Patent Publication Number: US-2022215208-A1

Title: Domain Adaptation Using Simulation to Simulation Transfer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/692,509, filed on Nov. 22, 2019, which claims the benefit under 35 U.S.C. § 119(a) of the filing date of Greek Patent Application No. 20180100527, filed in the Greek Patent Office on Nov. 23, 2018. The disclosure of the foregoing applications are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     This specification relates to controlling robotic agents. 
     In some cases, a robotic agent interacts with an environment by performing actions that are selected by a system in response to receiving observations that characterize the current state of the environment. 
     Some 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 are deep neural networks that 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 generally describes a system that trains a generator neural network to process input images of scenes in a real-world environment to generate a respective adapted image for each input image. The respective adapted image for each input image is an image that appears to be an image of the same scene as in the input image but in a canonical simulation of the real-world environment. A simulation of a real-world environment is one or more computer programs that model the dynamics, semantics, and appearance of the real-world environment. A canonical simulation is a simulation that models the real-world environment, i.e., the dynamics, semantics and appearance, without any noise or randomization being inserted. 
     Once the generator neural network has been trained, the trained generator neural network can be used in controlling a robotic agent interacting with the real-world environment to cause the robotic agent to perform a robotic task. 
     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 described systems train a generator neural network to adapt images from the real-world to simulation using training data that includes only or mostly simulated images, i.e., images from a canonical simulation and corresponding images from a randomized simulation. This is advantageous because images of the real-world can be unavailable or difficult to obtain while simulated images can be generated easily. 
     As a particular example, large amounts of simulated training data with accurate labels for robotic control can be generated relatively easily. Simulated training data is training data that is generated using a computer program simulation of a real-world environment. Real-world training data for robotic control, i.e., data gathered through actual interaction of the robotic agent with the real-world environment, on the other hand, is time-intensive and resource-intensive to collect, results in mechanical wear on the robotic agent, and can have noisy labels. 
     However, simulated training data is in practice difficult to employ in effectively training a control policy for use in controlling a real-world robotic agent interacting with the real-world environment. That is, directly using the simulation training data for training the control policy without accounting for the fact that the simulation training data is drawn from a different domain (simulation) than the intended domain (real-world) will generally result in the trained control policy performing poorly at controlling the real-world robot. 
     Some of the reasons for this poor performance can include that a specific simulation may not generalize well to an entire real-world environment and that the simulated data is systematically different from counterpart real-world data, e.g., because the real-world environment is less structured and predictable than the simulated environment, because images of the real-world environment appear differently from images of the simulated environment, and because dynamics of the real-world robotic agent can differ somewhat from the simulated version of the agent. 
     By training the generator neural network as described in this specification, the control policy can be effectively trained in simulation and then later used to control the real-world agent, i.e., by adapting the real-world images using the trained generator neural network. Being able to train the control policy in simulation greatly increases the amount of high-quality training data available for training the control policy. This results in improved performance of the trained network and of the robotic agent in performing the robotic task, even when the control policy is used to control the robotic agent using only monocular images of the environment captured by the sensors of the robot. 
     By using the generator neural network to adapt images, the need to generate large labeled datasets of real-world environment interactions is greatly reduced. Therefore, mechanical wear on the robotic agent during learning of the control policy is avoided. In fact, both the generator and the control policy can be trained using few or no real-world images. 
     The details of one or more embodiments of the subject matter described in 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 how a generator neural network is used in controlling a robotic agent. 
         FIG. 2  shows an example neural network training system. 
         FIG. 3  is a flow chart of an example process for training the generator neural network. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     This specification generally describes a system that trains a generator neural network to process input images of scenes in a real-world environment to generate a respective adapted image for each input image. The respective adapted image for each input image is an image that appears to be an image of the same scene as in the input image but in a canonical simulation of the real-world environment. A simulation of a real-world environment is one or more computer programs that model the dynamics, semantics, and appearance of the real-world environment. A canonical simulation is a simulation that models the real-world environment, i.e., the dynamics, semantics and appearance, without any noise or randomization being inserted. 
