It's time to create the animated masks that serve as sliding windows over the footage. To make it easier for you to keep track of which mask in the Timeline represents a mask in the composition, you'll turn on a preference that's new in After Effects 6.5:
Choose Edit > Preferences > User Interface Colors, and select Cycle Mask Colors.
Now, every time you draw a new mask, one of five colors will be used to identify the mask in the Timeline and in the Composition or Layer window. This will come in handy in the next section when you begin animating the masks.
Go to time 0;00, and double-click the Sequence Comp layer in the Master Comp Timeline to open it in a layer window. (Alternatively, you can complete this section in the Composition window; but the Layer window gives you more control.)
Click the Rectangular Mask tool in the Tools palette. Draw a vertical column starting below the bottom edge of the composition area and extending above the top edge of the composition.
Drawing the first mask in the Sequence Comp's layer window
Draw four more columns that also span beyond the top and bottom edge of the comp. Place them in other areas of the composition, and make them varying widths.
Five masks with unique colors in the Sequence Comp layer
Animating the columns
Your next task is to animate the masks horizontally and randomly across the screen. You'll use each mask's Mask Shape property as you would a Position property to animate its location over the Sequence Comp:
At time 0;00, expand the Sequence Comp in the Timeline, select all the masks, and then press M to display their Mask Shape properties.
With all the masks still selected, click the stopwatch next to any Mask Shape property to add a keyframe to all the masks.
Go to time 2;00, and click Mask 1's name in the Timeline to select the mask.
Using the Selection tool, drag Mask 1 to the opposite side of the comp.
Move the other four masks to new locations on the layer.
Remember to select the mask's name in the Timeline first in order to select all four vertices before moving the mask. Otherwise, you'll reshape the mask instead of moving it.
Go to time 4;00, and again move each mask to a new location.
Select the Mask Shape keyframes for all five masks, and copy them.
Go to time 6;00, and paste the keyframes.
Pasting all the Mask Shape keyframes
To add some definition to the boundaries of the masked areas, go to the Master Comp Timeline, select the Sequence Comp layer, and choose Effect > Render > Stroke.
Check the All Masks option in the Effect Controls window, and set Color to white (RGB: 255, 255, 255). Now each mask has a white outline.
Settings for the Stroke effect in the Effect Controls window