After Effects for Absolute Beginners: Masks and Simple Reveals

What a Mask Is (and What It Isn’t)

A mask is a path you draw on a layer that controls which parts of that layer are visible. Think of it like cutting a hole in a piece of paper placed on top of the layer: the “hole” shows the layer, and everything else can be hidden (depending on the mask mode).

Masks are attached to a specific layer and affect that layer’s pixels. You can draw masks on footage layers, solids, and adjustment layers. Masks can be animated by keyframing their path, and you can soften or grow/shrink their edges using feather and expansion.

Masks vs. Shape Layers (Common Beginner Confusion)

• Masks live inside a layer (footage/solid/adjustment) and primarily control visibility (or define areas for effects). They are not their own “graphic object.”
• Shape layers are layers made of vector shapes with their own fills/strokes and dedicated shape controls. They’re ideal for clean, editable graphics.
• Rule of thumb: If you want to reveal/hide part of an existing layer, use a mask. If you want to create a graphic element (like a button, icon, or animated line), use a shape layer.

Where You Can Use Masks

1) Masks on Footage

Use masks on video clips or images to reveal only a portion of the footage, hide unwanted areas, or create a “window” that moves across the shot.

2) Masks on Solids

Solids are great for learning masks because they’re simple and predictable. A masked solid can become a wipe, a vignette, or a custom matte.

3) Masks on Adjustment Layers

An adjustment layer applies effects to layers beneath it. When you add a mask to an adjustment layer, the effects only apply inside (or outside) the masked region. This is perfect for localized color correction, blur, glow, or sharpening.

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Creating a Mask (Tools and Basics)

To draw a mask, select the target layer first, then use one of these tools:

• Rectangle/Ellipse tools: Create a rectangular or elliptical mask quickly.
• Pen tool: Draw a custom mask with points and curves.

Once a mask exists, you’ll find it in the layer’s properties under Masks. Each mask has controls you’ll use constantly: Mask Mode, Mask Feather, Mask Expansion, and Mask Path.

Mask Modes: Add and Subtract (and Why Your Layer “Disappeared”)

Mask modes determine which side of the mask is visible.

• Add: Shows the area inside the mask and hides the outside.
• Subtract: Hides the area inside the mask and shows the outside (an “inverted” reveal).

If you draw a mask and your layer suddenly vanishes, it’s usually because:

• The mask mode is set to Subtract when you expected Add.
• The mask is tiny or off-screen.
• You drew the mask on the wrong layer (common when multiple layers are selected).

Feather and Expansion: Refining the Edge

Mask Feather

Mask Feather softens the edge of the mask by creating a gradual transition. This is essential for natural-looking reveals on real footage (hard edges often look like a cut-out).

Practical guidance:

• Use small feather values for crisp graphic reveals.
• Use larger feather values for organic footage reveals, especially with handheld shots or soft lighting.

Mask Expansion

Mask Expansion grows or shrinks the mask uniformly.

• Positive values expand the visible area (in Add mode).
• Negative values contract it.

This is a fast way to “nudge” an edge without editing points. It’s also useful after feathering, because feather can make the reveal feel slightly smaller than intended.

Animating Reveals by Keyframing Mask Paths

To create a moving reveal, you animate the Mask Path. This means the mask’s points change over time, so the visible area changes.

When to Animate Mask Path vs. Move the Mask

• Animate Mask Path when the shape of the reveal changes (for example, a mask that bends around an object or changes size/contour).
• Move the mask (by selecting the mask and dragging it) when you want the same shape to slide across the layer. This still creates keyframes, but it’s often simpler than reshaping points.

To keyframe a mask path: open the mask properties and click the stopwatch for Mask Path, then change the mask at a later time (move points or the whole mask). After Effects will create new keyframes.

Exercise: Video Reveal Through a Moving Mask

You’ll create a simple “window reveal” where a moving mask reveals a video layer, then you’ll refine the edge using feather and expansion.

