Elli Mayhem (ellimayhem) wrote in fau_digicomp,
Elli Mayhem

Mattes; The Alpha Channel

As we have already discussed, the Alpha channel is a greyscale image that maps the transparency of an image. White is opaque; black is see-through; greys are varying degrees of opacity. This is essential to making your composites work.

Maya's rendering engine, along with most other 3D packages, gives you the option to have images rendered include an alpha channel and this is by default set to on. So images you have rendered out of Maya will come into shake or other compositing packages with an alpha channel already in place, or premultiplied. It's called premultiplied because it is the R, G and B channels multipled my the alpha channel that allows the compositing software to imterpret the image thusly. However, sometimes if you are rendering in layers you will find that these premultipled images actually create a conflict in the course of putting your composite together. We will go over optinos to rectify this in Maya as well as some simple tricks for dealing with these problems in compositing.

Most bitmap or digital video images are not premultiplied and thus if they are to be placed in a composite must have steps taken to create a similar transparency map for them. This created alpha channel is called a matte. We will go over a few ways to create and apply mattes to unpremultiplied images.

The techniques we will look at today are for dealing with 3D situations and creating mattes by hand. Tuesday we will look at matte methods involving color keys and special matting tools that incorporate matting blue/green screen elements and are commonly used in special effects compositing.

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