Chun Yu et al., “Air finger: enabling multi-scale navigation by finger height above the surface,” in Proceedings of the 13th international conference on Ubiquitous computing, UbiComp ’11 (New York, NY, USA: ACM, 2011), 495–496

Defination

1. Virtual touch uses cameras to capture a user's hand and enables the user to manipulate an interface component with the hand image while the real hand operates on a separate surface (air medium in my case)

Fact

1. Direct hand interaction input enables natural and high degree-of-freedom input, but comes at the cost of known issues such as low accuracy and limited control range. To address the issues, several indirect approach have been proposed which allow touches be performed indirectly at the back of the device either by using separate control item or movement/gesture tracking.
2. Virtual hand enables beyond surface interaction and thus offers chances for designing richer touch based interface.

Method

Example of working states: