Original subtitles

Hi and welcome to Greenscreen Keying Fundamentals.

Today we're going to take a look at chroma keying, and exactly what that is.

Chroma keying's definition is a special effects or

post-production technique used for compositing two images.

So we're going to take two images and put one over top of the other.

Let's take a look at what that is exactly.

First thing we're going to do is a very simple technique.

We're going to take this element, we're going to pull a key,

and we're going to create a mask.

Here we've created a matte.

So let's just take a look at that alpha channel.

Here's a black and white alpha channel that we've created,

and it's created a cut-out, or a matte.

We're going to compare the two, and see what that exactly looks like.

So wherever we have sampled, and the alpha has turned black,

there is no RGB image visible here, which we can see.

And wherever the alpha channel's 100% white, our RGB image is 100% visible.

So that's the very, very basics of chroma keying,

but let's look into it in a lot more depth.

Chroma keying can be used on different colored backdrops.

The most common ones of course are a bluescreen,

which we have here, and a greenscreen.

You're going to hear the word backing color, a lot.

And the definition of a backing color is a screen color.

So that's what this is here.

This is a greenscreen and this is a bluescreen backing color.

Now throughout these modules and these clips you will

notice that I use the word greenscreen and bluescreen for

the most part interchangeably.

Just so that you don't really get confused between what I'm talking about there.

Let's take a look at exactly what keying is in more depth.

So let's go back to the fishing greenscreen image and we're

going to pull a chroma keyer through a Keylight.

So with the Keylight, I'm going to sample the greenscreens over here,

there's a picker.

And I'm going to Control + Shift.

Let's just try that one more time.

There we go.

And it's going to select that green.

Now let's take a look at that alpha channel, and see what that looks like.

I'm going to deselect the picker, so that it doesn't sample a second time.

You're going to notice that wherever that sample was taken

and the pixels that surround it that are identical it will

turn the alpha 100% black.

Wherever they are close but not quite the same as the sample area,

it will turn gray.

And wherever it doesn't match that sample area at all, it will turn white.

So green will turn black in the alpha,

and wherever it isn't green will turn white.

Let's take a look at how we can look at that in a little bit more depth.

I've already pre-created some things here,

so here's back to our greenscreen, and I've pulled a key.

And there's two basic type of keys that I can pull.

The first one is a soft matte which we've already pulled.

So here's our soft matte.

And it's called a soft matte because it has varying levels of the alpha channel.

So if I hover over top of here it is 13%.

Here it is almost 100%, so not quite opaque.

And here of course, it's almost 100% black.

So a soft matte is one that doesn't necessarily become 100% black or white.

So let's take a look at what that looks like in detail.

So here's a nice soft matte.

We have nice falloff on his edges.

And now we're going to take a look at a hard matte.

So with the hard matte you can see that this is 100% black and white.

Again let's sample.

100% white and 100% black.

Now let's take a look what that looks like in the colors.

So we're going to compare the two, I have a switch here.

You can see that this is a hard matte,

and the other one when I switch is a soft matte.

And you can see already what's happening down here.

Let's look at our actor's hair in more detail.

So a hard matte.

And a soft matte.

So you can see the nice falloff of the two,

and the benefits of both of them there.

So that's the difference between a hard matte, and a soft matte.

There's plenty of things that you can refer to in here,

and there's the variation.

So again, our hard matte, our nice solid alpha.

And I'm going to switch that over to our soft matte,

and we have the nice detail in our edges,

our transparency, and perhaps even wispy hair.

Next we're going to take a look at despill.

What exactly is despill?

Despill is, the definition I should say,

of despill is the removal of unwanted color that is reflecting from

your backing image onto your foreground subject.

So that means that this green is reflecting onto our actor.

Now let's take a look in more depth.

Here I have oversaturated on purpose.

Here's our source.

Here's our despilled image.

If you look really closely, you can see that up here is green,

and down here there's no green.

And we can switch back and forth between the two very

quickly and it might become more apparent.

So here is the original and the despilled.

Watch very carefully down here on his arm.

So again we have the green, and no green.

So keyers do two things, or can do two things,

that is that it pulls a key or an alpha channel,

and it can also despill the image.

Now not all keyers are alike, and not all keyers can do that.

But we will take a look at that in a little bit more depth.

Some other things to take a look at of course is,

let's go back to our hard matte,

and we can see that it's not 100% black and white.

In fact, we have some unwanted elements over here in the alpha channel.

And we're going to take what's called a garbage matte

and we're going to remove them.

Now the definition of a garbage matte is a hand-drawn,

quickly made matte that is used to exclude parts of an

image that is made by another process, such as bluescreen or greenscreen keyers.

So here we're going to take it and remove it.

It's created very quickly, does not have to be accurate,

and it is removed in our RGB image.

