Recently, 360° content has emerged as a new method for offering real-life interaction. Ultra-high resolution 360° content is mapped to the two-dimensional plane to adjust to the input of existing generic coding standards for transmission. Many formats have been proposed, and tremendous work is being done to investigate 360° videos in the Joint Video Exploration Team using projection-based coding. However, the standardization activities for quality assessment of 360° images are limited. In this study, we evaluate the coding performance of various projection formats, including recently-proposed formats adapting to the input of JPEG and JPEG 2000 content. We present an overview of the nine state-of-the-art formats considered in the evaluation. We also propose an evaluation framework for reducing the bias toward the native equi-rectangular (ERP) format. We consider the downsampled ERP image as the ground truth image. Firstly, format conversions are applied to the ERP image. Secondly, each converted image is subjected to the JPEG and JPEG 2000 image coding standards, then decoded and converted back to the downsampled ERP to find the coding gain of each format. The quality metrics designed for 360° content and conventional 2D metrics have been used for both end-to-end distortion measurement and codec level, in two subsampling modes, i.e., YUV (4:2:0 and 4:4:4). Our evaluation results prove that the hybrid equi-angular format and equatorial cylindrical format achieve better coding performance among the compared formats. Our work presents evidence to find the coding gain of these formats over ERP, which is useful for identifying the best image format for a future standard.

中文翻译：

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使用客观质量指标评估360°图像投影格式的编码性能

近来，360°内容已成为提供现实互动的一种新方法。360°超高分辨率内容映射到二维平面，以适应现有通用编码标准的输入以进行传输。已经提出了多种格式，并且正在做大量工作来使用基于投影的编码在联合视频探索小组中研究360°视频。但是，用于360°图像质量评估的标准化活动受到限制。在这项研究中，我们评估了各种投影格式的编码性能，包括最近提出的适合JPEG和JPEG 2000内容输入的格式。我们概述了评估中考虑的九种最新格式。我们还提出了一种评估框架，以减少对原生等边矩形（ERP）格式的偏见。我们将降采样后的ERP图像视为真实情况图像。首先，将格式转换应用于ERP映像。其次，每个转换后的图像都要经过JPEG和JPEG 2000图像编码标准，然后解码并转换回降采样的ERP，以找到每种格式的编码增益。针对360°内容设计的质量指标和常规2D指标已在两种子采样模式（即YUV（4：2：0和4：4：4））中用于端到端失真测量和编解码器级别。我们的评估结果证明，混合等角格式和赤道圆柱格式在比较的格式中具有更好的编码性能。