     In particular, the system trains the generator to accurately adapt the real-world images even when no real-world images or only a small number of real-world images are available during the training. 
     Instead, the system trains the generator to adapt images of a randomized simulation of the real-world environment into images of the canonical simulation. The randomized simulation is one in which one or more characteristics of a scene simulated in the canonical simulation are randomly modified. As a result of the training, the generator neural network is able to accurately adapt real-world images into simulated images, even when no real-world images were used in the training. 
     Once the generator neural network has been trained, the trained generator neural network can be used in controlling a robotic agent interacting with the real-world environment to cause the robotic agent to perform a robotic task. 
       FIG. 1  shows how a generator neural network  130  is used to control a robotic agent  150  that interacts with a real-world environment. 
     The robotic agent  150  can be, e.g., a robotic arm or another, more complex robot that is controllable to change position within the environment. 
     In order for the agent  150  to interact with the real-world environment, a system receives one or more images characterizing the current state of the environment and selects an action to be performed by the agent using the received images using a control policy, e.g., a neural network or other trainable policy. In particular, the system attempts to select the 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  150  to interact with the real-world environment for some specific purpose. 
     As shown in  FIG. 1 , the system trains the control policy in simulation, i.e., while controlling a simulated version  140  of the agent  150  during training of the control policy. In particular, during this simulated training, the generator neural network  130  receives input images of a randomized simulation and adapts the images to images of a canonical simulation. The images of the canonical simulation can then be used to control the simulated version  140  of the real-world robotic agent  150  as the system learns the control policy. 
     During “testing” or during inference after training the control policy, the system controls the real-world agent  150  while the real-world agent interacts with the real-world environment. 
     In particular, in the real-world and after both the control policy and the generator neural network  130  have been trained, the system receives an input image of the real-word environment captured while the robotic agent is interacting with the real-world environment and processes the input image of the real-world environment using the trained generator neural network  130  to generate an adapted image from the input image, i.e., an image that appears to be from the canonical simulation. 
     The system then provides the adapted image as input to the control policy for the robotic agent to select the subsequent action to be performed by the robotic agent. 
     In some cases, as shown in the example of  FIG. 1 , the control policy is provided as input both the adapted image and the input image to generator neural network  130 . 
     Thus, the control policy can be trained in simulation and, by making use of the generator neural network  130 , can then be used to control the robotic agent in the real-world. That is, the control policy requires only adapted images generated by the generator neural network as input and does not directly receive as input any real-world images. In order to allow the generator neural network  130  to be suitable for this purpose despite limited or no real-world data being available for training, the system trains the generator  130  as described below. 
     In some cases, if real-world data is available, the control policy, the generator  130 , or both can be fine-tuned on the real-world data after being trained in simulation. However, the generator  130  can be trained using the techniques described in this specification to accurately adapt real-world images even if very little or even no real-world data is available. 
     In particular, after training the generator neural network  130 , the system can either use the generator neural network  130  directly in controlling the agent or provide data specifying the trained generator neural network  130 , i.e., data identifying the architecture and trained parameter values of the neural network, to another system for use in adapting input images while controlling the robotic agent. 
       FIG. 2  shows an example neural network training system  100 . The neural network training 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. 
     As described above, the system  100  trains the generator neural network  130  to process input images of scenes in a real-world environment  116  to generate a respective adapted image  132  for each input image. 
     The respective adapted image  132  for each input image is an image that appears to be an image of the same scene as in the input image but in a canonical simulation of the real-world environment. A simulation of a real-world environment is one or more computer programs that model the dynamics, semantics, and appearance of the real-world environment. A canonical simulation is a simulation that models the real-world environment, i.e., the dynamics, semantics and appearance, without any noise or randomization being inserted. 
     For example, the canonical simulation can be one that standardizes the appearance of the environment, e.g., by applying uniform colors to the background and to objects in the environment while leaving the textures for the objects from the randomized version or the real-world in-place, by fixing the light sources in the environment, or both. 
