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Hi and welcome to Greenscreen Keying Fundamentals.
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Today we're going to take a look at chroma keying, and exactly what that is.
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Chroma keying's definition is a special effects or
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post-production technique used for compositing two images.
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So we're going to take two images and put one over top of the other.
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Let's take a look at what that is exactly.
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First thing we're going to do is a very simple technique.
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We're going to take this element, we're going to pull a key,
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and we're going to create a mask.
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Here we've created a matte.
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So let's just take a look at that alpha channel.
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Here's a black and white alpha channel that we've created,
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and it's created a cut-out, or a matte.
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We're going to compare the two, and see what that exactly looks like.
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So wherever we have sampled, and the alpha has turned black,
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there is no RGB image visible here, which we can see.
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And wherever the alpha channel's 100% white, our RGB image is 100% visible.
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So that's the very, very basics of chroma keying,
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but let's look into it in a lot more depth.
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Chroma keying can be used on different colored backdrops.
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The most common ones of course are a bluescreen,
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which we have here, and a greenscreen.
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You're going to hear the word backing color, a lot.
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And the definition of a backing color is a screen color.
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So that's what this is here.
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This is a greenscreen and this is a bluescreen backing color.
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Now throughout these modules and these clips you will
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notice that I use the word greenscreen and bluescreen for
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the most part interchangeably.
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Just so that you don't really get confused between what I'm talking about there.
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Let's take a look at exactly what keying is in more depth.
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So let's go back to the fishing greenscreen image and we're
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going to pull a chroma keyer through a Keylight.
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So with the Keylight, I'm going to sample the greenscreens over here,
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there's a picker.
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And I'm going to Control + Shift.
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Let's just try that one more time.
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There we go.
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And it's going to select that green.
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Now let's take a look at that alpha channel, and see what that looks like.
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I'm going to deselect the picker, so that it doesn't sample a second time.
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You're going to notice that wherever that sample was taken
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and the pixels that surround it that are identical it will
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turn the alpha 100% black.
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Wherever they are close but not quite the same as the sample area,
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it will turn gray.
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And wherever it doesn't match that sample area at all, it will turn white.
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So green will turn black in the alpha,
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and wherever it isn't green will turn white.
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Let's take a look at how we can look at that in a little bit more depth.
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I've already pre-created some things here,
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so here's back to our greenscreen, and I've pulled a key.
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And there's two basic type of keys that I can pull.
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The first one is a soft matte which we've already pulled.
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So here's our soft matte.
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And it's called a soft matte because it has varying levels of the alpha channel.
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So if I hover over top of here it is 13%.
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Here it is almost 100%, so not quite opaque.
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And here of course, it's almost 100% black.
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So a soft matte is one that doesn't necessarily become 100% black or white.
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So let's take a look at what that looks like in detail.
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So here's a nice soft matte.
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We have nice falloff on his edges.
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And now we're going to take a look at a hard matte.
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So with the hard matte you can see that this is 100% black and white.
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Again let's sample.
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100% white and 100% black.
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Now let's take a look what that looks like in the colors.
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So we're going to compare the two, I have a switch here.
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You can see that this is a hard matte,
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and the other one when I switch is a soft matte.
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And you can see already what's happening down here.
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Let's look at our actor's hair in more detail.
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So a hard matte.
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And a soft matte.
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So you can see the nice falloff of the two,
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and the benefits of both of them there.
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So that's the difference between a hard matte, and a soft matte.
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There's plenty of things that you can refer to in here,
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and there's the variation.
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So again, our hard matte, our nice solid alpha.
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And I'm going to switch that over to our soft matte,
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and we have the nice detail in our edges,
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our transparency, and perhaps even wispy hair.
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Next we're going to take a look at despill.
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What exactly is despill?
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Despill is, the definition I should say,
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of despill is the removal of unwanted color that is reflecting from
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your backing image onto your foreground subject.
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So that means that this green is reflecting onto our actor.
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Now let's take a look in more depth.
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Here I have oversaturated on purpose.
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Here's our source.
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Here's our despilled image.
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If you look really closely, you can see that up here is green,
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and down here there's no green.
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And we can switch back and forth between the two very
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quickly and it might become more apparent.
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So here is the original and the despilled.
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Watch very carefully down here on his arm.
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So again we have the green, and no green.
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So keyers do two things, or can do two things,
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that is that it pulls a key or an alpha channel,
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and it can also despill the image.
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Now not all keyers are alike, and not all keyers can do that.
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But we will take a look at that in a little bit more depth.
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Some other things to take a look at of course is,
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let's go back to our hard matte,
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and we can see that it's not 100% black and white.
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In fact, we have some unwanted elements over here in the alpha channel.
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And we're going to take what's called a garbage matte
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and we're going to remove them.
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Now the definition of a garbage matte is a hand-drawn,
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quickly made matte that is used to exclude parts of an
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image that is made by another process, such as bluescreen or greenscreen keyers.
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So here we're going to take it and remove it.
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It's created very quickly, does not have to be accurate,
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and it is removed in our RGB image.
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So now we have a pretty good key to work with overall.
