# Multimodal Natural Human-Computer Interfaces for Computer-Aided Design: A Review Paper
## 4 Ways to Implement Natural HCI for CAD
### Eye Tracking-Based Interaction for CAD  
The human gaze is defined as the direction to which the eyes are pointing in space, which could indicate where the attention of the user is focused [76,77]. Eye tracking is a technique in which eye-trackers record a user's eye movements to determine where on the screen the user's attention is directed when looking at the screen [78, 79, 80]. Ever since 1879, eye tracking has been used extensively in psychological science in the field of reading and image perception, and more recently in neuroscience and computer applications, especially HCI applications [80]. In the area of HCIs, eye tracking offers an important interface for a range of input methods including pointing and selecting [81], text input via virtual keyboards [82], gaze gestures [83], and drawing [84]. By tracking the direction and position of human eyes, the user's intention could be judged and analyzed directly and rapidly [80,85,86]. For example, [PERSON] et al. [87] developed two eyeLook applications (i.e., seeTV and seeTXT) based on a gaze-aware mobile computing platform. For seeTV, a mobile video player, gaze is used as a trigger condition such that content playback is automatically paused when the user is not looking. seeTXT, as an attentive speed-reading application, can flash words on a display, advancing the text only when the user is looking. Similarly, [PERSON] et al. [88] integrated an eye-tracking-based interface into a handheld mobile device, through which the user is able to move the cursor and interact with a mobile device through their gaze. Furthermore, [PERSON] and [PERSON] [89] demonstrated the capability of a smart phone to track the user's eye movements across the phone. They presented a prototype implementation of EyePhone on a Nokia phone, enabling the user to operate a phone in touch-free manner. Incorporating such a facility in the process of interaction enhances the entertainment value and practicability, and benefits the development of natural HCIs. For CAD applications, eye tracking could be used to interpret the user's intentions for modeling and operating a 3D model effectively in a touch-free manner [34,39]. For example, model selection is one of the fundamental operations and also the initial task for the most common user interactions in CAD modeling [41]. Using eye-tracking technology, the user could select models in 2D/3D environments directly and quickly. [PERSON] et al. [6] and [PERSON] et al. [69] adopted eye tracking as an extra channel of input and employed the user's gaze to find the intersection point with the surface of the target to select 3D objects in a virtual environment. In addition, eye tracking could also be used in certain CAD operation tasks as an independent pointing interface, such as rotation, zooming, and translation. In traditional CAD systems, free rotation is obviously difficult, requiring additional commands for setting the rotation center [34]. With an eye-tracking-based interface, the user in the CAD environment can easily choose the rotation center without much effort just by fixating their gaze on it. For the zooming task, in order to explore detailed information through zooming, the user needs to preset the point of interest to the target that the model view camera moves towards in CAD applications. To ensure intuitiveness of operation, the location of interest is fixated and recorded by the eye-tracker, while the zooming operation could be controlled by another modality, e.g., a hand gesture [34]. Furthermore, the direction of translation