     In particular, the system  100  trains the generator  130  to accurately adapt the real-world images even when no real-world images or only a small number of real-world images are available during the training. 
     Instead, the system  100  trains the generator  130  to adapt images  118  of a randomized simulation of the real-world environment into images of the canonical simulation. The randomized simulation is one in which one or more characteristics of a scene simulated in the canonical simulation are randomly modified. Randomizing images is described in more detail below with reference to  FIG. 3 . 
     As a result of the training, the trained generator neural network  130  is able to accurately adapt real-world images into simulated images, even when no real-world images were used in the training. 
     The generator neural network  130  can have any appropriate neural network architecture that allows the generator neural network  130  to map an input image to an output image. For example, the generator neural network  130  can be a fully-convolutional neural network that first contracts the spatial dimension of the input image through a series of contracting convolutions and then expands the spatial dimensions through a series of up-sampling convolution. An example of such a neural network architecture is the U-Net architecture, described in more detail in O. Ronneberger, P. Fischer, and T. Brox. U-net: Convolutional networks for biomedical image segmentation. In Intl. Conference on Medical Image Computing and Computer Assisted Intervention. Springer, 2015. 
     During the training, the system  100  repeatedly updates the values of the parameters of the generator neural network  130  (“generator parameters”) to determine trained values of the generator parameters by optimizing an objective function that includes one or more terms that encourage adapted images generated by the generator neural network  130  by processing randomized simulated training images to be similar to the corresponding canonical simulated training images. 
     More specifically, the system  100  trains the generator neural network jointly with a canonical-randomized discriminator neural network  150 . The canonical-randomized discriminator neural network  150  is a neural network having parameters (“canonical-randomized discriminator parameters”) that is configured to process input images in accordance with the canonical-randomized discriminator parameters to classify each input image as either being a canonical simulated image or not, i.e., an adapted image or a randomized simulated image. 
     The discriminator  150  can have any appropriate neural network architecture that allows the neural network to map an input image to a classification, i.e., a score that indicates whether the input image is from the canonical simulation. For example, the discriminator  150  can be a convolutional neural network that has an output layer that has a single neuron that generates the classification. 
     Generally, the objective function used to train the generator  130  and the canonical-randomized discriminator  150  includes a first term that (i) penalizes the generator neural network for generating training adapted images that are accurately classified by the canonical-randomized discriminator neural network and (ii) encourages high classification accuracy of the canonical-randomized discriminator neural network on the canonical and randomized simulated training images. 
     By training the two neural networks  130  and  150  jointly on this objective function, the system  100  forces the generator  130  to continue to generate more accurate adapted images that cannot be properly classified by the canonical-randomized discriminator  150 . 
     To improve the training of the generator  130 , during training, the generator  130  can also be configured to make one or more auxiliary predictions for each input image that is processed by the generator  130 . 
     One example of an auxiliary prediction that the generator  130  can be configured to make is to predict a segmentation mask  134  of an input image. That is, the generator  130  can be configured to process the input image to generate not only an adapted image  132  but also a predicted segmentation mask  134  of the input image. A segmentation mask of an input image assigns each pixel in the input image to one of a predetermined set of categories. For example, the categories can include different object categories and a background category that indicates that the pixel belongs to the background of the input image. 
     Another example of an auxiliary prediction that the generator  130  can be configured to make is to predict a depth map  136  of an input image. That is, the generator  130  can be configured to process the input image to generate not only an adapted image  132  but also a predicted depth map  136  of the input image and, further optionally, the predicted segmentation mask  134  of the input image. A depth map assigns a depth value to each pixel in the input image that represents the predicted distance of the scene that is depicted at that image from the camera that is capturing the input image. 
     Training the generator  130  and the discriminator  150  is described in more detail below with reference to  FIG. 3 . 
