- [Instructor] So far, we've just been looking
at color and light,
and one way or another, we've been using three channels
of color information for each pixel.
Let's use RGB for now.
There's another piece of information that's important
about pixels, and that's how transparent they are.
The words used to describe how opaque a pixel is vary.
Sometimes, it will be opacity, transparency or alpha.
Regardless of the name, when you make an adjustment to
the opacity of a pixel, you're actually adjusting
a separate channel, the alpha channel.
Adjustments made to the alpha channel are
completely separate from adjustments
made to the color channels.
If you reduce the alpha channel level,
the colors will stay the same,
but some of whatever is behind the pixel will show through.
Because most video editing timelines are black
in the background, reducing the alpha channel,
which is often described in software as
lowering the opacity, will appear to make the image darker.
And this isn't really true.
In fact, you're seeing a combination of the color
information in the foreground pixels,
that's the ones you're adjusting,
and the black of the background pixels
immediately behind them.
Adding another image behind the foreground
will create a new composition,
and this can be used to great dramatic effect.
If you're working with Photoshop, or any design
application with similar options,
you can display a so-called transparency grid
in areas that the image would be transparent.
This grid of checkered squares is a visual reminder
that the image would show what's behind it
if it were included in the composition.
There are a number of ways to adjust the alpha channel
for pixels.
One is to use a direct control provided by software.
Another is to automatically adjust the alpha channel
based on the color or brightness of the pixels.
And we'll talk more about this in the lesson
on masking, chroma key and luma key.