window (i.e. in this example is captured either before time step t−n or after time step t+n). An example of anchor, positive, and negative observations captured by a single modality at a single viewpoint is illustrated in  FIG.  4    (described below). 
     The training system  200  trains the time-contrastive neural network  130  on the triples in the training input data to determine trained values of the parameters of the time-contrastive neural network  130  from initial values of the parameters by minimizing a “triplet loss”. The triplet loss depends on, for a given triple of observations, the distance between the embedding of the positive observation and the negative observation and the distance between the embedding of the positive observation and the anchor observation. In particular, in some implementations the system  200  trains the time-contrastive neural network  130  to minimize the following loss function: 
         L=Σ   i   N [∥ f (x i   a )− f (x i   p )∥ 2   2   −∥f (x i   a )− f (x i   n )∥ 2   2 +α] + ,
 
     where f( ) represents the time-contrastive neural network  130 , i.e., f(x) is an embedding generated by the time-contrastive neural network  130  for a given observation x. The time-contrastive neural network  130  may embed an observation x into an embedding space such as a d-dimensional Euclidean space. x i   a  is the anchor observation  208 , x i   p  is the positive observation  210 , and x i   n  is the negative observation. N is the number of triples of observations in the training input data. α is a constant value that defines the desired margin between the two distance in the loss function. 
     This triplet loss will also be referred to in this specification as a “mirror loss” or a “time-contrastive loss”. 
     In particular, for a given triple including anchor, positive, and negative observations, the training system  200  determines a gradient of the triplet loss and then uses the gradient to update the values of the parameters of the time-contrastive neural network  130 . 
     Specifically, the system  200  processes the anchor observation  208  using the time-contrastive neural network  130  in accordance with the current values of the network parameters to generate a first embedding  214 . The system  200  processes the positive observation  210  using the time-contrastive neural network  130  in accordance with the current values of the network parameters to generate a second embedding  216 . The system  200  processes the negative observation  212  using the time-contrastive neural network  130  in accordance with the current values of the network parameters to generate a third embedding  218 . 
     The system  200  then determines the triplet loss  220  from (i) a first distance between the first embedding  214  and the second embedding  216  and (ii) a second distance between the first embedding  214  and the third embedding  218 . 
     The system  200  can adjust the current values of the network parameters using conventional neural network training technique, e.g., a gradient descent-based training technique. For example, the system backpropagates the gradient of the objective function, i.e., of the triplet loss, to determine a respective updated value for each of the network parameters of the time-contrastive neural network  130 . 
     By updating the values of the network parameters to minimize the triplet loss, the system  200  ensures that the trained time-contrastive neural network  130  can generate numeric embeddings that are invariant to certain transformations, e.g., viewpoint, occlusions, motion-blur, lighting, background or object instances. Thus, the embeddings generated by the trained time-contrastive neural network  130  can be employed to improve performance on a variety of tasks that require an accurate characterization of the state of an environment, including the tasks described above with reference to  FIG.  1   . The invariance to transformations such as viewpoint, occlusions, motion-blur, lighting, background or object instances, can therefore improve performance of robotic agents. 
       FIG.  3    shows an example triple of observations including an anchor observation  302 , a positive observation  304 , and a negative observation  306  that are used to train the time-contrastive neural network  102 . The observations  302 - 306  are images captured by two different modalities (and from two different viewpoints). Such an arrangement may be cheaper, simpler, and more practical than arrangements involving human labeling, for example. 
     In this example, a first camera captures a sequence of images  308  of a hand (of an agent) pouring liquid into a cup from the first viewpoint. The agent can be a human or a robotic agent. The first viewpoint (view 1) can be a first person view, i.e., a view captured by a camera of the first agent that is pouring the liquid into the cup would see. Simultaneously, a second camera captures a sequence of image  310  of the same hand from a second viewpoint. The second viewpoint (view 2) can be a third party view, i.e., a view that a second agent, who is observing the first agent pouring the liquid into the cup, would see. 
