Ever since I started putting up After Effects tutorials, I have gotten an increasing number of questions where people seem to get stuck on some of the more fundamental techniques rather than the actual visual effect. So I decided to start a small tutorial series to cover these After Effects Basics for all the new starters out there.

The first topic is masking and I hope you enjoy it :)

What is masking

All visual effects are created by combining different visual elements together. This allows you to add or remove items to or from your footage and create complex visual illusions for your audience.

In Adobe After Effects, each of these visual elements is represented by a ‘layer’. Layers are placed on top of each other with each one obscuring the layers beneath by default.

However, you can create masks to cut out certain areas of a layer so the ones below will show through. This is called masking :)

Let’s look at how you can create masks in Adobe After Effects!

Drawing Masks

There are different tools available for masking in Adobe After Effects: shape tools (for rectangles, circles, etc) and a pen tool for creating custom mask shapes. You can find these tools in the toolbar.

Select a shape or the pen tool, make sure your layer is selected and use the tool to draw and drag a mask onto the layer.

When using shape tools for masking, you can hold down the Control key to center the new mask in the middle of the layer. Hold down the shift key to scale it uniformly in width and height when you drag your mouse.

When using the pen tool, make sure to close the mask shape by placing the last point exactly on top of the start point for the shape.
Also, when you click to place a point and then, without releasing the mouse, start dragging, you will reveal the bezier handles that allow you to add smooth curves to your masks :)

If you suddenly see a solid shape being created when you try to draw a mask, hit undo and ensure that the layer you are drawing the mask on is actually selected. Otherwise it will create what is called a shape layer.

Mask properties

Once you’ve added masks to a layer, you can reveal their properties by selecting the layer and pressing the ‘m’ key. Alternatively, you can expand the properties of the layer by clicking on the twisty next to the layer name to get to the masks.

In After Effects CS5, each mask has 5 properties:

Mask mode

The mask mode determines how a mask is applied.
In Add mode, only the contents of the mask will be visible, the rest of the layer will be discarded.
In Subtract mode, the contents of the mask will be removed from the layer.

There are other modes like Intersect or Difference that only apply if you have multiple overlapping masks on the layer. Play
around a little to learn what they do :) Personally, I don’t really use them much.

Mask path

The mask path is the shape of the mask, consisting of the vertices as well as their connecting lines. As you will see in a little bit, this path can be animated which is often used to cut out animated elements with masking to create more complex effects.

Mask feathering

Mask feathering allows you to smoothen out the edges of the mask. This is very useful for blending the masked out elements gently back into the scene :)

Mask opacity

The mask opacity determines how translucent the masked out elements will be. Note that this will affect everything that is NOT in the mask if you’re using the Subtract masking mode. Also, sometimes this may behave differently to what you expect if you only have a single mask on your layer.

Mask expansion

Mask expansion allows you to expand or contract the shape of your mask. You won’t actually see the mask path change, but you will see more or less content being included within the mask as you change this parameter.

Animating mask in After Effects

One of the powers of masking is that you can animate the masks. This is useful to trace and follow moving elements in your footage and either cut them out or add them into your scene :)

You can enable the stopwatch icon next to the mask path property and then move the mask around or change the vertices around and the mask will be animated and interpolated between the keyframes.

To modify a mask, here are a few things to keep in mind:
When all the vertices are solid squares, dragging will move the entire mask. You can get into this ‘mask drag’ mode by Alt-clicking on the mask.
When all vertices are circles, dragging will either move the individual vertex or the selected edge. You can get into this mode by selecting the mask and then clicking outside of the mask shape.

Masks and masking is a fundamental and very powerful tool to create some rather complex effects. I guarantee there will hardly be a visual effect you work on that does not require masking in one form or another :)