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Let's move on to the premultiplication.
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We have our premultiplied image, which we have already created previously.
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And essentially what we've done is we have removed it from our greenscreens.
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So here we have a greenscreen.
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It has been removed,
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and now we can take that element and place it onto a new background.
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So let's take a look.
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Here is our greenscreen.
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Course our element has been separated,
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so he no longer belongs to that greenscreen.
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And I can place it on a new background.
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I can move him around, in this case with a transform tool.
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And now we have two separated elements.
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Next, we're going to take a look at premultiplication.
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Premultiplication is how we cut out our images,
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and how we apply the alpha channel to that RGB image.
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So let's take a look at that math.
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Here we have a 100% red constant that we have.
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You can't really tell where I sampled it because the box is red,
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but that's okay.
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We have an alpha channel here.
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So if we take the math of the alpha channel,
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which is 100% times the red channel, which is also 100%,
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one times one means 100% opacity in our red channel.
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So our element is 100% visible in our red channel.
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Let's take a look at another example.
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Here is our green constant.
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It is 50% green, and we have 100% in our alpha channel.
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We premultiply and our green channel remains at 50%.
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So 0.
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5 times one equals 50% of opacity in our green channel.
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One last example, we have blue.
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Blue is 25%.
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And our alpha is also 25%, 0.
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25 times 0.
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25 is 0.
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0625 in our premultiplied image, in our blue channel.
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Now that we understand how premultiplication works,
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and how to extract a matte, let's look at the main keyers inside of Nuke.
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So here we go back to greenscreen image that we may have looked at earlier,
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and it is a fan.
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We're going to go look at three basic chroma keyer tools: That is the Primatte,
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the Keylight, and the hue keyer.
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Now the Primattes and the Keylight are very similar,
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and we're going to take a look at those now.
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So here I have previously already pulled a key with a Primatte.
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You can see that it pulls a pretty good alpha channel to begin with,
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without a lot of revisions.
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And we're going to compare A against B.
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So here we can see that here's our original image,
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it has not only cut it out, but it has also despilled the image.
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So no more green.
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Green is most apparent down here at the bottom of the image.
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Keylight works very, very similar, but a slightly different result.
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So here is our Keylight,
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we pull a key and we shuffle out the alpha just take a look at it.
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So again, here is before, after.
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Primatte versus Keylight.
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And let's take a look at those RGBs, so slightly different in the despill.
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Looks like the Primatte was a little bit more aggressive with that despill.
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But both tools are very much usable.
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And we'll take a look at which one works best in different
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situations in later modules and clips.
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Next, let's take a look at the hue keyer.
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So here's the hue keyer, again, it will pull keys based on this backing color.
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So here we pull a hue keyer,
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but you will notice that it does not premultiply the image by default.
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Nowhere does it despill the image.
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So here is our alpha from our hue keyer,
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and again, it does a pretty good job,
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but not as good of a job as these initial two keys.
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So these are the two keys that I will tend to stick to.
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However this has it's time and place to be used as well.
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So now that we have gone through the keyers and the basics of keying,
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let's look through a couple of other examples.
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So again, we're going to stick with the same image.
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We've already done chroma keying.
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Let's look at a couple of other ones.
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We'll extract a matte based on the light and the dark values in the image.
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For the most part, this fan is darker than the background,
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and the background is lighter.
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But let's take a look at what it does.
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So by default we can see that it pulls an inverse matte,
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so let's just invert that for us, and then pulls a key.
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Again, it's not going to premultiply the image unless we force it to.
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It doesn't do a despill.
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It doesn't premultiply the image.
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It has to be forced a little more, so it is usable.
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It's mostly good for pulling stuff like highlights,
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sometimes reflections and so forth.
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But generally speaking, again, that Keylight and the Primatte are best for RGB.
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So it takes the image and it takes the luminance.
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The luminance are again, the dark versus light values.
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You can see the luminance down here.
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To sample it and it will extract a matte based on that.
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Our last example is the difference keyers.
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Now there are two basic types of difference keyers that we will look at here.
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The first one is the ultimatte.
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Now the ultimatte takes a clean plate.
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And a clean plate is essentially an image that has no foreground element.
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So it has been extracted, it has been removed from the backing color.
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So here is just a sampled constant.
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We have sampled the basic color of that backing color.
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And we have done the difference.
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So what is the difference between this image, and this image?
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The difference is the alpha.
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So let's take a look at that.
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Here's before, here is after.
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Before, after.
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And it also despills the RGB image.
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The last one which you will see often in these clips
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and modules as well is the IBK keyer.
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The IBK is a two-part keyer.
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It creates its own clean plates based on the IBK color,
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and then uses the IBK gizmo to extract the difference keyer.
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It also does a despill.
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So there you go.
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There's the basics of keying for today's lesson.
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And let's just do a quick recap summary of what we have learned.
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So first we reviewed the fundamentals of chroma keying,
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and premultiplication, how do we extract that image,
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how do we move it around, what is a hard matte,
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what is a soft matte.
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Next, we looked at the differences between chroma keying,
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difference keying and luma keying.
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