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There are several factors that engineers must consider when designing a HUD:

• field of vision — Since a person’s eyes are at two different points, they see two different images. To prevent a person’s eyes from having to change focus between the outside world and the display of the HUD, the display is “Collimated” (focused at infinity). In automobiles the display is generally focused around the distance to the bumper.
• eyebox — displays can only be viewed while the viewer’s eyes are within a 3-dimensional area called the Head Motion Box or “Eyebox”. Modern HUD Eyeboxes are usually about 5 by 3 by 6 inches. This allows the viewer some freedom of head movement. It also allows the pilot the ability to view the entire display as long as one of his eyes is inside the Eyebox.
• luminance/contrast — displays must be adjustable in luminance and contrast to account for ambient lighting, which can vary widely (e.g., from the glare of bright clouds to a moonless night approach to a minimally lit field).
• display accuracy — aircraft HUD components must be very precisely aligned with the aircraft’s three axes – a process called boresighting – so that displayed data conforms to reality typically with an accuracy of ±7.0 milliradians. Note that in this case the word “conform” means, “when an object is projected on the combiner and the actual object is visible, they will be aligned”. This allows the display to show the pilot exactly where the artificial horizon is, as well as the aircraft’s projected path with great accuracy. When Enhanced Vision is used, for example, the display of runway lights must be aligned with the actual runway lights when the real lights become visible. Boresighting is done during the aircraft’s building process and can also be performed in the field on many aircraft.[3] Newer micro-display imaging technologies are being introduced, including liquid crystal display (LCD), liquid crystal on silicon (LCoS), digital micro-mirrors (DMD), and organic light-emitting diode (OLED).
• installation — installation of HUD components must be compatible with other avionics, displays, etc.

Via Wikipedia