If you are editing or compositing video footage shot on a Green or Blue Screen and want to remove elements from the footage, you need to create a garbage matte. For the purposes of this explanation, imagine a single person standing in the center of your video frame.
- Open After Effects and import your footage into a Timeline.
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Step 2
Select the clip that you want to create a garbage matte on by clicking on the footage under “Layer Name” in the Timeline.
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Step 3
In the tools menu select the “Pen Tool.”
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Step 4
In your composition window “draw” a circle or shape around the person in the middle of the frame, by left clicking on the footage. Make sure all of your dots are connected in a complete circle or shape.
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Step 5
Back in the Timeline, expand the options related to your clip by clicking on the little arrow to the left of the footage name. Then click the arrow next to “Mask” to expand it’s options, and click the arrow next to “Mask 1″ (the mask you just created) to expand it’s options.
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Step 6
Click the little stopwatch next to “Mask Shape.” This adds a “Keyframe” to your footage, essentially saving your mask shape from frame to frame. In the pull down menu to the right of “Mask 1″, make sure that “Add” is selected. This tells the mask to remove everything in the frame that is outside the mask.
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Step 7
Scrub through your footage by dragging the “Time Indicator” in your Timeline through the footage. Make sure your person or object stays centered in your mask. If they are moving, it is likely they will not. You can adjust each dot of the mask individually by selecting it and dragging it to expand the mask.