Title of Invention: SENSOR-BASED MEDIA DISPLAY SYSTEM AND APPARATUS FOR MOBILE VEHICLES Abstract: Systems, techniques, and apparatuses are described for displaying digital media packages on mobile vehicles, including manned and unmanned autonomous ground and air vehicles. Apparatuses describe media display and mounting modalities in a variety of form factors, as well as component configurations for audience detection using sensor arrays. Systems and techniques describe mechanisms for selecting and displaying media packages aligned with the dynamic environmental, temporal, and geolocation conditions of client media display apparatuses. Detailed Description: RELATED APPLICATIONS This application is a continuation-in-part application of and claims priority under 35 U.S.C. § 111(a) of U.S. Patent Application Serial No. PCT/IB2020/056896 filed on Jul. 22, 2020 and titled SENSOR-BASED MEDIA DISPLAY SYSTEM AND APPARATUS FOR MOBILE VEHICLES, which in turn application of and claims priority under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 62/878,500 filed on Jul. 25, 2019 and titled SENSOR-BASED MEDIA DISPLAY SYSTEM AND APPARATUS FOR MOBILE VEHICLES. The contents of these applications are incorporated herein by reference. BACKGROUND Although drivers and passengers of mobile vehicles, as well as pedestrians, have become accustomed to being presented with media such as advertisements on billboards, taxis and buses, many problems remain to be solved in the efficient and effective display of digital media to persons on or near roadways. BRIEF SUMMARY An apparatus is described for displaying digital media packages on a mobile vehicle, the apparatus comprising: a protective enclosure comprising a base platform and a cover affixed on top of the base platform, the cover having at least one partially transparent side; mounting hardware operable to affix the protective enclosure to the mobile vehicle; and one or more audiovisual display units housed inside the protective enclosure, wherein the one or more audiovisual display units are connected, via an interface system, to a computing system comprising the interface system, a processing system, and non-transitory computer-readable storage media. A media display client system is described for displaying digital media packages on a mobile vehicle, the system comprising: non-transitory computer-readable storage media; a processing system; an interface system, including a communications interface and an audio-video interface; a GPS component; an object sensor component array; one or more audiovisual display units; program instructions stored on the computer-readable storage media that, when executed by the processing system, direct the processing system to: periodically, at a media package request time interval, (i) obtain GPS sensor data from the GPS component, wherein the GPS sensor data comprises location data, date data, and time data, and (ii) via the communications interface, send a media package request comprising the GPS sensor data to a media selection service; and in response to receiving, from the media selection service via the communications interface, a media package comprising an ordered set of media according to a media schedule, and media presentation parameters for each of the media including a geofence region constraint, a date range constraint, and a timeframe range constraint, initiate a media session by: determining a selected media from the ordered set of media corresponding to the media schedule and media presentation parameters; rendering the selected media on the one or more audiovisual display units; periodically, at a monitoring time interval, gathering session monitoring data comprising (i) object sensor data obtained from the object sensor component array comprising audience estimation data points, and (ii) updated GPS sensor data comprising updated location data, speed and direction data, updated date data, and updated time data obtained from the GPS component; storing the session monitoring data in a session data package on the computer-readable storage media; and if either the updated location data is outside the geofence region constraint, the updated date data is outside the date range constraint, or the updated time data is outside the timeframe range constraint, then (i) terminate the media session by at least: (a) terminating the rendering of the selected media on the one or more audiovisual display units, (b) terminating the gathering of session monitoring data, and (c) terminating the comparison of the updated GPS sensor data and the media presentation parameters; (ii) transmitting the session data package to the media management system/service via the communications interface; and (iii) initiate a next media session. A system is described for dynamic selection of digital media for presentation at a remote device, comprising: a processing system; non-transitory computer-readable storage media; a communications interface; and program instructions stored on the computer-readable storage media that, when executed by the processing system, direct the processing system to: in response to receiving, via the communications interface, a media package request from a media display client, wherein the media package request comprises GPS sensor data including location data, date data, and time data of the media display client: select media for a media package, wherein the location data, date data, and time data match media presentation parameters for each media, the media presentation parameters comprising a geofence region constraint, a date range constraint, and a timeframe range constraint; determine, using the media and media presentation parameters for the selected media, a media schedule and arrange the media as an ordered set; send, to the media display client via the communications interface, the media package comprising the selected media arranged as the ordered set, the media schedule, and the media presentation parameters for each media in the ordered set; and in response to receiving, from the media display client, a session data package, store, on a session data store located on the computer-readable storage media, the session data package. