Mosaic Effect in After Effects: Pixelate Video

The Mosaic effect in Adobe After Effects is a built-in stylization tool that degrades an image's resolution by dividing it into a grid of solid-colored squares. This article covers what the Mosaic effect is, how its core parameters work, and the exact step-by-step process required to selectively pixelate and track specific regions—such as faces, license plates, or sensitive information—within your video projects.

What Is the Mosaic Effect?

The Mosaic effect simulates a low-resolution display by sampling pixel data and grouping neighboring pixels into larger, uniform rectangular blocks. Located under Effect > Stylize > Mosaic, it is widely used in video post-production for two primary purposes: creative retro styling (mimicking 8-bit graphics) and practical censorship (obscuring identities, logos, or private data).

How the Effect Controls Pixelation

When applied to a layer, the Mosaic effect provides three primary settings:

  • Horizontal Blocks: Sets the number of columns of pixels across the width of the layer. Lower values produce larger blocks (heavier pixelation), while higher values produce smaller, finer blocks.
  • Vertical Blocks: Sets the number of rows of pixels along the height of the layer. Setting this value equal to or proportional to the composition aspect ratio maintains square pixel blocks.
  • Sharp Colors: Toggles color averaging. When unchecked, it averages the colors within each block. When checked, it samples the color of the pixel at the center of each block, resulting in a crisper, high-contrast blocky appearance.

How to Pixelate Specific Regions

To apply the pixelation to only a selected area rather than the entire frame, follow these steps:

1. Create an Adjustment Layer

  1. Place your footage onto the timeline.
  2. Go to Layer > New > Adjustment Layer (or press Ctrl+Alt+Y on Windows, Cmd+Opt+Y on Mac).
  3. Ensure the Adjustment Layer sits directly above your target video layer.

2. Apply the Mosaic Effect

  1. With the Adjustment Layer selected, navigate to the Effects & Presets panel.
  2. Search for Mosaic and double-click it.
  3. In the Effect Controls panel, decrease the Horizontal Blocks and Vertical Blocks (e.g., between 15 and 30) until the content becomes sufficiently obscured.

3. Mask the Region

  1. Select the Adjustment Layer.
  2. Choose the Ellipse Tool (for faces) or the Rectangle Tool (for screens or license plates) from the top toolbar. You can also use the Pen Tool for custom shapes.
  3. Draw the shape over the area you want to pixelate. The Mosaic effect will instantly be restricted to the boundaries of the mask.
  4. Press F on the timeline to adjust Mask Feather if you want to soften the edges between the pixelated area and the rest of the image.

4. Track Moving Regions

If the object you are censoring moves across the frame:

  1. Open the Tracker panel (Window > Tracker).
  2. With the mask selected on your Adjustment Layer, set the tracker method to Position (or Position, Scale, Rotation).
  3. Click the Track Selected Mask Forward button (the play icon in the Tracker panel).
  4. After tracking finishes, review the playback to ensure the pixelated mask stays aligned with the target object. Make manual keyframe corrections if necessary.