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Using alpha channels generated in Photoshop is a great way to add transparency to a layer in AE, but unfortunately it only works well if you are working with still images. If you need moving transparency, a static alpha channel just won't cut it. However, the only easy way to create a moving alpha channel is to work with a 3D animation program. Most 3D apps can automatically generate an alpha channel for each frame of your animation. What if you need to add moving transparency to live video though? In AE, you can do this with masks.

Masks can be applied to each individual layer. You can apply as many masks as you like to a layer, and they can be combined in a variety of ways. Once you have applied a mask to a layer you can turn on keyframing for that mask and change it's shape over time. You can also soften the edges of a mask, and vary it's opacity over time. Masks are not quite as flexible as alpha channels for creating complex transparency, but when it comes time to start animating your transparency masks are faster to work with.

Masks are drawn using 'Bezier splines', and work just like paths in photoshop or the freeform drawing tools in Illustrator or Freehand. The shape of the mask is defined by points, which are then connected by a line which describes the outline of the mask. Each point on the mask can have a curve applied to it by adjusting handles which are tangent to the point, and these handles can be toggled on or off to change a point from a smooth curve to an angular corner.

Applying masks

Masks can be applied to a layer in one of two basic ways:

Once you add a mask to a layer there are a couple of properties you may want to change. You can see a mask in the timeline using the keyboard shortcut "M". To view it's properties, flip down the little arrow next to it's name on the timeline.

Each time you draw a new mask on a layer, it is by default added to the selection area of the other masks. However, you can also choose different ways to apply masks to each other. Next to the mask name in the timeline there is a drop down menu which should say "add". This is the default setting, but other commonly used settings are:

Animating Masks

Animating a mask is similar to animating anything else in just need to turn on keyframing for the mask shape (using the stopwatch icon next to the mask shape property). Once you have done this you can go to a different point on your timeline and change the shape of the mask by moving it's points. AE will blend smoothly from one shape to the next.