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a high-level example of a system/component environment in which some implementations of systems, apparatuses, and techniques for sensor-based media display management can be carried out. FIGS. 1A-1C show examples and embodiments of system/component environments for sensor-based media display management that have varying configurations of media display client systems. FIG. 2 shows an example view of a media display client apparatus mounted on a mobile vehicle. FIG. 3 shows an example exploded view of an embodiment of a three-sided media display client apparatus. FIG. 4 shows an example outbound viewpoint from the inside of the protective enclosure of a media display client apparatus. FIG. 5 shows an example view of a media display client apparatus from a perspective facing the front of a mobile vehicle. FIGS. 6A-61 show various example perspective views and configurations of a media display client apparatus. FIGS. 7A-7D show example process flows that may be executed in some embodiments of a media display client system. FIG. 8A depicts an example of a process flow for receiving a media package request from a media display client system, selecting appropriate media for the media display client system, and sending a media package to the media display client system. FIG. 8B depicts an example of a process flow for receiving and storing a session data package from a media display client system. FIG. 9A shows an example representation of a media package store, organized as a table in a relational database. FIG. 9B shows an example representation of a session store, organized as a table in a relational database. FIG. 9C shows an example representation of a user data store, organized as a table in a relational database. FIG. 10 shows an example interface with user interface elements so that a user-operator can manage the configuration and operations of, and monitor the status of, the media display client system/apparatus. FIGS. 11A-B show street maps with various representations of geofence region constraint. FIG. 12 shows an example diagram of the object sensor data points of several objects as shown from the outbound viewpoint of a media display client apparatus. FIG. 13 shows a block diagram illustrating components of a computing device or system used in some embodiments of techniques, systems, and apparatuses for facilitating sensor-based media display management. DETAILED DESCRIPTION Existing technologies for the presentation of media to persons on or near roadways have lacked methods and structure to support the display of digital media based on dynamic parameters, and the methods and technological features for the estimation of audience levels or viewership of the digital media. Systems and apparatuses are presented to facilitate the selection and display of media to audiences using mobile vehicles, as well as to dynamically adjust the selection and rendering of digital media in real-time based on location, date, time and other parameters as mobile vehicles move around. Apparatus arrangements are described to support the presentation and rendering of media sessions on mobile vehicles while they are in motion. Embodiments of systems and apparatuses described herein further include technological features for gathering data to support estimation of audience size and quality in dynamically changing physical, real-world viewing environments. Further advantageous technical effects are described below with respect to detailed embodiments. Various classifications of media may be presented to audiences using the systems and apparatuses herein, for example, advertisements, public service announcements, entertainment offerings, and instructions and warnings. FIG. 1 shows a high-level example of a system/component environment in which some implementations of systems, apparatuses, and techniques for sensor-based media display management can be carried out. In brief, media display client system/apparatus 120 connects to a media management system/service 160 via network 140. Media display client system/apparatus 120 is connected to client interface 130 via network 140. Generally, media display client system/apparatus 120 is a system or apparatus composed of computing system elements, elements for displaying media, elements for detecting the location of the system/apparatus, elements for detecting the presence of viewers and other objects (e.g., obstructions), and elements for housing the computing system and attaching the apparatus to a mobile vehicle. It should be noted that media display client system/apparatus 120 has many possible configurations, numerous examples of which are described in more detail below. Media display client system/apparatus 120 performs various processing activities such as generating a media package request, receiving the media package and selecting media, rendering media on its audiovisual display units, conducting data gathering and other telemetry for audience estimation, and sending media presentation session data to a media management system/service 160. This broad description is not intended to be limiting, as the various processing activities of a media display client system/apparatus 120 are described in detail subsequently in relation to FIGS. 7A-7D. Media management system/service 160 performs various activities, described in more detail below, to process a media package request originating from a media display client system 120, determine an appropriate media package and schedule, and send the media package to a media display client system/apparatus 120. Example process flows describing the processing activities of media management system/service 160 are described in more detail with respect to FIG. 8A-8B and elsewhere, and thus this broad description is not intended to be limiting. Media management system/service 160 may, for example, have logical subcomponents such as service components (e.g., 162, 164) and data stores (e.g., 151, 152, 153). For example, in the example embodiment of FIG. 1, media management system/service 160 has a media selection service 162 and a session service 164. Media selection service 162 can, in conjunction with a media package store 151, perform processing activities related to determining a media package and schedule from a media package request received from a media display client system 120. A media package store 151 may house, for example, indexed media files/streams, media schedules, media presentation parameters, and other data (the discussion surrounding FIG. 9A describes a media package store in more detail). In some embodiments, other subcomponents/subservices of 160, such as a session service 164, performs activities related to processing a session data package received from a media display client system 120—containing, for example, audience estimation data and other telemetry. A session data package may be stored by the session service 164 in a session store 152, for example (the discussion surrounding FIG. 9B describes a