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TUTORIAL: G Matte: Tidying a composite using a garbage matte

Posted: Thu, 15th Apr 2004, 10:24am

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Simon K Jones

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Chromanator tutorials: Garbage mattes

Tidying a composite using a garbage matte

After using tools such as the Colour Difference Key, you may find that whilst the vast majority of your image has been keyed successfully, there may be areas left over still. This may be due to the greenscreen not being large enough to fill the entire frame, or it could be due to lighting problems.

Using the garbage matte you can select these areas and manually remove them. This example will use this problematic greenscreen shot:

As you can see, there are areas in the top left and top right of the frame that have not keyed properly, due to poor lighting in the original shot. When working in the garbage matte mode, the canvas only shows the current object, to help you identify problem areas.

Careful planning can often make post-production a much smoother experience. In this example, one option would be to draw two separate masks, one around each of the trouble areas. However, as the actor is perfectly isolated, time can be saved by drawing just a single mask around him, and inverting it. This will remove everything from the image except the actor.

  • Planning effects carefully can often speed up the work and make it easier.

Drawing the mask
The actor is not interacting with anything, so the garbage matte does not need to be too precise: a simple box shape will suffice.

  • The garbage matte toolset can be accessed by selecting an object on the timeline and switching to the Object View, then selecting the Key and Garbage Matte tabs.
  • First I create a new section, by clicking on the button.
  • Points can be added by clicking directly onto the canvas, in a square shape.
  • The final point can be dragged to meet the first so that the mask is closed and turns blue.

  • The mask is locked by clicking on the Animate tab in the mask toolset, at which point the mask will turn red.
  • Activating the invert option will select everything except the actor.

A quick scrub through the frames will check that the actor is inside the mask shape at all times. If the mask shape is not big enough at any point the control points can be adjusted as necessary. This will place new keyframes on that frame and the mask shape will animate smoothly between each keyframe.

This will result in a perfect composite. In some cases more than one mask will be needed to build up the garbage matte. With an number of multi-point masks, almost any keying job is possible in Chromanator – all it requires is a little patience.

Last edited Wed, 21st Apr 2004, 2:02pm; edited 2 times in total.