       FIG. 3  is a flow diagram of an example process  300  for training the generator neural network. For convenience, the process  300  will be described as being performed by a system of one or more computers located in one or more locations. For example, a neural network training system, e.g., the neural network training system  100  of  FIG. 1 , appropriately programmed, can perform the process  300 . 
     The system obtains a plurality of canonical simulated training images and a corresponding randomized simulated training image for each canonical simulated training image (steps  302  and  304 ). 
     Each randomized training simulated image is generated by randomizing one or more characteristics of a scene depicted in the corresponding canonical simulated training image. In particular, the system can generate the canonical and corresponding randomized simulated images during interaction of the simulated agent with the simulated version of the environment, e.g., during simulated training of the control policy for the agent. 
     For example, during the interaction of the simulated agent with the simulated version of the environment, the system can periodically randomize one or more of: lighting characteristics of the scene, texture characteristics of the scene, or object properties of objects in the scene, e.g., position or size of the objects, to generate randomized simulated images. This can in include, in some cases, randomizing properties, e.g., size or position or color, of the simulated agent. 
     The system can also capture an image of the same scene but with the randomization removed to generate the corresponding canonical simulation images. 
     When the generator neural network makes one or more auxiliary predictions, the system also generates a respective ground truth prediction for each pair of simulated images, i.e., a ground truth segmentation mask, a ground truth depth map, or both. Because the images are being captured in simulation, the system is able to accurately generate the ground truth predictions without any external supervision, i.e., because the information required to generate the ground truth segmentation mask and the ground truth depth mask is already being used by the simulation to render the scene. 
     The system trains the generator neural network to optimize an objective function (step  306 ). 
     Generally the optimization function includes multiple terms, one or more of which encourage adapted images generated by the generator neural network by processing the randomized simulated training images to be similar to the corresponding canonical simulated training images. For example, the objective function can be a sum or a weighted sum of the multiple terms. 
     In particular, as described above, the system trains the generator jointly with a canonical-randomized discriminator. Given this, the objective function includes a first term that (i) penalizes the generator neural network for generating training adapted images that are accurately classified by the canonical-randomized discriminator neural network and (ii) encourages high classification accuracy of the canonical-randomized discriminator neural network on the canonical and randomized simulated training images. 
     For example, the discriminator can output the likelihood that a given input image is from the canonical domain, i.e., is from the canonical simulation rather than the randomized simulation, and the first term can be equal to the expectation of the logarithm of the likelihood generated by the discriminator for a randomized image plus the expectation of the logarithm of one minus the likelihood generated by the discriminator for an adapted image. 
     The objective function can also include another term that encourages visual similarity between the adapted training images generated by the generator neural network and the corresponding randomized simulated images. For example, this term can be a mean pairwise squared error (MPSE) or other error, e.g., L1 or L2 error, between an adapted training image generated by the generator neural network and the corresponding randomized simulation image. 
     In cases where the generator is also configured to make one or more auxiliary predictions, the objective function also includes additional terms that make use of the auxiliary predictions and the corresponding ground truth auxiliary predictions to improve the training of the generator. 
     For example, when the generator is also configured to generate a predicted depth map of each input image, the objective function also includes a term that encourages similarity between the predicted depth maps generated by the generator neural network and the corresponding ground truth depth maps. For example, this term can be an L2 distance or other error, e.g., L1 distance, between a predicted depth map generated by the generator neural network and the corresponding ground truth depth map. 
     As another example, when the generator neural network is further configured to generate a predicted segmentation mask of each input image, the objective function includes another term that encourages semantic similarity between the predicted segmentation masks generated by the generator neural network and the corresponding ground truth segmentation masks. For example, this term can be an L2 distance or other error, e.g., L1 distance, between a predicted segmentation mask generated by the generator neural network and the corresponding ground truth segmentation mask. 
