# Multimodal Natural Human-Computer Interfaces for Computer-Aided Design: A Review Paper
## 3 Natural HCI for CAD
### The Reasons Why Natural HCI Is Required for CAD  
In the early 1960s, product design processes evolved from manual to computer-aided drawing, in parallel with the emergence of computer graphics display technology [17]. The initial technology based on 2D computer graphics evolved into 3D CAD systems around the 1970s, and the subsequent evolution of computer software and hardware allowed a rapid development of CAD technology [18; 19; 20]. CAD functions have incrementally been standardized and integrated, and artificial intelligence has been incorporated into their design [20; 21]. However, a key component of CAD systems has lagged in development: interactive interface technology still reflects the earlier stages of CAD technology.  
In most CAD applications, a text-based Command Line Interface (CLI) and WIMP-based Graphical User Interface (GUI) are still the mainly used components to date [20; 22; 23]. These traditional interaction modes have some obvious limitations for CAD applications as users have to create, modify, and manipulate 3D models with 2D devices (mouse and monitor), which make it difficult to meet users' needs for natural interactions.  
On the one hand, users are forced to frequently toggle certain on/off modules, and traverse across hundreds of menu items and toolbar buttons in the current interface, just to execute the desired CAD command [24]. Moreover, users need to memorize complicated hotkey combinations to control specific software products. Different CAD software programs usually have a variety of ways to represent the same command functions. This becomes even more challenging when designing across different 3D modeling applications, such as FreeCAD, AutoCAD, and SolidWorks [11]. As a result, current 3D CAD systems require a \"very steep learning curve\" for novice users to master the complex task of using a dense set of tool bars and menus [25].  
On the other hand, the visuomotor requirements of using a mouse and keyboard may interfere with the visuospatial resources of the design process. The user needs to switch between the 3D modeling environment to the 2D input and output space [3]. In this process, the cognitive load is further enhanced by planning ahead for the manipulations needed to build the model [26; 27]. Thus, the design intentions of users are frequently interrupted since considerable attention must be paid to finding the right commands via different button clicks [24].  
Therefore, to ease learning and increase modeling efficiency, there is a strong demand for natural HCIs that bypasses these difficulties and enables users to interact with the system in a natural manner.  
Natural HCIs for CAD maximally encapsulate the characteristics of human-human interactions (HHIs) in the modeling process [14; 16; 28]. As the next generation of natural HCIs, capturing original signals, such as human EEG, eye movements, gestures, and voices, is the premise and foundation for understanding users' model operation intentions accurately [29; 30]. Note that human-human communication and exploration behavior involve visual, auditory, and tactile processes, and thus are intrinsically multimodal. Multimodal interfaces therefore become an effective approach for realizing natural HCIs for CAD.