# High-Speed Dynamic Projection Mapping onto Human Arm with Realistic Skin Deformation +
## 5 Discussion  
Herein, we propose a high-speed dynamic projection mapping system for the human arm with realistic skin deformation. The developed system achieves a latency of 10 ms between the motion and projection, almost imperceptible by the human vision even under high-speed rotations and translations, as shown in Figure 7. This is the first projection mapping system that can realistically handle human arm surfaces with a very low latency, although the achieved latency is still a few milliseconds higher than that reported in [20]. We believe that our system can be beneficial for applications in many fields such as fashion, user interfaces, and education. For example, it is difficult to remove a tattoo after getting tattooed; however, by using the proposed system, people can enjoy the realistic tattoo digitally and intuitively find their favorite tattoo design, as shown in Figure 8. On-body interfaces can become more flexible and interactive owing to realistic skin deformation and low latency as shown in Figure 9. Special-effect makeup, typically considered complicated and costly, can be easily applied in the film industry, as shown in Figure 10. Physiotherapy students can gain a better understanding by directly projecting the anatomical structures on the body.  
Nevertheless, the proposed system has various limitations that remain to be addressed. For instance, the system uses a marker-based method, which complicates the setup. The registration of the joints requires careful manual operation to ensure that the coordinate systems of the registered rigid bodies and the joints are consistent. The error between the joints and the registered rigid bodies may lead to severe misalignments in the projection and error of the skin deformation. We believe that the problem can be alleviated if more joints are registered in the tracking system. The relationship of the skeletal structure can provide information to improve the registration accuracy.  
In addition, we only focused on the arm surface in this study, and the proposed system should be extended to other body parts to widen the applicability. Accordingly, the proposed accuracy compensation method should be carefully designed and modified because the neighboring joints of vertices of other body parts could be different from the forearm. For example, the torso of a body has more than two neighboring joints.  
The system described herein is user-specific in that the regression model was prepared in advance for a specific user. This limitation narrows the applications, and the regression model should also be extended to multiple persons. It is known that the SMPL model represents different body shapes through shape parameters [38]. If we collect the data of multiple persons with different shape parameters and regress the collected data with the shape parameters, the regression model can possibly be extended to multiple persons. In future work, we will implement the system using a marker-less method and widen its application to multiple persons simultaneously using learning-based methods.  
Conceptualization, H.-L.P. and Y.W.; methodology, H.-L.P. and Y.W.; software, H.-L.P.; validation, H.-L.P. and Y.W.; formal analysis, H.-L.P. and Y.W.; investigation, H.-L.P. and Y.W.; writing--original draft preparation, H.-L.P.; writing--review and editing, H.-L.P. and Y.W.; supervision, Y.W.; project administration, H.-L.P. and Y.W.; and funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript.  
Figure 10: Wound generated with physically based rendering texture (first column), wound effect projected on the forearm (second to fourth columns).  
**Funding:** This research was supported by JST Accelerated Innovation Research Initiative Turning Top Science and Ideas into High-Impact Values (JPMJAC15F1) and JSPS Grant-in-Aid for Transformative Research Areas (20H05959).  
**Conflicts of Interest:** The authors declare no conflict of interest.