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Using the Difference Keyer

Use the Difference Keyer to generate a matte clip from two source clips with the same background but different foreground elements. This allows you to remove an image from one context to add it to another.

Sometimes you do not have the luxury of shooting a subject in front of a blue screen. In this case, you can use the Difference Keyer to create a matte for the subject. To do this successfully, you need the clip of the subject, and a still image (or a clip) of the background with no subjects.

The difference matte effect calculates the difference between corresponding pixels of the two source images. The value of the pixel in the back image is subtracted from the value of the corresponding pixel in the front image, and the resulting absolute value is used in the difference matte.

The matte is created using the Tolerance and Softness values that you specify. The Tolerance value specifies the difference level that is considered black. A higher Tolerance value includes more black in the matte. The Softness value is used to soften the transition between the light areas and the dark areas of the matte by adjusting the amount of gray at its edges. Gray information is not included in the matte when the Softness is zero. Use a higher Softness value to increase the gray.

Difference Keyer Controls

The Difference Keyer Controls panel includes the following elements.

RGB or YUV

Click one of these buttons to specify the color model to use for the difference matte.

Output

Click a button under this heading to specify a difference key output. You can choose Front, Back, Matte, or Result.

Gain

Use this slider to boost the matte. The Difference Keyer multiplies the values of the pixels in the result clip by the Gain.

Lift

Use this slider to add the Lift value to all pixels in the difference matte.

R, G, or B

Enable the color channel(s) to use for the difference matte. You can use any of the Y, U, or V channels in the YUV color model, or any of the R, G, or B channels in the RGB color model.

Tolerance

Set the tolerance for the matte by clicking the color box next to the Tolerance field in the same row as the color channel you are using. Drag the color picker around the area of the image to matte out.

Note: You can also set the tolerance for the matte directly in the Tolerance fields.

Softness

Set the softness for the matte using either the Softness field or the color box beside its field.

Note: You can also set the softness for the matte directly in the Softness fields.

Back Source

The Back Source field displays the operator that you are comparing to the front layer. Click the Set button to pick a new Back Source.

Reset All

Click this button to reset all Difference Keyer parameters to their default settings.

To use the Difference Keyer:
  1. Load a front clip and a back clip into a composite.
  2. In the Workspace panel, select the front layer.
  3. Choose Operators | Keying | Difference.

    4. The Choose Background Source dialog appears.

  4. Select the operator you want to use for the background source. Usually, you would choose a Footage operator, but you can pick from anywhere in the workspace.

    5. The Difference Keyer Controls panel appears.

  5. Under the Output heading, click the Matte button to display the resulting matte clip.
  6. Choose the color model to use for the difference matte. You can use either RGB or YUV.
  7. Choose the color channel(s) to use for the difference matte.
  8. Set the tolerance for the matte by dragging the picker around the area of the image to matte out.
  9. Set the softness for the matte. Softness values should be around zero for each color channel to produce a clean matte.

    10. You can refine the edges later using matte controls. For more information, see Using Matte Controls Operator.

  10. Use Gain or Lift as needed.
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