# Off-Line Camera-Based Calibration for Optical See-Through Head-Mounted Displays
## 6 Conclusions and Future Work  
In this paper, we presented a method to easily calibrate OST HMDs based on the use of a calibrated camera replacing the user's eye. The method, which could work with any type of OST HMD, only required the camera to observe a planar calibration pattern displayed on the imaging plane of the see-through display.  
The method exploited standard camera calibration and computer vision techniques for estimating the projection parameters of the off-axis display model for a generic position of the viewpoint camera. In a prior-to-use second phase, the projection parameters were then refined through a planar homography that encapsulated the shift and scaling effect (i.e., the parallax contribution) associated with the relative translation from the old camera position to the current user's eye position.  
To evaluate the efficacy of the calibration technique objectively, we measured the overlay error between real and virtual features observed by the viewpoint camera from different positions within the eye-motion-box of the display. Experimental results indicated that the average overlay error of our calibration was 2.23 pixel.  
Our future work involves the integration of an additional step in the off-line calibration procedure devoted to the estimation of the radial and tangential distortion coefficients due to the lens system of the OST display. This calibration step can be performed through a non-linear optimization method such as the Levenberg-Marquardt algorithm.  
We are also working on the integration of an eye tracking camera with our custom made OST HMD to perform user studies to evaluate the efficacy of the run-time refinement of the linear and non-linear calibration parameters and to design possible algorithm improvements that take into consideration the eye model.  
Finally, the flexibility of our camera based calibration procedure can pave the way toward the profitable use of consumer level OST HMDs also in those applications that demand high accuracy in the spatial alignment between computer generated elements and the real-world scene (e.g., in surgical or high precision industrial applications).  
Conceptualization, F.C.; methodology, F.C., U.F., and N.C.; software, F.C., U.F., and N.C.; validation, F.C., U.F., N.C., and V.F.; formal analysis, F.C.; investigation, F.C. and U.F.; resources, F.C. and V.F.; data curation, U.F. and N.C.; writing, original draft preparation, F.C.; writing, review and editing, F.C., U.F., and N.C.; visualization, F.C.; supervision, F.C. and V.F.; project administration, V.F.; funding acquisition, V.F. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the HORIZON2020 Project VOSTARS (Video-Optical See Through AR surgical System), Project ID: 731974. Call: ICT-29-2016 Photonics KET2016.  
The authors would like to thank [PERSON] for his support in designing and assembling the AR visor.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; nor in the decision to publish the results.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} AR & Augmented reality \\ HMD & Head mounted display \\ OST & Optical see-through \\ SPAAM & Single point active alignment method \\ FOV & Field-of-view \\ \end{tabular}