session store in more detail). Either or both services 162 and 164, and/or other subcomponents of the media management system/service 160 may interact with a user data store 153, which contains user-operator account data, configuration data, and other properties of each of the media display client system(s) 120 registered to use the media management service (the discussion surrounding FIG. 9C describes a user data store in more detail). Client interface 130 may provide a user-operator of a media display client system/apparatus 120 with various user interface elements for managing the configuration and operations of, and monitoring the status of, their media display client system/apparatus 120. Client interface 130 also connects to media management system/service 160 to provide user interface elements for performing media management functions and data review capabilities with respect to the user-operator's media display client system/apparatus 120 account. Client interface 130 may interact with subcomponents of the media management system/service 160, such as a user data store 153, to modify user-operator account information. A client interface 130 is a computing device that can have various form factors. Non-limiting examples and embodiments of a client interface 130 include a computing system, desktop computer, mobile device, tablet device, mobile phone, wearable, an interface screen that is dash-mounted inside the mobile vehicle, and a in-dash interface device installed in the mobile vehicle running software that provides the user interface elements. Examples of a client interface 130 include devices that can use a web browser to access a web page, or that have an “app” (or other software applications), to connect to a cloud service interface over the network 140. Network 140 can include, but is not limited to, a cellular network (e.g., wireless phone), a point-to-point dial up connection, a satellite network, the Internet, a local area network (LAN), a wide area network (WAN), a Wi-Fi network, an ad hoc network, a Bluetooth network, or a combination thereof. Such networks are widely used to connect various types of network elements, such as hubs, bridges, routers, switches, servers, and gateways. The network may include one or more connected networks (e.g., a multi-network environment) including public networks, such as the Internet, and/or private networks such as a virtual private network or secure enterprise private network. Access to the network may be provided via one or more wired or wireless access networks as will be understood by those skilled in the art. Media display client system/apparatus 120, media management system/service 160, and client interface 130 may connect to network 140 by employing one or more elements of a communications interface. Computing system and device components supporting network connectivity via a communications interface are described in detail with respect to FIG. 13. In embodiments, components or subcomponents of the media display client system/apparatus 120 and the client interface 130 facilitate the interaction between the media display client system/apparatus 120 (and/or client interface 130) and the media management system/service 160, for example through an application programming interface (API) of the media management system/service 160. An “API” is generally a set of programming instructions and standards for enabling two or more applications to communicate with each other. An API is an interface implemented by a program code component or hardware component (hereinafter “API-implementing component”) that allows a different program code component or hardware component (hereinafter “API-calling component”) to access and use one or more functions, methods, procedures, data structures, classes, and/or other services provided by the API-implementing component. An API can define one or more parameters that are passed between the API-calling component and the API-implementing component. The API and related components may be stored in one or more computer readable storage media. An API is commonly implemented as a set of Hypertext Transfer Protocol (HTTP) request messages and a specified format or structure for response messages according to a REST (Representational state transfer) or SOAP (Simple Object Access Protocol) architecture. JSON-formatted and XML-formatted content is often returned to the API-calling component in response messages. It should be noted that, while sub-components of media management system/service 160 are depicted in FIG. 1, this arrangement of the media management system/service 160 into components is exemplary only; other physical and logical arrangements of a media management system/service capable of performing the operational aspects of the disclosed techniques are possible. Various types of physical or virtual computing systems may be used to implement the media management system/service 160 (and related example subcomponents 162, 164, 151, 152, and 153) such as server computers, desktop computers, cloud compute server environments, laptop computers, tablet computers, or any other suitable computing appliance. When implemented using a server computer, any of a variety of servers may be used including, but not limited to, application servers, database servers, mail servers, rack servers, blade servers, tower servers, virtualized servers, or any other type of server, variation of server, or combination thereof. A computing system or device that may be used in some environments to implement a media management system/service 160 is described in FIG. 13. Further, it should be noted that aspects of the media management system/service 160 may be implemented on more than one device. In some cases, media management system/service 160 may include components located on media display client system/apparatus device(s) 120, client interface device(s) 130, and/or on one or more services implemented on separate physical devices. For example, a subset of a media package store 151 that has been cached or replicated may exist on media display client system/apparatus 120 to provide caching of media, lowering network bandwidth demands. FIGS. 1A-1C show examples and embodiments of system/component environments for sensor-based media display management that have varying configurations of media display client systems 120. The FIGS. 1A-1C illustrate media display client systems 120 whose component arrangements vary from self-contained systems (e.g., FIG. 1A), to systems wherein certain components are leveraged from within, or part of, an autonomous vehicle (e.g., FIG. 1B), to systems