A video with heterogeneous spatial quality is a video where some regions of the frame have a different quality than other regions (for instance, a better quality could mean more pixels and less encoding distortion). Such a quality-variable encoding is a key enabler of Virtual Reality application, with 360-degree videos. So far, the main technique that has been proposed to prepare spatially heterogeneous quality is based on the concept of tiling. More recently, Facebook has implemented another approach: the offset projection where more emphasis is put on a specific direction of the frame. In this paper, we study quality-variable 360-degree videos with two main contributions. First, we provide the theoretical analysis of the offset projection and show the impact of the parameter settings on the video quality. Second, we propose another approach which consists in preparing the 360-degree video from a Gaussian pyramid of downscaled and blurred versions of the video. We perform an evaluation of tiling, offset and Gaussian-based approaches in representative scenarios of heterogeneous spatial quality in 360-degree videos and highlight the main trade-off to consider when implementing these approaches.

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A video with heterogeneous spatial quality is a video where some regions of the frame have a different quality than other regions (for instance, a better quality could mean more pixels and less encoding distortion). Such a quality-variable encoding is a key enabler of Virtual Reality application, with 360-degree videos. So far, the main technique that has been proposed to prepare spatially heterogeneous quality is based on the concept of tiling. More recently, Facebook has implemented another approach: the offset projection where more emphasis is put on a specific direction of the frame. In this paper, we study quality-variable 360-degree videos with two main contributions. First, we provide the theoretical analysis of the offset projection and show the impact of the parameter settings on the video quality. Second, we propose another approach which consists in preparing the 360-degree video from a Gaussian pyramid of downscaled and blurred versions of the video. We perform an evaluation of tiling, offset and Gaussian-based approaches in representative scenarios of heterogeneous spatial quality in 360-degree videos and highlight the main trade-off to consider when implementing these approaches.