Computer Screens - Tracking and Motion
Continued

Sometimes, the screen you wish to film has an object in front of it, or the camera moves. In these cases a simple overlay won't work. This was a problem we had to deal with in shooting this film. The solution we came up with used either a matte (in this case a still frame of the movie) or a color key.
 
  As you can see, the TV screen is partly blocked by the Commanders arm.
     
After the matte and keying, the TV behind his arm shows the radar image wanted.
    
   
 
                 
      The matte was simply a still frame of the movie that had the action part cut away. More importantly, the screen itself is cut away. During video editing, the black parts of the original footage are told to be neutral, or nothing at all. The image to the right was placed behind the original footage and filled in the black gaps that weren't supposed to be holes.    
                 
         
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