wherein most of the computing system components are within, or part of, an autonomous vehicle (e.g., FIG. 1C). These variants of system 120 are denoted by elements 120A, 120B, and 120C in the figures. Each of these possible configurations, and several other variations, are described in detail below. However, certain components are common to all the systems in FIGS. 1A-1C. FIG. 1A shows an example media display client system 120A having logically and physically self-contained components. Components of media display client system 120A, for example, are housed within, on, or as part of an apparatus for displaying media packages that is attachable to a mobile vehicle 99. Embodiments and examples of such an apparatus are described in more detail below, for instance in relation to FIGS. 2-6. Mobile vehicle 99 may have vehicle system 100A. Vehicle system 100A may have software (including an operating system), hardware, firmware, and devices for operating the mobile vehicle 99. Vehicle system 100A, for example, may include user interface software and hardware panels mounted in the dashboard, as well as various sensor componentry for autonomous driving, vehicle assisted driving, and driver hazard warning systems. Media display client system 120A is an example of a computing device or system having certain components as described in regard to FIG. 13. System 120A includes system components such as computer-readable storage media 101, on which program instructions for a media session component 102A are stored, and processing system 103, which executes the program instructions 102A, transforming the general-purpose computing device into a specific media display client system 120A for displaying media packages and obtaining media session sensor data. Example media display client system 120A includes a GPS component 104. GPS component 104 may be utilized to access a GPS service in order to obtain location, time, speed, heading, and distance data about the mobile vehicle that the media display client system 120A is affixed or installed upon. Processes employing data from a GPS component 104 are described in relation to FIGS. 7A-7D. Media display client system 120A includes one or more audiovisual display(s) 107 affixed to an apparatus affixed to the body of the mobile vehicle 99. Audiovisual display(s) 107 are operable to render/display digital media storages, files, and streams as images and/or video. In some embodiments, audiovisual display units also include speakers for rendering standalone audio media and/or the audio track of digital video media formats. Examples of audiovisual display(s) 107 include CRTs, LCD and LED displays and their various subtypes, instantiated in devices such as computer monitors/displays, televisions, touchscreen displays, tablets, etc. An audiovisual display 107 can be comprised of multiple, separate physical components, such as when the visual display component and speaker components for producing audible sounds are separated physically but connected by an interface to a common computing system. In some embodiments, an audiovisual display 107 can be comprised of a semitransparent surface on which an image is projected from an image-producing device (e.g., a portable projection device) behind it. The one or more audiovisual display(s) 107 may be affixed to the apparatus in a rear-facing aspect (i.e., facing outward from the back of the mobile vehicle, such that a driver or pedestrian behind the mobile vehicle can see the rear-facing audiovisual display). One or more audiovisual display(s) 107 may be mounted within the apparatus in a side-facing aspect (i.e., facing outward from the side of the mobile vehicle, such that a driver or pedestrian on the left or right side of the mobile vehicle can see the side-facing audiovisual display, as in, e.g., FIG. 6C). One or more audiovisual display(s) 107 may be affixed to the apparatus in a front-facing aspect (i.e., facing outward from the front of the mobile vehicle, such that a driver or pedestrian in front of the mobile vehicle can see the front-facing audiovisual display, as in, e.g., FIG. 6B). One or more audiovisual display(s) 107 may be affixed to the apparatus in an upward-facing aspect (e.g., to allow viewers in buildings with higher floors to see the media) and/or a downward-facing aspect (e.g., to allow viewers situated lower than a taller mobile vehicle such as a truck to see the media). Any combination of rear-facing, side-facing, front-facing, and/or upwardfacing and downward-facing audiovisual displays are possible, so long as at least one audiovisual display 107 is present. One or more of the audiovisual display(s) 107 may be tilted to intermediate angles, e.g., downward facing at a 45-degree angle from the horizontal. In certain embodiments, the audiovisual display(s) 107 can be housed outside the other media display client system components yet remain interfaced to the components via audio-video interface components, such as wired connectors, as described with respect to interface system 110. In some embodiments, audiovisual display(s) 107 are physically integrated with other computing system components as, for example, when an audiovisual display device 107 is part of a tablet device. Object sensor component array 106 includes one or more devices or components capable of detecting the presence of an object of interest in a particular proximity to the mobile vehicle 99. An object of interest includes other vehicles conveying human observers (e.g., passenger vehicles, public transport, trucks, vans, motorcycles), other forms of transportation conveying human observers (e.g., cyclists, skateboards, scooters), and human observers that are pedestrians. Data collected about an object of interest can include a data-defined signature that indicates the type of object (e.g., vehicle, vehicle type, human being), the object's heading, the object's speed, and the distance of the object of interest from the object sensor. This data may be used, for example, to perform audience detection/estimation as well as to record the duration the object remains within the optimal viewing distance from the object sensor, as described further in regard to FIGS. 7A-7B. The proximity of an object of interest as an audience member may vary in accordance with dynamic factors relating to the characteristics of a specific instance of a media display client system, including, for example, the type and capabilities of object sensor, the size, clarity, or brightness of the audiovisual display unit(s), and local viewing conditions (e.g., viewing conditions affected by weather). Object sensors