1. Introduction

1. Scene visualisation is mostly 2D (on the monitor), but displayed objects are three-dimensional. Usage of projection helmets and glasses  is not common practice because of low resolution and high price. Usage of lasers or volumetric displays for displaying 3D images is in development and is not suitable for general usage yet.
2. Input devices are mostly constructed for using in plane only. Mouse and tablet can be used for interaction in 3D space, but there is a need for some additional support from software and abstraction of the user. Typical is employment of three views to the scene - one top-view, one front-view and one from the side (3D Studio from Autodesk). Another approach is one perspective view combined with one additional view defined by the user (TrueSpace from Caligari). Both this systems employ various constraints to cursor movement, such as snap-modes or movement in one or two axes only. These are needed to simplify the interaction with objects in the scene for the user.

• Enhancing existing input devices. For example "SpaceBall", joysticks with spinning handles, force-feedback devices etc.
• Using devices, which were constructed for 3D interaction from start. Examples are data-gloves and 3D mice.
• Motion--capture systems. These are used mostly for animations and are too expensive for the common use.

1.1 Types of 3D locators

1. Optical and opto--mechanical systems. Optical systems use markers, which are placed on the moving object and tracked with cameras. They are used mostly in motion-capture systems. Opto-mechanical systems employ glass or plastic fibers, acting as waveguides. When the fiber is deformed, light evades from it and this can be detected as change in the intensity of the light on fibre's end. Typical usage is in data-gloves and data-suits.
2. Mechanical systems. These use complicated system of arms and ankles with several degrees of freedom. Each ankle has sensor for detecting angular movement, some systems are equipped with force-feedback. They are accurate and relatively cheap, but rather unwieldy. They are used mostly in robotics and simulators.
3. Locators employing acoustic waves. Usually work with ultrasonic signal, idea is very simple - they measure Time Of Flight (TOF) of the emitted signal from transmitter to several receivers. Using at least three non-collinear receivers enables calculation of the position in the working volume. This principle was used in the design of the 3D mouse.
4. Magnetic trackers. Magnetic sensors are widely used because they are cheap, but they have several disadvantages - low accuracy, sensitivity to interference caused by large metallic objects and small range.
5. GPS based systems. These are used mostly in robotics and navigation, have only limited usage in VR, because they don't work inside of buildings, have low resolution (several meters) and are very expensive.