# Multimodal Natural Human-Computer Interfaces for Computer-Aided Design: A Review Paper
## 4 Ways to Implement Natural HCI for CAD
### Speech Recognition-Based Interaction for CAD  
With the rapid development of speech synthesis and recognition technologies in recent years, speech-based HCIs have been extensively employed in a variety of household, automobile, office, and driving applications [48; 97]. In particular, voice-enabled intelligent personal assistants (IPAs), such as Amazon's Alexa, Apple's Siri, Google Assistant, and Microsoft Cortana, are widely available on a number of automatic devices [97]. Using a speech-based interface, routine operations can be efficiently executed with intuitive voice commands and the ease of use of the system can be improved. As well as in the household, speech-recognition technology can be applied in the context of CAD.  
CAD modeling involves the development of a virtual object, always using step-by-step commands of instantiating primitives and using operations such as move, copy, rotate, and zoom for model manipulation and assembly [11]. These CAD commands could be realized naturally and directly through a speech-based interface. Therefore, some studies tried converting the names of menus and icons into voice-controlled commands [48; 49; 50; 51; 52; 53]. Most of these studies were aimed at using fixed expressions to perform modeling operations. For example, [PERSON] et al. [98] developed a prototype system with more than 80 speech commands which were primarily used to activate different CAD operations, for instance, selecting various operations of the system or switching from one mode to another. Similar research was also carried out by [50; 52; 53].  
However, for all these speech-based CAD systems, CAD commands can only be activated when users utter the exact corresponding words or expressions [51]. So, novice users still have to devote considerable time to familiarize themselves with these systems and remember all the fixed vocabularies or preset expressions, which limits the utility of these speech-based CAD systems [54]. In order to implement a more flexible modeling operation, [PERSON] and [PERSON] [24; 54] proposed integrating a semantic inference approach into a speech-based CAD system, so users will no longer be constrained by predefined commands. In such a system, instead of using one-to-one mapping from word expressions to model actions, users can, for example, employ various expressions, such as \"Rectangle\", \"Draw a Rectangle\" and \"Create a Rectangle\", all with the same effect, namely, to generate a rectangle. This frees users from the need to memorize a host of predefined voice commands [24]. Whereas it is plain to see how this would work for simple commands, recognition methods should be further advanced for a speech-based CAD system to deal flexibly and intelligently with complex verbal commands [24; 55].  
Note that, as an effective interaction modality for CAD modeling, speech has been mostly used in conjunction with other interaction interfaces, such as \"speech and gesture\" [62], \"speech and EEG\" [3], and \"speech and eye tracking\" [32]. For this topic, more detailed studies will be reviewed in Section 4.5.