How Do I Move Content Inside a Mask in After Effects?

To move content within a mask in Adobe After Effects, you'll typically be adjusting the position of the layer the mask is applied to, or manipulating the mask's shape or position to reveal different parts of that layer. Here's a breakdown of how to do it:

1. Moving the Layer Underneath the Mask:

• Select the Layer: In your composition panel, select the layer that has the mask applied to it.
• Use the Position Property: Press P to reveal the Position property. Adjust the X and Y values to move the entire layer around within the constraints of the mask. This is the most common method.

2. Moving the Mask Itself:

• Select the Layer: Ensure the layer with the mask is selected.
• Reveal Mask Properties: Press M to reveal the mask properties.
• Select the Mask Path: Click on the "Mask Path" property to activate it. You can now adjust individual points or the entire mask.
• Move the Mask: Use the Selection tool (V) to click and drag the mask. Make sure you don't click on the content that's being masked; click on the mask path itself. Alternatively, adjust the "Mask Path" property numerically in the Timeline panel.

3. Selecting and Moving Individual Mask Points:

This can be tricky.

• Select the Layer: Ensure the layer with the mask is selected.
• Reveal Mask Properties: Press M to reveal the mask properties.
• Select the Mask Path: Click on the "Mask Path" property to activate it.
• Deselect All Points (Crucial Step): Click anywhere outside of the mask on the composition panel. This deselects all mask points.
• Select Individual Points: Now, click on the specific mask point(s) you want to move. Selected points will appear highlighted.
• Move the Point(s): Drag the selected point(s) to reshape the mask, thus revealing or hiding different parts of the layer underneath.

Important Considerations:

• Mask Mode: The "Mask Mode" (Add, Subtract, Intersect, etc.) affects how the mask interacts with the layer. Make sure the mode is set appropriately for your desired effect.
• Feathering: The mask's feathering can influence the appearance of the edges where the layer is revealed or hidden. Adjust the "Mask Feather" property as needed.
• Mask Expansion: This setting will increase or decrease the size of your mask. A negative value will shrink the mask.
• Animation: All mask properties, including Mask Path and Mask Position, can be animated to create dynamic effects.

Example:

Let's say you have a video clip of a person walking (the layer) and you want to reveal only their head using a mask. You'd:

1. Draw a mask roughly around the person's head.
2. If the mask isn't perfectly positioned, you could either:
   • Move the video layer itself (using the Position property) so the head is properly aligned within the mask, or
   • Move the mask to a new position to cover the head properly
3. Animate the Mask Path over time to follow the head as it moves.

By understanding these methods, you can effectively control how content is revealed or concealed by a mask in After Effects.