Imagine you have to make an animation where you want to show that a character is dreaming something. Let's get creative and say, in addition, that the subject is about to wake up and everything in the dream gradually fades away until it disappears completely. How would you accomplish this effect?
Well, let's start by saying that first, you should have two scenarios at the same time: one where the main character appears and another that shows the dream gradually disappearing. Let's focus on the latter scenario because, to achieve this trick, you must know how to fade out a specific part of a video in After Effects.
To understand the concept behind working with two scenarios so you can choose only a part of one of them while leaving the rest of the image of the second footage visible, it is necessary to create two solid backgrounds of different colors.
To do that, simultaneously click Ctrl + Y or Cmd Y, choose a color, and hit Ok. Then, repeat the process one more time to create the second solid.
If one background completely overlaps the other, change the order in which their layers appear on the Timeline, so the second solid is below the first one. After doing so, move both solids so that each one occupies one-half of the canvas.
Now that you have two backdrops visible on your composition screen, draw a Mask on the first solid you created:
Play with the different transformation options that appear in your layer’s properties, so you can see how, by placing several keyframes on the Timeline, you can create multiple effects.
Once you have mastered this logic, you are ready to move on to a more professional level in motion graphics and After Effects.