How to take high quality chroma key in Adobe After Effects CS5 ? (Part 1-Creating a garbage matte)

When using a green or a blue screen,the only area that matters is the area around the subject.However,while filming a video you intentionally or unintentionally include extra elements in the video frame.The first step in the chroma keying process is to create a garbage matte,which is a mask that you draw to remove the unnecessary areas of the video frame and to isolate the subject and the area immediately around it.The garbage matte has to completely enclose the subject.If the subject is moving it is important to animate the vertices of the mask to accommodate the moving subject in the video frame.

Perform the following steps to create a garbage matte:

1. Drag the files into a new composition in the project panel.In our case we have created a new composition called chroma keying and placed two footage items in the timeline panel.
   
    
2. Select the layer containing the green/blue screen background in the timeline panel.
3. Move the playhead to the desired time mark on the timeline,where the subject takes maximum area on the screen.
4. Select the tool that you want to use to create the garbage mask.in our case we have selected rectangle tool.
   
5. Select rectangle tool and click and drag the mouse pointer around the subject to create the garbage mask.
   
6. Select File ->Save from the Menu bar to save the project.

You can use the pen tool to create the garbage mask in case you want to mask out complex areas.It is always preferred to create a large garbage mask to not cut out any of the subject's movement.Alternatively,you can create  a smaller mask that matches the contours of the figure of subject and then animate the position of the vertices on the path using the Mask Path property.

   In the next part you will learn how to create a chroma key like a pro.

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