  360-degree streaming videos can provide a rich immersive experiences to the
users. However, it requires an extremely high bandwidth network. One of the
common solutions for saving bandwidth consumption is to stream only a portion
of video covered by the user's viewport. To do that, the user's viewpoint
prediction is indispensable. In existing viewport prediction methods, they
mainly concentrate on the user's head movement trajectory and video saliency.
None of them consider navigation information contained in the video, which can
turn the attention of the user to specific regions in the video with high
probability. Such information can be included in video subtitles, especially
the one in 360-degree virtual tourism videos. This fact reveals the potential
contribution of video subtitles to viewport prediction. Therefore, in this
paper, a subtitle-based viewport prediction model for 360-degree virtual
tourism videos is proposed. This model leverages the navigation information in
the video subtitles in addition to head movement trajectory and video saliency,
to improve the prediction accuracy. The experimental results demonstrate that
the proposed model outperforms baseline methods which only use head movement
trajectory and video saliency for viewport prediction.