     To train the generator neural network to optimize this objective function, the system can obtain batches of simulation training inputs that each include a set of canonical simulated images and their corresponding randomized simulation images. The system can then update the generator parameters using each batch of inputs by performing an iteration of a machine learning technique. In particular, the system determines a gradient of the objective function with respect to the generator parameters for each batch, e.g., through backpropagation, and then computes an update to the generator parameters using the gradient, e.g., by applying an optimizer, e.g., a stochastic gradient descent learning rule, the Adam optimizer, or rmsProp, to the gradient. 
     By repeatedly updating the generator parameters on batches of training data using this objective function, the system trains the generator neural network to accurately adapt input images to images of the canonical simulation of the environment. 
     Because the discriminator is being trained jointly with the generator, the system also needs to update the discriminator parameters during the training of the generator. Thus, as part of optimizing the objective function, the system repeatedly alternates between (i) performing an iteration of a machine learning training technique that updates the generator parameters to minimize the objective function by determining gradients with respect to the generator parameters for a batch of simulation training inputs (as described above) and (ii) performing an iteration of a machine learning training technique that adjusts the canonical-randomized discriminator parameters to maximize the objective function by determining gradients with respect to the canonical-randomized discriminator parameters for a batch of simulation training inputs. For example, the system can first perform (i) and then (ii) for each batch of simulation training inputs or first perform (ii) and then (i) for each batch of simulation training inputs. 
     Optionally, if any real-world images are available, the system can also obtain real-world training images of the real-world environment (step  308 ). For example, the system can receive or capture images of the real-world environment as the real-world agent interacts with the environment. During this interaction, the agent can be controlled by a different control policy, e.g., a random control policy that selects random actions or a fixed control policy that is not being trained. 
     If real-world images are available, the system also trains the generator neural network on the real-world images (step  310 ). For example, the system can fine-tune the generator neural network on the real-world images or include one or more additional terms in the objective function that encourage adapted images generated by the generator neural network by processing the real-world training images to appear to be images of the canonical simulation while maintaining the semantics of the corresponding real-world training images. 
     This specification uses the term “configured” in connection with systems and computer program components. 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 storage medium for execution by, or to control the operation of, 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. 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 term “data processing apparatus” refers to data processing hardware and 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 also be, or further include, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). The apparatus can optionally include, in addition to hardware, code that creates an execution environment for computer programs, 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, an app, 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 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 data communication network. 
     In this specification, the term “database” is used broadly to refer to any collection of data: the data does not need to be structured in any particular way, or structured at all, and it can be stored on storage devices in one or more locations. Thus, for example, the index database can include multiple collections of data, each of which may be organized and accessed differently. 
     Similarly, in this specification the term “engine” is used broadly to refer to a software-based system, subsystem, or process that is programmed to perform one or more specific functions. Generally, an engine will be implemented as one or more software modules or components, installed on one or more computers in one or more locations. In some cases, one or more computers will be dedicated to a particular engine; in other cases, multiple engines can be installed and running on the same computer or computers. 
     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 special purpose logic circuitry, e.g., an FPGA or an ASIC, or by a combination of special purpose logic circuitry and one or more programmed computers. 
     Computers suitable for the execution of a computer program 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. The central processing unit and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 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. 
     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 device in response to requests received from the web browser. Also, a computer can interact with a user by sending text messages or other forms of message to a personal device, e.g., a smartphone that is running a messaging application, and receiving responsive messages from the user in return. 
     Data processing apparatus for implementing machine learning models can also include, for example, special-purpose hardware accelerator units for processing common and compute-intensive parts of machine learning training or production, i.e., inference, workloads. 
     Machine learning models can be implemented and deployed using a machine learning framework, e.g., a TensorFlow framework, a Microsoft Cognitive Toolkit framework, an Apache Singa framework, or an Apache MXNet framework. 
     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, a web browser, or an app 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. In some embodiments, a server transmits data, e.g., an HTML page, to a user device, e.g., for purposes of displaying data to and receiving user input from a user interacting with the device, which acts as a client. Data generated at the user device, e.g., a result of the user interaction, can be received at the server from the device. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or on the scope 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 be 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 and recited in the claims 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 some cases, multitasking and parallel processing may be advantageous.