- [Instructor] The alpha channel for a pixel
uses the same bit depth as the color channels.
The numbers are the same,
so it's fairly straightforward
to translate one channel figure into another.
You can designate areas that should be transparent
manually using a mask,
and when you do that
you're using a shape to set an adjustment
to just the alpha channel on the pixels selected.
But you can also automatically apply adjustments
to the alpha channel on pixels
based on colors or luminance.
Let's look at a simple example of this, lumakey.
Put simply, when you lumakey pixels,
you convert the luminance level of each pixel
to its alpha channel.
Unless you make any adjustments to the effects settings,
only pixels that are perfectly black
will be completely transparent,
because only an alpha channel of zero
is totally invisible.
The brighter each pixel is, the more opaque it will be,
until pixels are completely white
and therefore completely opaque.
Lumakey can produce some lovely effects,
if they're planned for.
You'll normally specify a minimum luminance,
below which all pixels are completely transparent,
and a maximum luminance,
above which all pixels are completely opaque.
And this'll give you a cleaner result.
In any case, the principle is very simple.
Chromakey works in a very similar way,
except that instead of using the luminance of a clip,
the alpha channel for a pixel is changed
if it's close to a particular color.
And it's often the color green.
Green is used so often for this
that green screen is synonymous with chromakey.
Any pixel that perfectly matches
the selected color in the shot
will be set to alpha zero.
The more pixels vary from the selected color,
the higher the alpha channel value,
making them more and more opaque
until the block the background completely.
As with lumakey,
you'll set minimum and maximum limits to the selection
when using chromakey.
And there are often multiple controls available
to clean up the resulting selection.
The selection that's defined
is often called a mask, or I suppose a key.
And a mask can sometimes define an area
that's visible or transparent,
or can also just define a selection.
In this instance, it's kind of both.
Pixels are being selected,
and that selection is used
to define the alpha channel value
which changes the opacity.
The key part of the name chromakey or lumakey
indicates that something is going to be made transparent.
There are a number of other kinds of key,
like track matte or a garbage matte key.
Just know that anything that involves keying
means adjusting the alpha channel somehow.