How to Blur Backgrounds in After Effects

By Nick Flegg

Use the Gaussian Blur effect to blur the background layer of your clip.
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When blurring a project in After Effects that has a separate background layer, you can add a blur filter over the background layer to differentiate it from the foreground. If the foreground and the background are all in one layer, you need to rotoscope the clip. After Effects has a Roto Brush tool that enables you to paint part of a clip and apply that selection across a number of frames. When you complete your selection, you can add a blur filter to the background.

Step 1

Duplicate your clip by selecting it, and then pressing "Ctrl-D." Double-click the top clip to open it in the Layer panel.

Step 2

Press "Alt-W" to activate the Roto Brush tool. Scan through the clip until you find the frame in which the foreground takes up most of the frame.

Step 3

Paint over the object you would like to define as the foreground by clicking and dragging in the Layer panel. Paint as much of the foreground object as possible, as accurately as possible. When painting the foreground object, your cursor should be a green circle with a plus sign in the center.

Step 4

Hold down the "Alt" key and paint the background of your clip to define it as the background. When painting the background, your cursor should be a red circle with a minus sign in the center. A magenta line defines the line between your foreground and background objects. Ensure your selection is as accurate as possible, as the primary frame affects all other frames in the clip.

Step 5

Press the "Page Down" button to move forward one frame. Correct any flaws in your rotoscope selection in the current frame by using your Roto Brush to paint the new foreground and background objects in your clip. Continue to navigate through your clip frame by frame until you've checked the entire clip.

Step 6

Navigate to the "Effect Controls" panel, and then select the "Refine Matte" option. The Effect Controls panel enables you to fine-tune your rotoscope selection by smoothing or feathering the edges.

Step 7

Select the bottom clip in your Timeline panel. Select the "Effects" menu, and then choose "Blur and Sharpen." Click the blur filter that best fits your requirements, such as "Gaussian Blur." Go to the "Effect Controls" panel and use the slider to increase the amount of blur to the clip.