Visual discomfort is a serious challenge for stereoscopic videos to become prevalent. However, the assessment of stereoscopic visual discomfort is a complicated issue. In this paper, we have studied four of the major factors which affect visual comfort by conducting extensive subjective assessments. Namely, foreground disparity, size of foreground object, disparity distribution and in-depth motion. Relationships between visual comfort and these four factors are analyzed and four conclusions are drawn according to the experimental results. Firstly, when the influences of disparity magnitude and range of disparity function together and clash, the latter is in the dominant position. Secondly, the degree of visual comfort increases with the increment of the object size in a certain range and then levels off or decreases slightly after a certain threshold. Thirdly, better visual comfort is always obtained when the bottom part was perceived nearer to the viewers than its upper part. Last but not least, the variation of parallax over time might be one of the major factors which affect visual comfort. And its influence is complicated and can be reflected by the object's velocity, motion direction and starting plane.
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