Masking and Keying

 You can use the Pen or shape tool (in the tool palette) to draw a mask

mask tools

Layer - Mask menu items (Mask Shape, Feather, Opacity, Reset, Remove)

mask menu

Animating a Mask - Open the arrow under the layer and open the Mask arrow
  Set keys on Mask Path shape, Mask Opacity, etc.

mask animation

Color keying

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

Luma Matte

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.