Projecting a full-size image on the windshield would in no way be safe. And using a projector on a windshield there is no way to really see how well the image would end up. Heads up displays are projected in a very small area with simple digits that have lots of empty areas, not a map going across the windshield. The projector would be connected with a video cable not with USB or bluetooth. I thought you would use this for when the vehicle was stopped and you are going over the map.
There are plenty of small projectors you can take a look at, many cheap Chinese things mostly done as toys then anything else.
https://www.google.com/search?q=por...2j69i57j0l5.3688j0j4&sourceid=chrome&ie=UTF-8
I would have to dispute your statement that projecting a full size (10") display "would in no way be unsafe". The safety of what is projected is dependent upon the opacity, and the location on the windshield. In many cases, a full-size projection would enhance safety. I can say, predicated upon some substantial experience, that experience in the use of Heads-Up Displays (HUD) is an important component of safety of use. Additionally, determining a correct degree of opacity predicated upon the specific location of the image, size, and slope/angle of the windshield, etc. are experience-dependent. The principle factor in their use is to effectively proportion the duration of the driver's visual scan as a fixation upon the projection, e.g. "Target Fixation" so as to permit proper allocation within the process of situational awareness. The next factor in proper utilization of HUD's is to adapt the scale of the object to the operational factors such as vehicle speed so as to adapt the scale to the speed, variations in terrain, etc., and complexity of the vehicle operation. Obviously, the scale to be projected would be as vastly different in interstate highway operations where a speed exceeding 1 mi./min. versus a complex trail operation, e.g. rock crawling where speeds are <~42 ft./min. (many times with an external spotter).
The principal alternative to a windshield projection would be the more conventional mounting of a laptop computer, e.g. a Panasonic Toughbook CF-31 is one of the most popular types of computers and is frequently used within law enforcement vehicles. In these vehicles, the computer is generally mounted so as to place the screen area below eye level, generally close to the level of the instrument panel of the vehicle or slightly above same, and they create a visual obstruction that makes the use of the screen, and data contained thereon, unavailable to the driver while in motion. This is not a problem when the computer is being used when the vehicle is not moving, however, even when the vehicle is moving at an extremely slow speed it is a major compromise of safe operation even if it is being used by a second officer in the passenger seat position within the vehicle.
The ergonomics of computers, and many other devices, being utilized while operating an automobile are a subject of much discussion by both behavioral sciences, engineers, and drivers themselves. The issues are further complicated by the infinite variables within the context of "operations" that make it difficult, if not impossible to make definitive judgments, quantitatively as well as qualitatively in respect to what actually constitutes an actual safety factor, and what is required for the integration of contextual and situational usage, e.g. does a driver who is typing a short text message on a cell phone while stopped at a traffic light that the driver knows has a long duration in each function, as opposed to a driver performing the same function while driving at a high(er) speed on a congested freeway during rush hour. Many scientists oppose even "hands-free" use of a cell phone while driving, concluding that the thought processes constitute a distraction that reduces driver attentiveness to conditions of traffic, weather, and the roadway itself, yet these same scientists draw a different conclusion when they are faced with the use of a police radio, which requires that the driver/Trooper hold the microphone in their hand with an additional "push-to-talk" requirement. We now have a recommended procedure that a second officer/vehicle join the pursuit at which time that Trooper would take over all communications responsibilities, thus allowing the Trooper initiating the pursuit to concentrate on the safety factors relating to the operation of not only the patrol vehicle but the same considerations as to the threat to civilian drivers on the roadway, and will necessarily include a judgment call as to the skill of the driver operating the vehicle being pursued as well as the severity of the underlying offense, etc.
I apologize in advance if this reply to your question as to the safe operation of a vehicle with a full-screen HUD display but the question you posed was worthy of an exploration in-depth, not only as it may relate to driving with a large display HUD, but is also applicable to the design of newer vehicles which includes progressively large displays, including "touch-screen displays", many of which are mechanically deployed by motorized mounts which, while perhaps not 100% analogous to a HUD, present an equal, if not greater, element in the obstruction of driver vision and moreso as a distraction from safe vehicle operation.
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