# A Review on Methods and Systems for Remote Collaboration
## 5 Communication Methods and Cues
### Combining Visual Communication Cues  
The use of annotations and body cues has been studied separately, but there are several other studies that focus on the mixed use of visual communication cues, both related to body gestures and annotations.  
[PERSON] et al. [126] studied the use of hand gestures and annotations overlaid on a live 360-degree video. The system was based on MR; the local user wore an AR HMD, and the remote user wore a VR HMD. The remote user's gestures were tracked using a sensor and displayed in a 360-degree video feed as virtual hands. Besides the hand gesture cues, the user was allowed to use annotations, and his pointing gestures were captured and displayed as a ray pointer as seen through the local user's AR device. The annotations had fixed spots both in the virtual and the real environment, even if the local user's head direction changed. The study showed that the participants could finish the task faster and understand the remote instructions better when using visual annotations.  
[PERSON] et al. [127] researched the effect of combining visual communication cues such as hand gestures, sketches, and pointing in a mixed reality remote collaboration. The study showed that using sketches and hand gestures gave the best results regarding task performance, but the pointing cues did not improve the performance significantly. Moreover, the sketches and the pointing cues involved higher mental effort for the users.  
The research of [PERSON] et al. [128,129] focused on proposing communication cues when using an MR system based on a 360-degree panorama reconstructed in a 3D environment. The remote user could control a virtual ray pointer using hand gestures or add sketches (drawings), which were displayed through the AR HMD of the local user and overlaid onto the 360-degree panoramas seen by the remote user.  
[PERSON] et al. [130] modeled the eye gaze of the remote user as a virtual ray cast line and the remote user's hand as a 3D mesh; both were overlaid onto the local user's AR view. In order to help the users to identify their partner's location and viewing direction, the authors proposed the use of a 3D arrow cue and a virtual avatar as a virtual head frustum that points to the location of the other user. The head frustum represents the other user's head direction, while the 3D arrow is modeled as a pin arrow pointing to the other user's avatar head. The results showed that when combining hand gestures and eye gaze, the task completion time was reduced, and the users had a better feeling of co-presence. For the local user, the required mental effort was considered lower when using the gesture cues, but both users said that they preferred combined cues over gesture or gaze alone.  
In Table 3, we classify the publications that address the use of communication cues in remote collaboration and assistance by the application and the communication method employed to accomplish the given task. In this case, the synthesis in Table 3 allows the straightforward identification of scientific papers that present a method applied in a certain field. With the advent of new 5G communications technologies, it is possible that researchers' efforts will focus on such methods. Moreover, many more applications will be developed, and the global state of the pandemic will accelerate research in these areas.  
Surveying the scientific literature, it can be observed that efficient communication presupposes sound, image, and orientation towards the object being discussed. Digitization together with remote assistance and collaboration to accomplish tasks will be the future challenges for technological development.