E3.5: Garbage Mattes and Basic Compositing

Returning to one of the first exercises of this unit, the rotating chair, bring this into your editing software to use as the basis of a compositing exercise.

Find a still image from the internet of an animal or object, or photograph your own.
Bring this in to your editing software as a new layer and use the matte function to overlay this image onto your rotating chair.

You can work as simply or as intricately as you like. The result need not be slick or believable. The purpose of the exercise is for you to explore the basics of using matte’s and to gain an understanding of compositing. If you like you can bring in a moving image to apply the matte to, but a still image will suffice.

Bear in mind the matte feature may be called ‘mask’ in your editing software (in iMovie it is often referred to as ‘green screen’ or ‘blue screen’). Almost all editing software will have some level of basic matte or masking that allows you to work with at least two visual layers.

“A video matte is the electronic equivalent of two pieces of
construction paper in the hands of a talented child wielding a pair of
safety scissors.
If one piece of paper is blue and the other is yellow, little Johnny can
simply position the blue on the yellow, or the yellow on the blue. Unless
the top paper completely covers the bottom, the result is going to be a
blue/yellow composite picture.
If Johnny knows his way around those scissors, he can cut a shape out
of the yellow card and place it over the blue, or cut a hole in the blue
and position it over the yellow.
If Johnny is really creative, he’ll ask the teacher for more colors of
paper and build himself a collage – layers of colored paper stacked and
arranged into a pleasing picture. Add a little colored tissue or
cellophane scraps and Johnny’s visual construction tool kit can even
include partial transparency.
If you transform the construction paper into video signals, you’ve
entered the world of electronic mattes and masks. And just as a paper
and cellophane collage can consist of any combination of shapes and
forms, the possibilities for building electronic mattes are equally
infinite. Depending on your editing software, you’ll have a wide variety
of convenient matte-generating tools at your disposal.
These include geometric mattes in squares, circles and diamonds,
along with customizable shapes, such as four- or eight-point garbage
mattes. These are so named because they’re particularly useful as
quick and dirty ways to mask out stuff you want to hide in the
underlying video. Text and titles, which you can think of as masks
shaped like fonts, are variations of a geometric matte.
You can, in fact, make a matte out of anything from static shapes to
moving images.”
Bill Davies, Computer Editing: Keying, Alpha Channels and Mattes (2002) Videomaker.com
Once you are comfortable using a matte/mask, you can extend this exercise
further by exploring other ways of compositing. For example, try bringing in a
line drawing and use a ‘multiply’ function to overlay this drawing onto your chair
animation. Upload any extra experiments onto your learning log.