can take a variety of forms, including RADAR, LIDAR, and camera components. LIDAR object sensor(s) can be used to very accurately determine the distance of an object from the LIDAR sensor. In some cases, object type analysis can be performed using LIDAR data. Different types of LIDAR include, for example, mechanical lidar and solid state lidar. RADAR-type object sensor(s) can be used to determine the speed, distance, and/or direction of objects near the mobile vehicle 99 conveying the media display client system. In some embodiments, RADAR data may be analyzed to determine the shape of objects in order to classify them by object type. Camera components include one or more cameras capable of capturing an image or video from an outbound viewpoint from the apparatus affixed to the mobile vehicle 99 on which the apparatus is housed. In some embodiments, the camera components include at least one camera operative to capture a still image or video from an outbound viewpoint of at least one of the audiovisual display unit(s) 107 (see, for example, the outbound viewpoint of apparatus 200 shown in FIG. 4). Camera components implement the visual imagery data-gathering aspect for performing audience detection, e.g., detection of the existence of human observers of the media via the periodic capturing of images and/or video, the process of which capturing is described with regard to FIG. 7A-7B. In some embodiments, images and/or video captures from camera components are used to classify objects into object types that are relevant to audience estimation. For example, images and video captures may be analyzed to perform face identification and eye gaze tracking within the image or videos, indicating the presence of an audience member within viewing range of the selected media. In certain embodiments, camera components implement the visual imagery data-gathering aspect for performing obstruction detection, e.g., detection of the existence of obstructions to the audiovisual displays, such as other vehicles, trees, roadway structures, signs, debris, or sun glare, that may impede the viewing of the media being displayed on the audiovisual display(s) by human observers in one or more viewing angle relations to the audiovisual display(s) 107. The object sensor component array 106 can be made up of many combinations of forms of object sensor, for example, RADAR only, LIDAR only, camera components only, RADAR and LIDAR, RADAR and camera components, LIDAR and camera components, and RADAR, LIDAR and camera components (i.e., all three types). The object sensor components forming the object sensor component array 106, including those of the same object sensor type (e.g., camera, LIDAR, RADAR), may also be part of various systems; i.e., certain object sensor components may be part of the media display client system, while other object sensor components are leveraged from the vehicle system (see, e.g., FIGS. 1A-1C). In examples and embodiments, object sensors comprising the object sensor component array 106 are located variously, including on the mobile vehicle 99 (e.g., webcams and similar devices, LIDAR units, and RADAR units mounted on the roof top, sides, hood, trunk enclosure, truck bed enclosure (or “topper”) roof, sides, or back, pickup tailgate or latch, or other external surface; within the external frame of the mobile vehicle 99, such as inside the front grill, in the rear frame, as for a backup camera, or other frame location) and within the mobile vehicle 99 (e.g., placed in the rear or front window dashboard area, mounted to a side window, within the bed of a “pickup truck,” or tailgate or latch of the pickup truck bed). Additional examples and embodiments locate one or more object sensors affixed to or within the media display client apparatus itself, such as mounted on the external top or sides of the media display client apparatus, or within the protective enclosure of the media display client apparatus. In some examples (see, e.g., FIGS. 1B-1C), one or more object sensors in the object sensor component array 106 are installed on the mobile vehicle 99 and are utilized by interacting through an interface system 110 with software components installed the mobile vehicle system (e.g., 100B and 100C from FIGS. 1B and 1C, respectively). In some embodiments, one or more tablets (such as an iPad® or Android® device) may be utilized as audiovisual display unit(s) 107. In such embodiments, certain camera components may already be present on the standard tablet, pointing outward from the tablet display panel. A plurality of tablets might be synchronized or paired with one another to show the same media simultaneously and capture different outbound viewpoints from each of the audiovisual displays 107. The activity of the plurality of tablets might be coordinated by a tablet acting as the media display client system 120A that resides, for example, inside the mobile vehicle. Alternatively, one of the tablets (e.g., a “master tablet”) may act as an audiovisual display 107, a camera component in the object sensor component array 106, and as the media display client system (e.g., 120, 120A-C) which coordinates the display of media on the other tablets acting as audiovisual displays. It should be noted that any combination of object sensor component locations described above, e.g., on the vehicle, in the vehicle, and on or inside the media display client apparatus, can comprise an object sensor component array. Further, camera components may be capable of more than one outbound viewpoint from a single mounting point. Some embodiments of a media display client system may include a graphics processing system 108. A graphics processing system 108 may include, for example, a graphics processing unit (GPU) and the bus architecture (e.g., VESA, PCI) and or hardware driver software required to integrate the GPU into a computing system with a processing system and other components. The graphics processing system 108 may be used to process media for rendering on the audiovisual display unit(s) 107, as well as (in some cases) to interpret images and video captures from the camera components. Embodiments also have an interface system 110, which in some embodiments may include several subtypes, such as a communications interface, an audio-video interface, and a power interface, as well as other types of serial, parallel, and specialized interfaces supporting various components. A communications interface may be used to provide communications between systems, for example over a wired or wireless network 140 (e.g., Ethernet, WiFi, a personal area network, a wired area