Hi Min,
The Tobii EyeX Engine (and the Tobii EyeX SDK) only works with a monitor attached to the system. With the Tobii Gaze SDK, on the other hand, a monitor is not required. With the Gaze SDK you always have to write your custom screen setup and calibration procedure, and you could define a virtual screen plane. The tricky part would be to implement the calibration procedure where you have to make the user look at pre-defined points on this virtual screen plane for time intervals that are dynamic depending on how long time the eye tracker needs to collect enough data for each point. Both the EyeX SDK and the Gaze SDK are available via the Downloads page.