Subjective Evaluation of the 360-Degree Projection Formats Using Absolute Category Rating

Recently, the 360-degree image is widely used in the expanding virtual reality market. The 360-degree image has the advantage of creating a virtual space that is similar to the real scene easily and quickly in the existing multimedia market. Many projection formats, which generate 2D plane from 3D space, are being published and studied along with the rapidly spreading 360-degree image applications. However, the equi-rectangular projection (ERP) format is merely being used in many applications based on its ease of use whereas the quality evaluation for various projections is not actively carried out. In this study, four projection formats including ERP are briefly described and a subjective test of the four projection formats is conducted. We defined a subjective test procedure for evaluating the image quality of the 360-degree image using an absolute category rating method. We proposed a methodology for the subjective test of 360-degree images and conducted the subjective test performed by 22 observers. Interestingly, the subjective test results show that the ERP format is not marked as the high-quality format. The octahedron projection format shows the best quality. Our research will help market efforts to provide the best quality 360-degree image services.