network, an intranet, the Internet, Bluetooth, etc.). The communications interface may be composed of several components, such as networking cards or modules, wiring and connectors of various types, antennae, and the like. Synchronized tablets may communicate over a wireless network such as via Bluetooth, Wi-Fi, or cellular. Further information about communications interfaces may be found in regard to FIG. 13. An audio-video interface may be used to connect audiovisual display unit(s) 107 to the media display client system components, e.g., to the graphics processing system 108. In some embodiments, the audio-video interface may include connectors and wiring of a standard interface type, such as HDMI, VGA or SVGA, DisplayPort, and DVI, for connecting audiovisual display units (e.g., monitors) to GPU components. In some embodiments (e.g., those represented in FIGS. 1B and 1C), the audio-video interface may be of a more specialized type for connecting audiovisual display unit(s) to the vehicle system (e.g., 100B or 100C) of the mobile vehicle 99, e.g., via a special or custom interface port. Other components of the media display client system, such as object sensor components and the GPS, may be connected via other types of interfaces present in the interface system 110). These types of interface can adhere to categories such as parallel or serial (e.g., using interface standards such as USB, Firewire, etc.). In some embodiments (e.g., those represented in FIGS. 1B and 1C), the other interfaces may be of a more specialized type for connecting media display client system components together with components that are part of the vehicle system (e.g., 100B or 100C) of the mobile vehicle 99. Object sensor components (including camera components), GPS components, and audiovisual display units can be physically separate, but connected via the interface system, and still be part of same “system”. Some embodiments of media display client system 120 (e.g., 120A, 120B, and 120C) connect to vehicle system 100 to provision one or more capabilities. For example, media display client system 120 in some embodiments have direct-current (DC) power interfaces (e.g., wiring harnesses) connected to the DC power bus of the vehicle system 100 in order to provide power to one or more system components. Similarly, some embodiments may also have an AC-to-DC converter device that transforms DC power from the DC power bus of the vehicle system 100 to AC power that can be used for standard computing system components. Power interfaces, when present, are subtypes of the interface system 110. Some embodiments of media display client system 120 may optionally have their own power system, such as a battery cell like those found in a UPS (uninterruptable power supply). These embodiments may use power interfaces (including DC interfaces and AC-to-DC converter devices) to recharge the battery system in the UPS. In some embodiments, power for system components or power for recharging the UPS battery can be provided to the system by a solar panel arrangement affixed to the outside of the apparatus and the associated solar panel to AC or DC power interface components. Network 140, connectivity to which is provided by the interface system 110, may be accessed by the media display client system 120A to interact with media management system/service 160, and client interface 130, as described in relation to FIG. 1. Certain types of vehicle systems may have components that the mobile vehicle uses to perform autonomous driving, vehicle-assisted driving, or that are used in driver warning systems (e.g., pedestrian detection, backup camera warnings, side mirror object detection, automatic braking systems). Vehicle systems may also have GPS components to provide navigation functions. FIG. 1B shows an example embodiment of a systems environment in which media display client system 120B leverages certain components that are part of the vehicle system 100B of the mobile vehicle 99. In FIG. 1B, the media display client system 120B utilizes a GPS component 104 that is part of the vehicle system 100B. Object sensors in the object sensor component array 106 of the vehicle system 100B are also utilized in this example. So that the vehicle system components may be leveraged, connectivity between the media display client system 120B and the vehicle system 100B are provided by a paired interface system 110, present on both 120B and 100B. Element 141 is indicative of the local pairing connection between the systems 120B and 100B. Connection 141 may be implemented over a network (e.g., wired, wireless, Bluetooth) using a communications interface as described previously. In some embodiments, the other interfaces may be of a more specialized type for connecting media display client system 120B components together with components (e.g., 104, 106) that are part of the vehicle system 100B of the mobile vehicle 99. Specialized program instructions 102B on the computer readable storage media 101 of the media display client system 120B, when executed by processing system 103, perform these specialized operations for accessing and instructing vehicle system 100B components. In addition to these specialized program instructions, program instructions may also include those described in regard to various process flows of system 120B (e.g., in FIGS. 1, 1A, and 7A-7D) and/or those for controlling system components such as audiovisual display unit(s) 107, in conjunction with graphics processing system 108. Network 140, connectivity to which is provided by the interface system 110, may be accessed by the media display client system 120B to interact with media management system/service 160, and client interface 130, as described in relation to FIG. 1. In addition to object sensor and GPS components as described in FIG. 1B, certain types of vehicle systems have components that the mobile vehicle 99 uses for other functions, such as computing system components for processing object sensor data for vehicle control systems, providing user interfaces to drivers, and connecting with video display screens inside the vehicle (e.g., for passenger entertainments). FIG. 1C shows an example embodiment of a systems environment in which a simplified media display client apparatus 120C leverages several components that are part of the vehicle system 100C of the mobile vehicle 99. In FIG. 1C, the media display client apparatus 120C, such as the apparatus depicted in FIGS. 2-6, has audiovisual display unit(s) 107, and an interface system 110. The apparatus 120C utilizes a GPS component 104 that is part of the vehicle system 