# Multimodal Natural Human-Computer Interfaces for Computer-Aided Design: A Review Paper
## 4 Ways to Implement Natural HCI for CAD
### Multimodal HCI for CAD  
Multimodal interfaces aim to construct interactive systems that leverage natural human capabilities to communicate through different modalities such as speech, gesture, gaze, and others, bringing more accurate and robust intention recognition methods to human-computer interaction [29]. Thanks to advances in the development of pattern-recognition techniques (in natural language processing, machine vision, etc.) and hardware technologies of input and output (cameras and sensors, etc. ), there has been a significant increase in multimodal HCI research [107]. [PERSON] [29], [PERSON] [33], and [PERSON] [108] performed valuable surveys on the current status of multimodal HCI research. From these studies, we can easily see that the main goal of research on multimodal interactions is to explore a more transparent, flexible, efficient, and natural interface that can remove existing constraints on what is possible in the field of HCIs, and progress towards the full use of human communication and interaction capabilities. The realization of this aim depends on collaboration among researchers from different disciplines, such as computer scientists, mechanical engineers, cognitive scientists, and other experts [29]. Meanwhile, multimodal interfaces are considered to offer an improved user experience and better control than a unimodal input in the application of CAD modeling. The pioneering study by [PERSON] [63] in creating a \"Put That There\" system showed the potential and advantages of combining gesture and speech inputs for the selection and displacement of virtual models. This system proposed the use of commands such as \"Move that to the right of the green square\" and \"Delete that\", allowing users to employ vague language to activate CAD functions while disambiguating them with gestures. It is noteworthy that none of these commands can be interpreted properly from just a single input modality--at least two were required--but this multimodal interface created simple and expressive commands that are natural to users. The work by [PERSON] [63] led to a rapid development of many multimodal interfaces that typically used gesture, speech, eye-tracking, or BCI-based input modals for creating and manipulating CAD models, as is shown in Figure 1. From the survey results, \"Gesture and Speech\" is the most widely used interface in the application of CAD and drove the majority of multimodal interface research. Alternative formulations have also been followed, bringing new multimodal interfaces such as \"Gesture and Eye Tracking\" [34; 69], \"Gesture and BCI\" [70], \"Gesture, Speech and BCI\" [3] and others [30; 71; 72; 73; 74; 75]. For these multimodal interface-based CAD applications, there are three main types of combined modalities, as shown below. 1. Different modalities can be used to execute different CAD commands. For example, in [69], eye tracking is used to select CAD models and gesture is used to activate different types of manipulation commands. 2. Different modalities can be used to execute the same CAD commands at the same time. In another words, there is overlap in the set of CAD commands that can be implemented through different modalities. For example, the task of creating models can be completed both by BCI-enabled commands and speech commands [3]. In this case, users can choose different interfaces according to their preferences. Moreover, to a certain extent, this kind of overlap in commands improves the robustness and flexibility of the interfaces. 3. Different modalities can be combined to execute a CAD command synergistically. In this respect, the advantages of different interfaces can be leveraged to work together on a model manipulation task. For example, in most CAD prototype systems combined with speech and gesture [8; 12; 64], speech is used to activate specific model manipulation commands (translation, rotation, zooming, etc. ), while gestures are used to control the magnitude and direction of specific manipulations. Figure 1: Survey on multimodal interfaces for CAD.