Subjective Assessment of 360° Image Projection Formats

Recently, the use of virtual reality has been spreading rapidly in many industrial fields, such as games and entertainment, and 360° contents are widely used in various applications of the virtual reality (VR) market. The 360° contents create a virtual space comparable to real scenes. Coupled with the rapid growth of the 360° image/video field, many projection formats have been developed and published. However, the studies which identify the best format for a variety of applications are still limited. Therefore, it is desirable to evaluate the visual quality of the projection formats used in the current market, and the result of the evaluation can be used as an important consideration capable of providing 360° VR service of better quality. Here, we propose a methodology for the subjective test and briefly present ten 360° projection formats used in our experiment. The proposed methodology for the subjective test uses a combination of the Degradation Category Rating (DCR) and Pair Comparison methods. We performed the subjective test with 15 participants for each test step, the experimental set-up for which followed the proposed methodology. The results show that the equirectangular projection format (best known and widely used format in most commercial products) did not give the highest quality. Hybrid equiangular cubemap and equatorial cylindrical projection gave the best quality compared with the other eight formats. Based on the results of the DCR, it is noted that different monitors (size and resolution) do not affect the ranking of 360° image projection formats. Our work presents the ranking of ten published projection formats which is useful for identifying the best format for a future 360° image standard.