# Multimodal Natural Human-Computer Interfaces for Computer-Aided Design: A Review Paper
## 3 Natural HCI for CAD
### The Effects of Using Natural HCI in CAD  
Natural HCIs will drive a gradual shift in the product design process from a traditional system-centered design to user-centered design, and provide a more intuitive model operation experience for users [31]. Thus, research on natural HCIs will become a main development trend of CAD interaction technology. While natural HCI technology is not yet commonly used in current CAD systems in industry, it has been explored extensively in academic research and areas where natural HCIs could bring significant positive effects have been identified [32; 33]. The potential effects of natural HCIs in CAD are mainly reflected in three aspects described below.  
They can shorten the learning cycle and reduce the difficulty of learning for novice users.  
Since users traditionally have to spend a huge amount of energy on learning and memorizing dense software menus and complicated operation instructions in modeling, it is not easy for the novice user to learn and master a new CAD application. To overcome this shortcoming, a natural HCI provides users with a novel interactive pattern where users just need to model in the way that is most comfortable and natural for them. The users' intention of modeling will be automatically sensed and accurately recognized by the multimodal natural HCI without the user having to remember a lot of rules and instructions embedded in CAD systems. For instance, in order to manipulate a target model, users can select it just by directing their gaze at it and, at the same time, instruct the system to move the model by making a move gesture with their hands [34]. This interactive pattern is easy for novices to learn and could be quickly mastered regardless of experience level.  
A natural HCI also adds cross-platform features to CAD systems that enable users to control 3D models in different software environments. As a result, users do not need to relearn a large number of new commands when faced with a new CAD application. This could significantly shorten the learning cycle and quickly turn users from novices to experts.  
They enable a more intuitive and natural interaction process.  
Traditional WIMP interfaces use 2D subspace inputs such as mouse and keyboard to create and operate 3D models, which disconnect the input space from the modeling space and make the interaction process tedious and non-intuitive [3]. A natural HCI circumvents the limitations of traditional interfaces in terms of interaction space and dimensions, and provides a new end-to-end channel for direct interaction between users and 3D models. By capturing and recognizing the users' EEG, gestures, speech, and other original signals in 3D modeling, natural HCIs can provide a more familiar and direct interaction for users, meaning that the interaction between the user and model can be performed more intuitively and naturally [35].  
They can enhance the innovative design ability of designers.  
Current CAD systems have difficulty in supporting innovative design as they have been developed with too much emphasis on graphical representation techniques, and not enough on exploring how to support the processes that are essential for creative design [36,37]. Central to innovative design are ambiguity, uncertainty, and parallel lines of thought. All of this is limited by the circumscribed thinking and premature fixation from current CAD systems [38], which are readily supported by natural HCIs.  
For natural HCIs, the input and output devices that separate the user and the system should be as unobtrusive as possible or hidden, so that users do not notice them and instead pay more attention to the product design process [3]. In addition, a natural HCI gives designers the freedom to creatively express their ideas and allows them to rapidly generate a wide variety of concepts, which will increase designers' ability to innovate.