package cardboard

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* Message describing properties of a VR head mount device (HMD) which uses an interchangeable smartphone as a display (e.g. Google Cardboard). While some properties are certain (e.g. inter_lens_distance), others represent nominal values which may be refined depending on context (e.g. viewport_angles). Lengths are in meters unless noted otherwise. Fields are _required_ unless noted otherwise. Some context on why this set of parameters are deemed necessary and sufficient: * FOV scale can be reasonably approximated from lens-to-screen distance and display size (i.e. knowing lens focal length isn't crucial). * Lenses are assumed to be horizontally centered with respect to display. * The display is not necessarily vertically centered. For interchangeable phones where the device rests against a tray, we can derive the vertical offset from tray-to-lens height along with phone-specific bezel and screen sizes (supplied separately).

• optional string vendor = 1

  String identifying the device's vendor (e.g. "Google, Inc."). A device's [vendor, model] pair is expected to be globally unique.

• optional string model = 2

  String identifying the device's model, including revision info if needed (e.g. "Cardboard v1"). A device's [vendor, model] pair is expected to be globally unique.

• optional float screen_to_lens_distance = 3

  Distance from the display screen to the optical center of lenses. This is a required field for distortion rendering, and must be positive.

• optional float inter_lens_distance = 4

  Horizontal distance between optical center of the lenses. This is a required field for distortion rendering, and must be positive.

• repeated float left_eye_field_of_view_angles = 5

  Four-element tuple (left, right, bottom, top) of left eye's view extent angles relative to center, assuming the following: * eye is aligned with optical center of lens * display screen is equal or larger than extents viewable through lens * nominal eye-to-lens distance * mirrored field of view will be applied to the right eye These values are essentially used as an optimization to avoid rendering pixels which can't be seen. This is a required field for distortion rendering, and angles must be positive.

• optional DeviceParams.VerticalAlignmentType vertical_alignment = 11

  Set according to vertical alignment strategy-- see enum comments above. NOTE: If you set this to CENTER, see special instructions for the tray_to_lens_distance field below.

• optional float tray_to_lens_distance = 6

  If the phone is aligned vertically within the device by resting against a fixed top or bottom tray, this is the distance from the tray to optical center of the lenses. This is a required field for distortion rendering, and must be positive. NOTE: Due to a bug in initial versions of the SDK's, this field must be set explicitly to .035 when vertical_alignment = CENTER.

• repeated float distortion_coefficients = 7

  Coefficients Ki for pincushion distortion function which maps from position on real screen to virtual screen (i.e. texture) relative to optical center: p' = p (1 + K1 r^2 + K2 r^4 + ... + Kn r^(2n)) where r is the distance in tan-angle units from the optical center, p the input point, and p' the output point. Tan-angle units can be computed as distance on the screen divided by distance from the virtual eye to the screen.

• optional DeviceParams.ButtonType primary_button = 12

  Specify primary input mechanism of the HMD. Intended for advisory purposes only, to address simple questions such as "can HMD be used with apps requiring a physical button event?" or "what icon should be used to represent button action to the user?".

Used in: DeviceParams

• NONE = 0

  No physical button, and touch screen is not easily accessible.

• MAGNET = 1

  HMD has integrated magnet switch similar to original Cardboard.

• TOUCH = 2

  At least a portion of touch screen is easily accessible to user for taps.

• INDIRECT_TOUCH = 3

  Touch screen is triggered indirectly via integrated button on the HMD.

Used in: DeviceParams

• BOTTOM = 0

  phone rests against a fixed bottom tray

• CENTER = 1

  phone screen assumed to be centered w.r.t. lenses

• TOP = 2

  phone rests against a fixed top tray