that combine two sources are called
transitions, such as
dissolves and wipes.
You’ve probably seen wipes on TV and less frequently
in film, although they’re considered somewhat
artificial in film because they call attention to themselves (the
Star Wars films are probably the most prominent
films to use wipes, perhaps as a nod to old black-and-white adventure
films and weekly cliff-hangers).
cut from one scene to another is also a transition, but that doesn’t involve any kind of effect.
To show off a transition, this lab will open two movies and create a user-selected transition between them.
In coding terms, the only significant difference from a one-source effect is, predictably, that you need to set up an input map that references both source tracks for the effect.
But in terms of practicality, although you might apply a filter to a long sequence of video, a transition will be very short typically—only a few seconds at most. Because a video track used as a source to an effect is shown only as part of that effect, to show all of one video source transitioning into all of another, you need five tracks:
All of source A, up to the beginning of the transition (i.e., its last n seconds)
The portion of source A to be used for the transition
The portion of source B to be used for the transition
All of source B after the transition (i.e., everything but its first n seconds)
The effects track
So, to change the ...