Goal

A rectangular reveal travels across the frame, showing the footage only where the mask passes, with a clean but slightly softened edge.

Step-by-Step

1. Choose the layer to reveal. In your composition, select the footage layer you want to reveal.

2. Draw the starting mask. Select the Rectangle Tool. With the footage layer selected, draw a rectangle mask on the left side of the frame (a “window” that will move across).

   Confirm the mask mode is Add so only the inside is visible.

3. Animate the mask’s movement. Expand the layer properties to Masks > Mask 1. Turn on the stopwatch for Mask Path.

   Move the playhead forward in time. Select the mask (click “Mask 1” or click the mask outline), then drag the entire mask to the right side of the frame. This creates a second keyframe.

4. Preview the reveal. Scrub through the timeline to confirm the reveal travels across the footage.

5. Refine the edge with feather. Increase Mask Feather slightly (start small, then adjust). You’re aiming for a softer edge that doesn’t look like a harsh cutout.

6. Adjust coverage with expansion. If feather makes the reveal feel too tight, increase Mask Expansion a little. If the reveal feels too wide or bleeds too far, reduce expansion.

7. Optional: Make the reveal more interesting. Instead of a rectangle, use the Pen Tool to create a custom shape (like a diagonal edge). Animate the mask path so the shape slightly changes as it moves, creating a more dynamic wipe.

Edge Refinement Tips (What to Look For)

• Too sharp: Increase feather.
• Too blurry/washed: Reduce feather, then use a small positive expansion to keep coverage.
• Edge doesn’t align where you expect: Expansion is often faster than re-editing points.

Using Masks on Adjustment Layers (Selective Effects)

Try this quick variation to understand how masks behave differently on adjustment layers:

1. Create an Adjustment Layer above your footage.

2. Apply an effect like Gaussian Blur (or any obvious effect).

3. Draw a mask on the adjustment layer. In Add mode, only the masked region will be blurred; in Subtract mode, everything except the masked region will be blurred.

This is one of the most practical real-world uses of masks: controlling where an effect applies without cutting up your footage.

Troubleshooting Common Mask Problems

Problem: The reveal is inverted

Symptom: You expected to see inside the mask, but you see everything except the mask area.

Fix: Change Mask Mode from Subtract to Add (or vice versa, depending on your goal).

Problem: The mask is affecting the wrong layer

Symptom: You draw a mask, but a different layer changes visibility, or nothing happens to the layer you intended.

Fix: Undo, then click the correct layer to select it before drawing. Also make sure you don’t have multiple layers selected when you start drawing.

Problem: I can’t see or select the mask path in the viewer

Symptom: The mask exists in the timeline, but you can’t grab its points.

Fixes:

• Select the layer, then click the mask name (e.g., Mask 1) so its path becomes active.
• Make sure you’re not in a different tool mode that prevents selection; switch to the Selection Tool.
• If the layer is shy/locked or you’re viewing a different panel, unlock/select the correct layer and try again.

Problem: The edge looks “wrong” after feathering

Symptom: Feather makes the reveal look too transparent or like it’s fading more than expected.

Fix: Reduce feather and compensate with a small positive Mask Expansion. Feather softens by blending; expansion restores coverage.

Problem: The mask moves but the reveal doesn’t match what I animated

Symptom: You animated the mask, but it seems offset or not behaving as expected.

Fixes:

• Confirm you keyframed Mask Path (not a different property).
• If you intended to slide the mask, make sure you moved the entire mask (not individual points) consistently.
• Check if there are multiple masks on the same layer; you might be editing the wrong one.

Practice Variations (Quick Drills)

• Two-mask reveal: Add a second mask and set it to Subtract to “cut out” a notch from your reveal window.
• Spotlight effect: On an adjustment layer with blur or exposure, use an elliptical mask in Subtract mode so everything is affected except the subject.
• Organic wipe: Draw a wavy mask with the Pen tool and animate the path for a hand-drawn style reveal, then refine with feather/expansion.