Masking and Keying
You can use the Pen or shape tool (in the tool palette) to draw a mask
Layer - Mask menu items (Mask Shape, Feather, Opacity, Reset, Remove)
Animating a Mask - Open the arrow under the layer and open the Mask arrow
Set keys on Mask Path shape, Mask Opacity, etc.
Effect menu - Keying - Keylight (1.2)
Screen Colour - Use the color picker to select the color you want to get rid of
Despill Bias - if some of the color is spilling onto the objects in the scene
Screen Matte - Clip Black or Clip White to adjust the keying of the edge of the object
Screen Softness - Around the edge it will become blurry
We are using sand animation as an example.
The source footage should have good contrast and with white or black background.
Use the same source footage to create two video layers.
1. Duplicate Layer - Edit menu - Duplicate - Command D
Call the top layer Matte layer, and the bottom layer as Source layer
2. Select the name of the layer and hit RETURN - then type the new name
Add another image/ video as the background layer at the bottom.
Open the Transfer Controls
In timeline: it should look like this:
1. Turn off visibility for the matte layer (click the EYE BALL off)
2. Set the source layer TrkMat as Luma Inverted Matte "matte layer"
It will combine the images.