Advanced Video Editing

After Effects

Using Masks

A mask is a shape that you draw onto a layer, which shows you only a selected portion of the layer.

Remember, create a project folder and save your AE project to the folder.

Start by creating a New Composition - don't forget to name it. Go ahead and set the resolution to 1920 x 1080, the fps to 29.97 and the Duration at 10 seconds.

Then add a Solid

Layer > New > Solid

Set the Solid Settings to Make Comp Size and choose a colour other than black in the colour box.

In the Timeline, select the Solid layer and then select a Rectangle tool from the Shapes button in the tool palette. As long as your Solid layer is selected, than the Shape Tool will create a Mask. Click and draw the Rectangle on the Composition and you’ll notice that the Composition turns black except for the area where you draw the Rectangle. In other words, the mask is showing only a selected portion of the Solid Layer.

Back in the Timeline you’ll notice that Mask 1 was created. Pull down the mask’s disclosure triangle to reveal the mask options, which includes Mask Path, Mask Feather and so on. Each of these options has a stopwatch allowing you to animate them over the course of your clip.

The Mask also has Transform properties. Open the disclosure triangle to reveal the Transform options. Rotate the mask a few degrees and you’ll notice it rotates around the anchor point for the Solid layer, which is positioned in the middle of the Composition. In the image below, you can see the anchor point in the middle of the image, which is the middle of the Solid layer, not the Rectangle itself.

To rotate around the centre of the mask itself, you would need to relocate the anchor point. Simply click on the Pan Behind tool (located in the Tool palette), which allows you to take hold of the anchor point to move it. But getting the anchor point perfectly centred in your mask is tricky. A good way to measure more accurately the centre point is to use a ruler as a guide, which you can show in the Composition itself.

First, reset your Rotation - go to the Transform properties and you'll see to the right where it says Reset. Click it.

In the lower left of the Composition panel, click on the Guide button and then select Ruler from the options.

A ruler grid will appear in both the horizontal and vertical portions of the Composition panel.

Hover your pointer inside one of rulers until it becomes a double-arrow, then click and drag from the Ruler over the mask and place the line in the middle of the mask handles. Do this again using the other ruler. If you have a hard time positioning the line, go ahead and zoom in so you can see more closely what you're doing.

Where the lines intersect in the middle of the mask is where you can now drag your anchor point (don’t forget to click on the Pan Behind Tool first otherwise you'll move the entire mask). The anchor point should snap in the middle where the lines intersect.

Now you can Rotate the grid from the mask centre instead of from the Solid centre.

To change the shape of the Mask, make sure that Mask 1 is selected in your Timeline.

Click Command + T - this will add handles to the boundary box, which you can then grab with the pointer to push or pull and change the shape of the mask.

Animating the Mask Path is a great way to reveal items like the text in Lower Thirds.