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

The Difference tool calculates the difference between the 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 to create the difference matte.

The matte is created using Tolerance and Softness values you specify. The Tolerance value specifies what difference level is considered as black. Use a higher Tolerance value to include more black in the matte. The Softness value softens the transition between the light and dark areas of the matte by adjusting the amount of grey at the edges of the matte. No grey information is included in the matte when the Softness value is equal to zero. Use a higher Softness value to increase the amount of grey.

To create the difference matte:

1. Select FX from the A/V Tools box and then click Difference.
2. Select the Front and Back source clips (in that order).
3. Select destination for the difference matte.
4. Click Result to display the resulting matte clip in the image window.

   
   

5. Select the colour model you want to use to create the difference matte. You can work in either the RGB or YUV colour model.
6. Choose the colour channel(s) that you want to use to create the difference matte. You can work with 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 softness for each channel using either the Softness fields or the corresponding colour pots.
8. Manipulate the resulting matte, if necessary.

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

9. Click Process.

   The result clip appears in the selected destination.