VR world sway due to the time delay when using a HMD

Model shape	Box Fan	Head rotation
Delay		0 ms
Amplitude of rotation		\(\pm\) 0 deg.
Moving period		0 s
Interval		0 s
Time scale		1

[Japanese / English]

This page explains the "world sway" observed by a user wearing a HMD, when the delay from head movement to display exists.

This figure is a top view with the top of the window indicating the frontal direction. The inverted T-shapred mark at the bottom of the window indicates the direction of the user's head. The movement of the mesh shows the direction in which the virtual world is shown at each moment. The user's head rotates such that the angle from the forward is
\(\qquad\theta(t) = \pm A \sin \pi (t/T-1/2)\)
where \(A\) is the rotation angle, \(t\) is the time elapsed from the motion start, \(T\) is the moving time from the left to the right (and vice versa)．

At the beginning of movement, the world sticks to the head and turns, and when the head stops moving, the world turns in the opposite direction. When the VR world is updated at 60 fps, the world sway cannot be ignored even if just 1-frame delay (about 17 ms) exists. The greater the delay, the worse the world sway. This shows the importance of delay suppression (delay compensation) in the HMD system. The impact of the delay in HMD systems have been mentioned since the dawn of VR around 1990, and effective delay compensation solutions have been proposed and implemented since around 2000. Most HMD systems on the market today provide delay compensation functionality.

In fully immersive VR, the world sway due to delay is relatively less noticeable because the real world is hidden. However, especially in Augmented Reality using an optical see-through HMD, the delay compensation is more critical because deviation between the real object and the virtual object is directly visible.