simovni2:hmd:types [SimOvni 2]

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===== HMD Types =====

These standalone HMDs can be placed into 2 categories. 3DOF and 6DOF (degrees of freedom). All use an IMU (Inertial Measurement Unit).
  * 3DOF category : they only estimate their orientation solely by use of gyroscope + accelerometer (IMU) and magnetometer. They are cheaper.
  * 6DOF category : they fully locate themselves in space by use of gyroscope + accelerometer (IMU) and magnetometer AND by "looking" at the environment (using many cameras looking all around).  
    * In principle, all 6DOF HMDs should be able to run in a 3DOF degraded mode. 
    * Some of these HMDs can track the hands. This in itself is a little revolution. For instance, if the user points a direction with his index, the HMD can display to the user a virtual laser that extends his index to infinity. The HMD can thus extract the absolute direction pointed to by the index. 
    * Some of these HMDs can track the eyes. The accuracy is not very high (0.5°). But with some tricks, it should be possible to reach higher accuracies.

For our application, we don't always need the full 6DOF tracking. 3DOF tracking is enough. This is because quite often, the witnesses do not translate during the observation. 

To name a few (6DOF except if otherwise stated)
  * VR
    * Oculus Rift 
    * Oculus/Meta Quest 2 
    * HP Reverb
    * Samsung Gear VR type (3DOF category)
  * AR
    * Hololens 1 & 2
    * Magic Leap 1 & 2
    * Oculus/Meta Quest Pro & Meta Quest 3(S) 
    * Pico 4
    * Lynx-R1
    * Apple Vision Pro



===== Expected limitations =====


Many of the characteristics of current HMDs make them actually worse than a basic optical system like the SimOvni.

Due to the technologies involved, all HMDs have several of these issues

  * Very limited dynamic range of the brightness of virtual objects displayed
    * cannot display objects of very low luminosity
    * cannot display objects of high luminosity
  * Resolution still below human eye resolution, but we are getting really close
    * as an example : the oculus quest 2 has a resolution of about 20 pix/°, this is to be compared with the 1 arc minute (60 pix/°) of the 20/20 human eye.
  * Vergence-Accommodation Conflict : Unfortunately, almost all the HMDs on the market are designed to display the virtual images at a fixed distance from the user ([[simovni2:hmd:specs|lets say 2m]]). By this you should understand that the optical system creates a virtual image that is seen clearly if the user accommodates at that 2m distance. For 3d objects not located at 2 meters from the user's eyes in virtual space, this produces a "vergence conflict" in the visual system of the user. For example, when the HMD displays a 3d object "at infinity", the eyes are oriented parallel but still should accommodate at 2 meters (while the brain/eyes expects infinity). I don't know if studies have estimated the impact, but it may alter the estimations by the witness.
  * FIXME Still tracking in low light ? I do not know right now, the minimum of luminosity of the environment needed for the HMD to be able to track its pose in space accurately without too much noise. This remains to be estimated. 
  * FIXME More general tracking limitations  : What about tracking in a moving car or train ? This becomes even more challenging for the HMD. It may be able to track the inside, but not the outside and certainly not both at the same time.  

==== Using Virtual Reality HMDs (head mounted displays) ====

  * Very limited dynamic range for the display of the environment. Unable to show a dark sky. Especially with LCD screens. OLED screens can do much better.
  * This is not a limitation per se, but using VR HMDs imply that some modeling of the environment be done before a session with the witness.
    * It would involve typically as a minimum that a 360° photographic panorama be done and quite some work to create the transparency layer for the sky. It is quite time consuming in some situations where the sky is cluttered with trees for example.
    * The fact that such "modeling" is not needed in augmented reality is one of the advantages of AR over VR. 
==== Using Augmented Reality HMDs ====

All HMDs for augmented reality have the advantage of removing the need for the modeling of the environment. Only the phenomenon observed by the witness has to be modeled.

=== See through HMDs (where one can see reality through a transparent medium) ===

  * Bad color rendition of the virtual objects : the optical systems that use the technology of wave guides suffer from **terrible** color distortions. By terrible I mean that white objects actually appear pink, greenish or reddish and in a non uniform way.
  * Limited angle of view : the most advanced, standalone see through HMD, the Hololens 2, only has 35° of horizontal field of view. Fortunately, few UAP cases involve UAPs seen under angles of view higher than 35°. 
  * Very good resolution for Hololens 2: 47 pix/°
  * Quasi perfect environment display : by design.

=== Video through HMDs (where one can see reality through video cameras) ===
Below are limitations in addition to the limitations of HMDs for Virtual Reality
  * Bad resolution for the display of the environment due to the limited resolution of the video cameras. The video resolutions of AR HMDs are much lower than consumer cameras because AR requires a frame rate higher than 60fps. Here we are talking about 17pix/°.
  * Cameras unable to work in low light : expect big video noise in the dark.
  * Very limited dynamic range for the display of the environment. Video though HMDs combine the limitations of their cameras and displays. Unable to show a dark sky. Especially with LCD screens. OLED screens can do much better but cannot compensate for the noise of the camera. In principle, if the environment does not move much, denoising algorithms could be applied. But I doubt that this behavior comes free with the HMD. I don't expect "to be able"/"have the time" to develop this myself.

====== Candidate HMD Specs ======

[[https://ufowaves.org/enquetes/simovni2/doku.php?id=simovni2:hmd:specs|Candidate HMD specs]]