100C. Object sensors in the object sensor component array 106 of the vehicle system 100C are also utilized in this example. In addition, the graphics processing system 108, processing system 103, and computer-readable storage media 101 of the vehicle system 100C are leveraged by the media display client apparatus 120C. Some mobile vehicle systems have the capability for users or manufacturers to upload custom programs onto the vehicle systems. For example, “apps” can, in some mobile vehicles, be installed via the vehicle's user interface panel, which may run a variation of a common operating system such as ANDROID. In this manner, specialized program instructions 102C may be installed on the computer readable storage media 101 of the vehicle system 100C. When executed by processing system 103 it may perform these specialized operations for accessing and instructing vehicle system 100C components in accordance with processes herein. In addition to these specialized program instructions, program instructions may also include those described in regard to various process flows described herein (e.g., in FIGS. 1, 1A, and 7A-7D) and/or those for controlling system components such as camera components and audiovisual display unit(s) 107 of the media display client apparatus 120C that are outside the vehicle system 100C. Connectivity between components of the media display client apparatus 120C and the vehicle system 100C so that the vehicle system components may be leveraged are provided by a paired interface system 110, present on both 120C and 100C. Element 141 is indicative of the local pairing connection between the systems 120C and 100C. Connection 141 may be implemented over a network (e.g., wired, wireless, Bluetooth) using a communications interface as described previously. In some embodiments, the other interfaces may be of a more specialized type for connecting media display client apparatus 120C components (e.g., 107) together with the vehicle system 100C of the mobile vehicle 99. Network 140, connectivity to which is provided by the interface system 110, may be accessed by the vehicle system 100C to interact with media management system/service 160, and client interface 130, as described in relation to FIG. 1. User interface features may be presented on the mobile vehicle's 99 user interface panel, which serves as a client interface 130, via an app to show the driver operator various metrics and provide control and configuration options. In some embodiments of media display client system 120, including any example configuration in FIGS. 1A-1C, all or part of the processing system (e.g., 103), computer-readable storage media 101, and program instructions for a media session component (e.g., 102A, 102B, and 102C) may supported by a cloud computing arrangement, for example by being hosted by the media management system/service 160. In such embodiments, a remote computing system (e.g., consisting of a multiplicity of coordinating virtual cloud computing systems) may use their processing systems to execute certain program instructions, such as those implementing all or part of the media display client system process flows described in FIGS. 7A-7D. Connectivity to these cloud computing systems, which may be instances of computing systems as described in FIG. 13, may be provided by using the interface system 110 to connect with the media management system/service 160 over network 140. FIGS. 2-6G show example embodiments of components, configurations, and placement of a media display client apparatus for housing a media display client system and mounting it on a mobile vehicle. FIG. 2 shows an example view of a media display client apparatus mounted on a mobile vehicle. In FIG. 2, media display client apparatus 200 is mounted to the roof of mobile vehicle 99. It should be noted that other top-surface mounting points on a mobile vehicle are possible, e.g., on the trunk of a passenger car, the top of a delivery truck, and the open bed of a pickup truck or top of a pickup “topper”-FIG. 2 shows only one possible mounting point of media display client apparatus 200 in which the mounting hardware is bolts through the vehicle roof. Mounting of apparatus 200 on other surfaces is described in respect to subsequent figures. FIG. 3 shows an example exploded view of an embodiment of a three-sided media display client apparatus. In FIG. 3, a protective enclosure is formed by the assembly and attachment of a triangular roof piece 320, to three transparent sides/panels 350 positioned vertically or nearly vertically, to a base platform 380. It should be noted that, while the triangular protective enclosure formed by the triangular roof piece 320, matching triangular base 380, and three transparent panels/sides 350 constitutes one possible shape of a media display client apparatus, other shapes are possible—e.g., foursided protective enclosures using a rectangular-shaped roof and base and four panels/sides; pentagonal or hexagonal protective enclosures with five- or six-sided roofs and bases and five or six panels/sides, respectively; oval or elliptical-shaped protective enclosures using oval-shaped roofs and bases and two curved sides; and protective enclosures with slanted or curved sides. The base 380 and roof 320 can be made from a variety of materials, including high durability plastic, UV-resistant plastic, and metal alloys. Transparent sides/panels 350, for example, can be composed of shatter-resistant glass (e.g., tempered) or high-durability transparent plastic, such as Acrylic/PMMA or clear polycarbonate. Some embodiments may have one or more non-transparent sides made of other durable but non-transparent plastics. It should be noted that any panel/side in any embodiment may be only partially transparent, as for instance when part of surface of the side is transparent and the remainder of the surface of the side is opaque. Housed inside the protective enclosure formed by base 380, roof 320, and sides 350 are audiovisual display unit(s) 370. Audiovisual display unit(s) 370 are standard panel displays (e.g., LCD or LED) or tablet computers. Other variants (not shown) support a curved or bendable audiovisual display panel. The audiovisual display units face outward so that the transparent sides 350 allow them to be visible to persons looking at the media display client apparatus. Bracing pins or tabs 381 on the base 380 are shown, for illustrative purposes, for holding the audiovisual display units 370 vertical (or nearly vertical) and stable within the protective enclosure; these are not intended to be limiting, as there are many ways of