     A first image is selected from the sequence of image  308  as an anchor image  302 . The anchor image  302  can be randomly selected from the sequence of images  308 . A second image that was taken at the same time as the anchor image  302  but by the second camera is selected as a positive image  304 . A third image is selected from the sequence of images  308  as a negative image  306 . The negative image  306  can be selected randomly from the images captured by the first camera that are within a temporal neighborhood (i.e. within a predetermined time window) of the anchor image  302 , e.g., images that are taken two, five, or ten seconds after the anchor image  302  was taken. 
     As shown in  FIG.  3   , the first, or anchor, image  302  is captured at time step t, and the third, or negative, image  306  is captured at time step t+2, which is within the temporal neighborhood of the anchor image  302 . The negative image  306  is in the same sequence  308  as the anchor image  302 . Being trained on the triple of anchor image  302 , positive image  304 , and negative image  306 , the time-contrastive neural network  102  can learn to capture properties that vary over time but are consistent across views, such as hand poses and amounts of poured liquid, while becoming invariant to irrelevant transformations such as background or lighting. 
       FIG.  4    shows another example triple of observations including an anchor observation  402 , a positive observation  404 , and a negative observation  406  captured by a single modality (from a single viewpoint). 
     In this example, a single camera captures a sequence of image  408  of a hand pouring liquid into a target container from a single viewpoint (view 1, which is a self-view). A first image from the sequence  408  is selected as the anchor image  402 . A second, or positive, image  404  is selected from images that are within a margin range around the anchor image  402 . The positive image  404  can be randomly selected within the margin range. The margin range of the anchor image  402  defines a predetermined time window around the anchor image  402 . For example, as shown in  FIG.  4   , the anchor image  402  is captured at the time step t anchor , and the margin range of the anchor image  402  includes images that are captured from time step t anchor −2 to time step t anchor +2. 
     A third, or negative, image  406  is selected from images that are within a negative range of the anchor image  402 , i.e., a range that is outside of the margin range of the anchor image  402 . 
     The triple of images  402 ,  404  and  406  can be used as training input data to train the time-contrastive neural network  130  using the techniques described in detail above with reference to  FIG.  2   . By training the time-contrastive neural network  130  using images selected in this manner, the system  100  ensures that the neural network  130  can learn attributes in each observation and properties of an interaction. In the example of  FIG.  4   , the neural network  130  can learn to distinguish different poses of the hand, e.g., whether or not the hand is contacting the white cup. The neural network  130  can also learn the tilt of the white cup, or the amount of liquid currently in the target container or its viscosity. 
       FIG.  5    is a flow diagram of an example process  500  for training a time-contrastive neural network using observations captured by multiple modalities. The neural network is configured to receive an input observation characterizing a state of an environment and to process the input observation to generate a numeric embedding of the state of the environment. For convenience, the process  500  will be described as being performed by a system of one or more computers located in one or more locations. For example, a training system, e.g., the training system  200  of  FIG.  2   , appropriately programmed in accordance with this specification, can perform the process  500 . 
     The system can repeatedly perform the process  500  on different triples of observations to train the neural network. 
     The system obtains a first (or anchor) observation captured by a first modality (step  502 ). The first observation may be an image and the first modality may be a camera at a first viewpoint. 
     The system obtains a second (or positive) observation that is co-occurring with the first observation and that is captured by a second, different modality (step  504 ). The second observation may be an image and the second modality may be a camera at a second viewpoint. 
     The system obtains a third (or negative) observation captured by the first modality that is not co-occurring with the first observation (step  506 ). 
     In some cases, the system may select the third observation randomly from the observations captured by the first modality that are within the temporal neighborhood of the first observation. 
     In some other cases, the system may select as the third observation an observation that is a hard negative relative to the first observation from a sequence of observations captured by the first modality that are within the temporal neighborhood of the first observation. The hard negative observation is the observation within the temporal neighborhood that has an embedding that is farthest away from the embedding of the first observation. 
