In recent years, we witness dramatic growing attention in immersive media technologies like 360-degree videos and virtual reality (VR). However, measuring the quality-of-experience (QoE) for 360-degree VR videos is not a trivial task. Streaming such videos to head mounted displays (HMDs) is extremely bandwidth-demanding when compared to traditional 2D videos. In HTTP adaptive streaming, QoE tends to deteriorate significantly during fluctuating network conditions, which results in various bitrate changes and causes multiple stalling events during playback. Thus, understanding how the human visual system perceives 360-degree video with the effect of stalling and different bitrate levels becomes inevitable. In this paper, we investigate the impact of stalling on users QoE under different bitrate levels and the interaction between stalling event and bitrate level for 360-degree videos in VR. To aim this, we first build a 360-degree videos database by encoding videos in three different bitrate levels (1, 5, and 15 Mbps) with 4K resolutions (3840 × 1920 pixels). We then simulate various stalling events in the videos and conduct a subjective experiment in a virtual reality environment to investigate the human responses. Finally, we use a Bayesian method to estimate and predict the QoE while measuring the quality drop owing to various stalling events and bitrate changes. Proposed solution and prediction results show a strong dependency between playback stalling and bitrate of 360-degree video in VR. Stalling always impacts the QoE of 360-degree videos, but the strength of this negative impact depends on the video bitrate level. The adverse effect of stalling events is more profound when bitrate level approaches to the high and low end, which is in close agreement with subjective opinion.

近年来，我们见证了沉浸式媒体技术（例如360度视频和虚拟现实（VR））受到越来越多的关注。但是，测量360度VR视频的体验质量（QoE）并不是一件容易的事。与传统的2D视频相比，将此类视频流传输到头戴式显示器（HMD）的带宽需求极大。在HTTP自适应流传输中，QoE在网络状况变化期间往往会显着恶化，这会导致各种比特率变化并在播放期间导致多个停顿事件。因此，了解人类视觉系统如何在停顿和不同比特率水平的影响下感知360度视频变得不可避免。在本文中，我们研究了在不同的比特率水平下停顿对用户QoE的影响，以及VR中360度视频的停顿事件和比特率水平之间的相互作用。为此，我们首先通过以三种不同比特率级别（1、5和15 Mbps）以4K分辨率（3840×1920像素）对视频进行编码来构建360度视频数据库。然后，我们在视频中模拟各种停滞事件，并在虚拟现实环境中进行主观实验以调查人类的反应。最后，我们使用贝叶斯方法估算和预测QoE，同时测量由于各种停顿事件和比特率变化而导致的质量下降。拟议的解决方案和预测结果表明，VR中360度视频的播放停顿与比特率之间存在很大的依赖性。拖延始终会影响360度视频的质量，但是这种负面影响的强度取决于视频比特率水平。当比特率水平接近高端和低端时，停顿事件的负面影响更加深远，这与主观观点非常一致。