# Multimodal Natural Human-Computer Interfaces for Computer-Aided Design: A Review Paper
## 4 Ways to Implement Natural HCI for CAD
### Gesture Recognition-Based Interaction for CAD  
Gestures are a form of body language that offer an effective way of communicating with others. People use a variety of gestures ranging from simple ones (such as pointing and pinching) to more complex actions for expressing feelings [90]. In recent years, the maturity of gesture-recognition technology based on depth vision has promoted the development of gesture interaction technology in applications of CAD [91]. Gestural interfaces aim to offer highly intuitive and free-form modeling modes that emulate interactions with physical products [42] and allow designers to create 3D conceptual models quickly, while just requiring a minimal amount of CAD software experience [43]. Approaches and modes using gestures to execute CAD modeling manipulation tasks will be covered in this section.  
According to their spatiotemporal characteristics, gestures used in CAD interactions can mainly be divided into static and dynamic ones [25]. Research on static gestures considers the position information of a gesture, while that on dynamic gestures needs to consider not only the change in spatial position of the gesture, but also that in the expression of a gesture during the time sequence. Therefore, static gestures could be mainly used to invoke CAD commands for model-creation tasks while dynamic gestures could be used for model manipulation and modification [27]. In some studies [3, 34, 92], static gestures and dynamic gestures are combined for CAD manipulation instructions. For example, users can translate a model by closing the left hand into a fist (i.e., static gesture) and moving the right hand in the direction they want to translate it (i.e., dynamic gesture) [92, 93]. In this process, the left-hand gesture plays the role of a trigger signal for the translation task while the right-hand gesture controls the direction and distance of the translation.  
In our literature review, we found that researchers designed various sets or repositories of gestures for carrying out various CAD model tasks in the past [7, 8, 25, 44]. The gestures in these repositories could, in principle, readily be used to test some gesture recognition algorithms and also directly as resources to select gestures in the application of CAD interfaces. However, most gestures in these repositories are typically chosen by researchers rather than users, often for the sake of ease of implementation with existing technology, while ignoring users' preferences. It could be difficult for users to remember this information and, as a result, their cognitive loads are enhanced rather than reduced [94]. To advance from this situation, user-based research has been established as an effective gesture elicitation method [7, 95]. [PERSON] [7] and [PERSON] [25] developed user-based gesture vocabulary for conceptual design via evaluating the user's activities in the modeling process in order to achieve a higher adoptability of gestures by inexperienced users and reduce the need for gesture learning.  
Moreover, some authors explored the use of simple prescribed, even if free-form, gestures, such as pinching or grasping with one's hand, to quickly create a variety of constrained and free-form shapes without the need for extensive gesture training [45, 46, 96]. Their evaluations demonstrated that it is possible to enable users to express their intents for modeling without the need for a fixed set of gestures. Therefore, a potential future gesture system for CAD could allow individual designers to use non-prescribed gestures that will support rather than inhibit their conceptual design thinking. Meanwhile, further research into both the procedural structure of CAD-based activities and the ways in which they might change depending on the shape being created could be conducted to explore better gesture-based design patterns that adapt to users and their specific workflows as they are used [7].