     The system determines a triplet loss that uses the first observation as an anchor example, the second observation as a positive example, and the third observation as a negative example (step  508 ). 
     In particular, the system processes the first observation using the neural network in accordance with the current values of the network parameters to generate a first embedding. The system processes the second observation using the neural network in accordance with the current values of the network parameters to generate a second embedding. The system processes the third observation using the neural network in accordance with the current values of the network parameters to generate a third embedding. The system determines the triplet loss from (i) a first distance between the first embedding and the second embedding and (ii) a second distance between the first embedding and the third embedding. 
     For example, the system can determine the triplet loss for a given triple of observation as follows: 
       Triplet loss=∥ f (x i   a )− f (x i   p )μ 2   2   ∥f (x i   a )− f (x i   n )∥ 2   2 +α,
 
     where f( ) is an embedding function that embeds an observation x into an embedding space (e.g., a d-dimensional Euclidean space) and can represent the neural network being trained, x i   a  is the anchor observation, x i   p  is the positive observation, and x i   n  is the negative observation. 
     The system then updates values of the network parameters to minimize the triplet loss (step  510 ). The system can update the values of the network parameters using conventional neural network training techniques, e.g., a gradient descent-based training technique. For example, the system backpropagates the gradient of the triplet loss to determine a respective updated value for each of the network parameters of the neural network. 
       FIG.  6    is a flow diagram of an example process  600  for optimizing a policy controller. For convenience, the process  600  will be described as being performed by a system of one or more computers located in one or more locations. For example, a reinforcement learning system, e.g., the reinforcement learning system  100  of  FIG.  1   , appropriately programmed in accordance with this specification, can perform the process  600 . 
     The system obtains a demonstration sequence that includes demonstration images of another agent performing a version of the specified task (step  602 ). As described above, the other agent can be any appropriate agent that is proficient at performing the task. For example, the other agent can be a human demonstrator or another robotic agent that has already been trained to perform the task or has been hard-coded to perform the task. The version of the specified task may be the same as the specified task to be performed by the agent or may differ in certain ways from the task to be performed by the agent. For example, if the task is to move an object to a destination location in the environment the version of the task performed in the demonstration images may move a somewhat different object from the object that the robotic agent will be tasked with moving. 
     The system processes each demonstration image in the demonstration sequence using the trained time contrastive neural network to generate a respective demonstration embedding for each demonstration image (step  604 ). The demonstration embedding is a numeric embedding of the respective demonstration image, wherein the demonstration embedding characterizes a state of the environment as depicted in the respective demonstration image. 
     The system then repeatedly performs steps  606  through  610  to optimize policy controller used to control the robotic agent as it performs the robotic (specified) task. 
     The system obtains a robot sequence (step  606 ). The robot sequence is a sequence of robotic images of the robotic agent performing the specified task by performing actions selected using the current policy controller. For example, the system or another system can cause the robotic agent to (i) perform the task by repeatedly selecting actions using the current policy controller and instructing the robotic agent to perform each selected action and (ii) while performing the task, regularly capture images using a camera sensor of the robotic agent. Each robot image in the robot sequence corresponds to a respective demonstration image in the demonstration sequence, i.e., is taken at approximately the same time during performance of the task as one of the demonstration images. 
     The system processes each robot image in the robotic sequence using the time contrastive neural network to generate a respective robot embedding for each robot image (step  608 ). The robot embedding is a numeric embedding of the respective robot image, wherein the robot embedding characterizes a state of the environment as depicted in the respective robot image. 
     The system updates the current policy controller (step  610 ). In particular, the system updates the policy controller by performing an iteration of a reinforcement learning technique to optimize, i.e., maximize, a reward function that depends on, for each demonstration image, a distance between the demonstration embedding of the demonstration image and the robot embedding of the corresponding robot image. That is, the reward for a given demonstration image—corresponding robot image pair is higher when the distance between the corresponding embeddings is shorter. Such a reward function can enable efficient reinforcement learning that is practical for real-world robotic applications. 
