title: How to convert Top-and-Bottom 3d video to side-by-side 3d video with FFmpeg
body: <p>I have a top-and-bottom 3d video, and i want to look at it with Gear VR, but Gear VR only support side-by-side video, so i need to convert it to side-by-side, while i don't know how to use ffmpeg to achieve it,does anyone knows ? thanks very much.</p>