# Multimodal Natural Human-Computer Interfaces for Computer-Aided Design: A Review Paper
## 4 Ways to Implement Natural HCI for CAD
### Other Interfaces for CAD  
In addition to the interfaces listed in the previous sections, a short overview of other interfaces developed is given in this section.  
Touch-based interfaces are those in which designers can input design information on a touchscreen with their fingers. [PERSON] et al. [71] developed a multimodal interface, \"MozArt\", which combines speech and touch for creating and interacting with CAD models. This system features a minimalist user interface and uses a touch table whose orientation can be changed depending on the needs of the user. While MozArt provides users with efficient results, the main problem with this interface is that the touch-based system is subject to ambiguity since it is two dimensional in nature. To resolve these ambiguities, EEG is considered here as the main method to resolve these ambiguities. Therefore, a novel system combining touch and BCI was proposed by [PERSON] [30] in order to reduce the ambiguity associated with 2D touch to perform 3D model manipulation. The experimental results have shown that BCI could help to identify the true intention of the users while performing the modeling tasks by touch [30].  
Virtual reality (VR) is providing a new perspective for human interaction with CAD models, allowing for the stereoscopic visualization of virtual objects on one-to-one scale within the working space of users [74]. In a VR-based system, the user can directly create, modify, and manipulate models without learning complex protocols and procedures [109]. Haptic interface, as a promising 3D HCI for various applications, could allow the user to feel the contact force in the process of free-form design and more realistically convey virtual reality to humans so as to aid the design process [110, 111, 112]. To construct more immersive design environments for users, [PERSON] [113] and [PERSON] [114] combined haptic and VR in multimodal systems and found that haptic feedback can provide a more accurate manipulation of virtual objects and significantly improve the working efficiency of CAD systems. On this basis, a novel multimodal application, \"VRSolid\", was proposed by [PERSON] [73], further extending the input modalities within the VR environment. The VRSolid system consists of five modalities, including two for output (haptic feedback and 3D visual) and three for input (gesture, speech, and motion tracking). Following this, [PERSON] [72] and [PERSON] [35] constructed some similar multimodal systems for CAD in a VR environment. All applications mentioned above are designed to provide an intuitiveness and natural environment for interaction with CAD models, while creating a common multimodal interface that can be integrated with the existing commercial CAD software, such as SolidWorks, Creo, and Parasolid [35,72,73].  
Additionally, [PERSON] et al. [74] tested \"pen and VR\" in an application called VR-cave, which has a physical pen for free-hand sketching in 2D and 3D immersive modeling environments. [PERSON] et al. [75] investigated the performances of four modes of CAD applications, such as, \"mouse\", \"pen\", \"pen and speech\", and \"pen, speech and mouse\".  
They found that \"pen, speech and mouse\" was the best multimodal combination in terms of drawing time, modification time, and subjective preferences among these four modes [75].  
All the studies reviewed in this section further emphasize the diversity of CAD interface development research and the difference in the outcome of the interface evaluations. Further exploring more natural interfaces and integration methods for CAD application will be an important issue for future studies.