bracing or affixing the audiovisual display units 370 within the apparatus. Similar bracing pins or mounts (not shown) may be provided in some embodiments to secure other components (e.g., computing system 360, battery cell/UPS 361) that are optionally present within the protective enclosure Many possible options exist for mounting the apparatus on a mobile vehicle 99 and the depicted example is not intended to be limiting. The example media display client apparatus shown in FIG. 3 includes mounting hardware (e.g., 390, 391) attachable to the base 380 for affixing the apparatus to the roof rack rails 395 already installed mobile vehicle 99. In FIG. 3, a two-part sliding unit 390 is shown, where one hollow component is attached to the base 380 (e.g., using bolts or other fasteners) and the other angular component is inserted into the hollow component attached to the base 380. The vertical part of the angular end of 390 is then paired with a component 391 that attaches vertically to the roof rack rails 395. Other types of mounting hardware include screws or bolts that affix the apparatus to the mobile vehicle surface (e.g., FIG. 2). Magnets may also be attached to the underside of the base 380 to magnetically clamp the apparatus to the metallic surface of the mobile vehicle (e.g., FIGS. 6E and 6G). In certain embodiments, the mounting hardware is operable to adjust the positioning of the apparatus on different possible mounting surfaces of the mobile vehicle. For example, the height of the base platform above the mobile vehicle mounting surface may be adjustable so that object sensors or cameras already on the mobile vehicle (e.g., on the roof of the mobile vehicle) can continue to operate without obstruction. The centering of the base platform on the mobile vehicle may also be adjustable. For instance, centering of the apparatus on the roof of the mobile vehicle is possible using the two-part sliding rail unit 390 paired to movable affixation components 391 that can be variably positioned on roof rack rails 395 depicted in FIG. 3. In some embodiments, the tilt of base platform can be adjusted to accommodate a sloping roof or trunk lid of a mobile vehicle. FIG. 3 shows an example of such mounting hardware in that the vertical part of the angular end of 390 may be paired adjustably with component 391 attaching to the roof rack rails 395. Other components depicted in FIG. 3 are illustrative of variations available in some (but not all) embodiments. As described elsewhere, object sensor component array 310 may be mounted or affixed to the top of the roof 320 of the media display client apparatus. Camera components (not shown) may also be mounted similarly on top of roof 320. A cooling unit 330 may be present in some embodiments that is capable of regulating the temperature and/or humidity levels within the protective enclosure. The cooling unit 330 can be of a small or portable size, as depicted in FIG. 3. The cooling unit 330 can, for example, use evaporative cooling technology to cool air by passing the air over a fan with self-contained evaporative cooling pads moistened by water from a reservoir. Cooling units such as these use little power (in some cases they can be powered by USB or even standard-size batteries (e.g., AA size) and are small in size (see, for example, the “WoneNice Portable Small Fan & Mini-air Conditioner, Runs On Batteries Or USB-Gray” sold on Amazon.com, which cools air by approximately 30° F. and runs on AA batteries or powered USB). These examples are of course not intended to be limiting; larger-sized HVAC and refrigeration technologies (e.g., portable A/C units and the types of compressor systems seen in small refrigerators) are also envisioned. Also depicted in this example embodiment is a battery unit 331 for powering the cooling unit 330. In other embodiments, the cooling unit 330 can attach (e.g., via powered USB or other power interface) to the computing system battery cell 361, computing system 360 (e.g., via powered USB port) or DC power system of the mobile vehicle. The cooling unit 330 and/or battery unit 331 may be supported by or separated from a divider platform 340 with vent 341 for positioning the cooling unit 330 and battery unit 331 above the compartment containing the audiovisual display units 370 and computing system 360. The vent 341 may allow airflow between the cooling unit 330 and the protective enclosure compartment and provide dust/particulate filtration with the addition of a filter. Vent 341 may allow for hot air to exit the protective enclosure compartment without intaking any outside elements such as water that can damage the components inside. In some embodiments, including the example in FIG. 3, a computing system 360 may be housed within the protective enclosure, for example in the empty space created between the panels of the audiovisual display units 370 or within the roof 320 or base platform 380. The computing system 360 includes, for example, certain components of a computing device as described in FIG. 13 (e.g., a processing system, computer-readable storage media, an interface system, a GPS component). Computing system 360 is capable of executing the program instructions of a media display client system (e.g., 120) as described in regard to FIGS. 1-1C, 7A-C, and elsewhere herein. The computing system 360 need not be a desktop-type device as suggested by FIG. 3; it could be a small, standard-model portable device such as a “Raspberry Pi.” If one of the audiovisual display units 370 is the screen of a tablet device, the computing system 360 structure/functions can be serviced by the tablet device. The front-facing cameras of tablet devices used as audiovisual display units 370 may also be part of the object sensor component array. In some embodiments, as in FIG. 3, a battery cell 361 may be housed within the protective enclosure that is operable to provide power to one or more of the audiovisual display units 370, the computing system 360, or the object sensor component array 310. The battery cell 361 can be, for example, a UPS-type battery cell that is charged continuously by the power system of the mobile vehicle through a power interface as previously described. FIG. 4 shows an example outbound viewpoint from the inside of the protective enclosure of a media display client apparatus 200. In the outbound viewpoint, an obstruction 440 (e.g., representing debris such as a newspaper or snow) of the view through the transparent panel is shown as a shaded area. Also depicted are a truck 420 and automobile 430, which may
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