How to Create a Subtract Mask Using Adobe After Effects

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If you are using the Chroma-Key effect in After Effects to make your video background transparent, you may run into a problem if you do not have a perfectly even Green Screen. If that is that case, you may need to create a Subtract Mask on the footage to remove any areas not easily removed with the Keylight Tool.

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Instructions

Difficulty: Moderate

Things You’ll Need:

Computer with Adobe After Effects
Digitized video footage

Step1

Follow the instructions in “How to Use Chroma-Key on Adobe After Effects.”

Step2

If there are areas that did not key all the way out, they will likely appear as black shadows.

Step3

Select the clip that you want to create the Subtract Mask on by clicking on the footage under “Layer Name” in the Timeline.

Step4

In the tools menu select the Pen Tool.

Step5

In your composition window, carefully “draw” a circle or shape around the area that needs to be removed by left clicking on the footage. Make sure all of your dots are connected in a complete circle or shape.

Step6

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.

Step7

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 “Subtract” is selected. This tells the mask to remove everything within the mask.

Step8

Scrub through your footage by dragging the “Time Indicator” in your Timeline through the footage. Make sure are stays within your mask from frame to frame of the entire clip. If there is motion in the footage, it is likely it will not stay within the mask. You can adjust each dot of the mask individually by selecting it and dragging it to expand the mask.

Step9

Give the Mask Feather Option (in the Timeline) a value of approximately “3.” Save your work.