So now we have a pretty good key to work with overall.

Let's move on to the premultiplication.

We have our premultiplied image, which we have already created previously.

And essentially what we've done is we have removed it from our greenscreens.

So here we have a greenscreen.

It has been removed,

and now we can take that element and place it onto a new background.

So let's take a look.

Here is our greenscreen.

Course our element has been separated,

so he no longer belongs to that greenscreen.

And I can place it on a new background.

I can move him around, in this case with a transform tool.

And now we have two separated elements.

Next, we're going to take a look at premultiplication.

Premultiplication is how we cut out our images,

and how we apply the alpha channel to that RGB image.

So let's take a look at that math.

Here we have a 100% red constant that we have.

You can't really tell where I sampled it because the box is red,

but that's okay.

We have an alpha channel here.

So if we take the math of the alpha channel,

which is 100% times the red channel, which is also 100%,

one times one means 100% opacity in our red channel.

So our element is 100% visible in our red channel.

Let's take a look at another example.

Here is our green constant.

It is 50% green, and we have 100% in our alpha channel.

We premultiply and our green channel remains at 50%.

So 0.

5 times one equals 50% of opacity in our green channel.

One last example, we have blue.

Blue is 25%.

And our alpha is also 25%, 0.

25 times 0.

25 is 0.

0625 in our premultiplied image, in our blue channel.

Now that we understand how premultiplication works,

and how to extract a matte, let's look at the main keyers inside of Nuke.

So here we go back to greenscreen image that we may have looked at earlier,

and it is a fan.

We're going to go look at three basic chroma keyer tools: That is the Primatte,

the Keylight, and the hue keyer.

Now the Primattes and the Keylight are very similar,

and we're going to take a look at those now.

So here I have previously already pulled a key with a Primatte.

You can see that it pulls a pretty good alpha channel to begin with,

without a lot of revisions.

And we're going to compare A against B.

So here we can see that here's our original image,

it has not only cut it out, but it has also despilled the image.

So no more green.

Green is most apparent down here at the bottom of the image.

Keylight works very, very similar, but a slightly different result.

So here is our Keylight,

we pull a key and we shuffle out the alpha just take a look at it.

So again, here is before, after.

Primatte versus Keylight.

And let's take a look at those RGBs, so slightly different in the despill.

Looks like the Primatte was a little bit more aggressive with that despill.

But both tools are very much usable.

And we'll take a look at which one works best in different

situations in later modules and clips.

Next, let's take a look at the hue keyer.

So here's the hue keyer, again, it will pull keys based on this backing color.

So here we pull a hue keyer,

but you will notice that it does not premultiply the image by default.

Nowhere does it despill the image.

So here is our alpha from our hue keyer,

and again, it does a pretty good job,

but not as good of a job as these initial two keys.

So these are the two keys that I will tend to stick to.

However this has it's time and place to be used as well.

So now that we have gone through the keyers and the basics of keying,

let's look through a couple of other examples.

So again, we're going to stick with the same image.

We've already done chroma keying.

Let's look at a couple of other ones.

We'll extract a matte based on the light and the dark values in the image.

For the most part, this fan is darker than the background,

and the background is lighter.

But let's take a look at what it does.

So by default we can see that it pulls an inverse matte,

so let's just invert that for us, and then pulls a key.

Again, it's not going to premultiply the image unless we force it to.

It doesn't do a despill.

It doesn't premultiply the image.

It has to be forced a little more, so it is usable.

It's mostly good for pulling stuff like highlights,

sometimes reflections and so forth.

But generally speaking, again, that Keylight and the Primatte are best for RGB.

So it takes the image and it takes the luminance.

The luminance are again, the dark versus light values.

You can see the luminance down here.

To sample it and it will extract a matte based on that.

Our last example is the difference keyers.

Now there are two basic types of difference keyers that we will look at here.

The first one is the ultimatte.

Now the ultimatte takes a clean plate.

And a clean plate is essentially an image that has no foreground element.

So it has been extracted, it has been removed from the backing color.

So here is just a sampled constant.

We have sampled the basic color of that backing color.

And we have done the difference.

So what is the difference between this image, and this image?

The difference is the alpha.

So let's take a look at that.

Here's before, here is after.

Before, after.

And it also despills the RGB image.

The last one which you will see often in these clips

and modules as well is the IBK keyer.

The IBK is a two-part keyer.

It creates its own clean plates based on the IBK color,

and then uses the IBK gizmo to extract the difference keyer.

It also does a despill.

So there you go.

There's the basics of keying for today's lesson.

And let's just do a quick recap summary of what we have learned.

So first we reviewed the fundamentals of chroma keying,

and premultiplication, how do we extract that image,

how do we move it around, what is a hard matte,

what is a soft matte.

Next, we looked at the differences between chroma keying,

difference keying and luma keying.