Original subtitles

[Autogenerated] in this clip, We're going to learn what an Alfa Channel is.

We've learned in the previous clip what RGB channels are Any image

inside of nuke will show us what channels it has,

such as this background plate shows us at it.

Contains Red data, Green Data and Blue Data.

That data is represented in each one of the channels.

So let's look at this.

We again we have our red evergreen weaver blue.

This image does not contain an Alfa does not have one here.

However, if we look at our helicopter, we can see that not only do we have red,

green and blue but we also have this white square at the

end which represents the Alfa Channel.

Now what is an Alfa channel?

An Alfa Channel is a representation of the occlusion of light.

You see,

RGB is the measure of the quantity of that particular wavelength of light

being reflected from the surface of any given object.

Where's the Alfa Channel represents the occlusion of that light.

Whether we see it or we don't Now let's take a look at what

this helicopters off a channel looks like.

There we go.

I hit the a key, which is the shortcut that allows us to see the Alfa Channel.

What we have is a pure white Matt that has anti leasing on the edge,

and it is solid white and solid zero black.

A good Alfa Channel should look something like this now.

I also said that it is the occlusion of light.

What that means is that light either is occluded and it does

not pass through where it does pass through.

So zero is a false reading,

so that means it passes through where one is positive.

It means that light does not pass through where it is solid.

It is commonly mistaken to call an Alfa channel a

representation of transparency.

This is not an accurate description of the way light behaves.

It is not transparent.

It is either occluded or not included.

Now let's take a look at how this can interact with our composites.

Well, we take emerge.

We connect the A input to the foreground object that has an Alfa

Channel and the B input to the background plate.

And let's view that when we view it,

we can see that our helicopters correctly layered on

top of or over the background.

That's not just a proposition.

The over is actually a mathematical function,

and its function is a plus B times one minus lower case A.

It takes the Alfa Channel of the A input and subtracts it from one now one.

If we remember before that,

when we had one in each channel 111 that was the color white.

So it takes the color white and subtract C Alfa.

So what this actually does is it turns this Alfa inside out and negates it.

So then it's white over here on black here.

Then it multiplies that by the B input, which is our plea.

And what ends up happening is it creates a black hole in the plate,

which then a is added to it.

So again, everything that we do in RGB color space is meant to be additive color.

Additive Color is the central theme of everything that we

do because it's RGB color channels and the Alfa Channel is

critical to this process.

Without the Alfa Channel being present in this image.

If we turn it off, if we get rid of it, if we tell it no, we will not process.

You grab an image that doesn't aven Alfa.

Here we go, beauty, No Alfa.

We'll load that up and right here you can see that this is the exact same image.

Except when I hit that a key.

There's nothing there.

So let's now go back to our composite and we're going to

switch the A input to the no Alfa version.

And what we see is a very different result because there's nothing there.

It treats all the pixels, is just additive,

and it just adds the brighter pixels from a to the pixels from B.

So if B is a darker image, it will add into a and it will be more obvious.

But because this is a bright background, it almost creates a ghostly image.