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Moving Split Screen to Show Before and After
I have watched several videos made by filter and effects companies where they demonstrate the impact their software has on a video clip by having a line move from screen left to screen right – as this line moves – the version of the clip with the effect applied is revealed. In other words, everything to the left of the line shows the clip with the filtered applied and everything to the right of the line is the original, unaltered clip. As the line moves then more and more of the clip has the filtered applied to it.
Simply put, how do they pull off this effect?
I can see that part of the answer is to stack the two versions of the clip (with and without filters applied) on the timeline but what happens after that? Is it some kind of mask or matte that is moving or changing?