Basic Masking

"Masking" refers to the technique of hiding part of an image to reveal another element behind it. Remember that the stack of layers in your video editor may be displayed in the timeline as being above and below one another but you should also think of that as representing the foreground and the background.

With masks you no longer need to thing of the visiblity of a layer as a binary option. You are not restricted to showing the an entire layer with the occasional fade out to blend it with another. If we want to isolate an element from the "top" layer so that it can appear within another layer which serves as our backgroudn then this is perfectly possible.

Of course it won't always be as simple as that. As usual it will be the details which will ruin it for everyone.

If you are familiar with the differences between raster and vector masks in video editing software then just know that most video editing suites lean towards vector masking.

Layered Masks

It is not impossible to use "raster" masks in most video editors. But they are typically used in the form of alpha channels that may be present in still images. For instance; if you were to make a mask for an image within Adobe Photoshop and save that image as a file format that supports transparency (such as .png) then that transparency channel will automatically be read by your video editing software and the appear as you intended within the program.

Unfortunately most video editors do not make it easy to use such masks for video footage or to move them between layers.

Rotoscoping

The phrase "Rotoscoping" is basically a fancy way of saying "traced". It involves isolating an element the same way that masks do but has the connotation of the mask being redefined on each new frame of the footage. From there the tracing can be used as a simple mask (as we will discuss here) but can also be the basis for animated versions of that element in other programs such as for cartoons and animated movies. It is not a monolithic phrase.

After Effects Masking

Mask tools

Possibly the most basic excercise one could try to illustrate how masks work would be a split-screen composition.

  1. Select the mask tool.
  2. Click and drag within the main window to define the area to be masked (hidden).
  3. Switch to the composition window if it is not already visible in the work space.

At this time if you were to play back the composition you should see both clips playing with the area you masked off hiding the upper clip and revealing the lower clip.

Masking with complex shapes

Masks become even more powerful through the use of multiple shapes and/or the pen tool. Just keep in mind that you can NOT define different properties for individual mask shapes. Mask properties will be applied across an entire mask layer that might be made up of multiple shapes.

Adding multiple shapes to a masking layer is as simple as drawing multiple shapes using the mask tools while the desired masking layer is selected.

Animating masks

Mask properties can be animated just as any other property with the stopwatch can be animated.

Mask shapes themselves can NOT be moved. Only an entire mask layer can have the x and y coordinates animated to move the mask around the layer.

Effects like the duplicated housecat in this video by Super Epic Cats might be achieved via masking off different parts of the screen (though it certainly is not the only tool used in this case).

https://www.youtube.com/watch?v=fm0KmBXgaFg