问题
As ffmpeg calculates ssim score by a frame-by-frame comparision,
how do I exactly align the reference video (say ref.mp4) with the video stream, so that only the corresponding frames (ith frame of both the videos, for all i) are compared by ffmpeg ? Please help.
来源：https://stackoverflow.com/questions/36156140/what-is-the-accurate-way-of-calculating-ssim-score-with-ffmpeg-between-a-video