     In other words, the system generates a reward for each demonstration image—corresponding robot image pair and uses a reinforcement learning technique that takes the rewards as input to update the current policy controller, i.e., updates the current policy controller using a reinforcement learning technique that updates the policy controller to increase the received rewards. The system can use any appropriate reinforcement learning technique that takes rewards as input to perform the optimization step. For example, the reinforcement learning technique can be a model-free technique, e.g., PI2, a model-based technique, e.g., LQR, or a technique that combines model-based and model-free algorithms, e.g., PILQR. 
     In some implementations, the reward for a demonstration image and corresponding robot image is based on the Euclidean distance between the demonstration embedding of the demonstration image and the robot embedding of the corresponding robot image. For example, the reward function can include a Euclidean distance term that is a square of the Euclidean distance. As another example, the reward function can include a Huber-style loss term that is a square root of a sum between a constant value and a square of the Euclidean distance between the demonstration embedding of the demonstration image and the robot embedding of the corresponding robot image. In some of these implementations, the reward function R is a weighted sum of the Euclidean distance term and the Huber-style loss term and satisfies: 
         R ( v   t   , w   t )=−α∥ w   t   −v   t μ 2   2 −β√{square root over (γ+∥ w   t   −v   t ∥ 2   2 )},
 
     where v t  is the demonstration embedding of the demonstration image in a t-th position in the demonstration sequence, w t  is the robot embedding of the robot image in a t-th position in the robot sequence, α and β are fixed weighting parameters, and γ is a small positive constant value. 
     For a system of one or more computers to be configured to perform particular operations or actions means that the system has installed on it software, firmware, hardware, or a combination of them that in operation cause the system to perform the operations or actions. For one or more computer programs to be configured to perform particular operations or actions means that the one or more programs include instructions that, when executed by data processing apparatus, cause the apparatus to perform the operations or actions. 
     Embodiments of the subject matter and the functional operations described in this specification can be implemented in digital electronic circuitry, in tangibly-embodied computer software or firmware, in computer hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions encoded on a tangible non transitory program carrier for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. The computer storage medium can be a machine-readable storage device, a machine-readable storage substrate, a random or serial access memory device, or a combination of one or more of them. The computer storage medium is not, however, a propagated signal. 
     The term “data processing apparatus” encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. 
     A computer program (which may also be referred to or described as a program, software, a software application, a module, a software module, a script, or code) can be written in any form of programming language, including compiled or interpreted languages, or declarative or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data, e.g., one or more scripts stored in a markup language document, in a single file dedicated to the program in question, or in multiple coordinated files, e.g., files that store one or more modules, sub programs, or portions of code. A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     As used in this specification, an “engine,” or “software engine,” refers to a software implemented input/output system that provides an output that is different from the input. An engine can be an encoded block of functionality, such as a library, a platform, a software development kit (“SDK”), or an object. Each engine can be implemented on any appropriate type of computing device, e.g., servers, mobile phones, tablet computers, notebook computers, music players, e-book readers, laptop or desktop computers, PDAs, smart phones, or other stationary or portable devices, that includes one or more processors and computer readable media. Additionally, two or more of the engines may be implemented on the same computing device, or on different computing devices. 
     The processes and logic flows described in this specification can be performed by one or more programmable computers executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). For example, the processes and logic flows can be performed by and apparatus can also be implemented as a graphics processing unit (GPU). 
     Computers suitable for the execution of a computer program include, by way of example, can be based on general or special purpose microprocessors or both, or any other kind of central processing unit. Generally, a central processing unit will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a central processing unit for performing or executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device, e.g., a universal serial bus (USB) flash drive, to name just a few. 
     Computer readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user&#39;s client device in response to requests received from the web browser. 
     Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back end, middleware, or front end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), e.g., the Internet. 
     The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system modules and components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.