Use the Difference command 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.

When you generate a difference matte, the difference between corresponding pixels of the two source images is calculated. 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 Tolerance and Softness values. The Tolerance value specifies the difference level that is considered black. A high 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 grey at its edges. Grey information is not included in the matte when the Softness is zero. Use a high Softness value to increase the grey.

To generate a difference matte:

1. Click Difference in the Processing menu.
2. Select the Front and Back source clips (in that order).
   NoteIf you want to use clips of different resolution, resize them according to the common destination resolution. See Resize.
3. Select the destination reel.

   The Difference menu appears.

   
   

   (a) Colour channels  

4. Select Result from the View box or press F4 to display the resulting matte clip.
5. Select the colour model to use for the difference matte: RGB or YUV.
6. Choose one or more colour channels to use for the difference matte. You can use any of the Y, U, or V channels in the YUV colour model, or any of the R, G, or B channels in the RGB colour model.
7. Set the tolerance for the matte by pressing on the colour pot beside the Tolerance field in the same row as the colour channel you are using. Drag the colour picker around the area of the image to create the matte.
   NoteYou can also set the tolerance for the matte directly in the Tolerance field.
8. Set the softness for the matte using either the Softness field or the colour pot beside the field.
9. Use the Gain and Lift fields to manipulate the resulting matte.

   Use:	To:
   Gain	Boost the matte. Difference multiplies the values of the pixels in the result matte by the Gain.
   Lift	Add the Lift value to all pixels in the difference matte.

10. Click Process.