A Center-to-Edge Progression for Equirectangular Projected 360° JPEG Images

Imaging devices capable of capturing 360° images were recently released into the market. The spherical images acquired by imaging devices, such as 360° cameras and multiple camera smartphones, are projected onto the 2D image plane to allow the use of the existing image coding standard. The 360° image market is currently dominated by the equirectangular projection format coded by the JPEG image coding standard. Spherical images are characterized by their large size, and they are typically viewed by displaying a default viewport of the entire image first, followed by navigation by the user. Therefore, progressive image coding capable of transmitting the default viewport first is in high demand. However, the progressive image coding methods proposed thus far, such as quality-based and resolution-based progressive coding, do not support the spatial region-based progressiveness necessary to first display the default viewport. This paper proposes a center-to-edge progressive mode for the JPEG image coding standard, which enables transmittance of the discrete cosine transform blocks including the default viewport first by reordering the blocks' scanning path. This progressive mode was designed to maintain all other encoding structures of the JPEG standard, such that use of the already established toolchain of the JPEG is maintained. Results of the test performed with our proposed mode using eleven 360° images in the equirectangular projection format and six objective image quality parameters show that transmission of default viewports is notably faster than existing JPEG modes and negligible effects on JPEG image coding efficiency.