All language subtitles for 4. Keying for Motion Blur

In this clip we're going to talk about keying for Motion Blur.

So when we pull the keys for Motion Blur we treat the edges much like we treat

the hair wispies or fine details in other particular shots.

So we key on frames with the Motion Blur and try to pull the

key as close to the Motion Blur as possible.

That's going to be a very light key.

So let's take a look at script.

So before we begin,

you will notice that we do have the complete workflow from our previous clip

with all of the steps that we've created so far for this image,

so for the car sequence.

So now let's take a look at how we treat that edge detail on the Motion Blur.

So here we have pirate.

We have a little bit of Motion Blur that is on his sword,

on his head and so forth.

And we want to take that Keylight and we're going to sample it.

Now keep in mind, this image has not been preprocessed.

That would be in other modules and clips.

So take a look at that.

However,

we do just want to pull a rough key to get an idea of where

we're going to pull that key on the Motion Blur.

So again, zoom in, Control + Shift + Left Mouse Button to click and drag.

And again a close to that edge as possible,

which should yield a pretty good alpha.

So we want to do the core and fringe and so forth.

However, we're just looking at the edge details in this example.

So again, a little bit of a halo.

Some artifact is, well that will be rid in our preprocessing workflow.

However let's take a look at this when it's been revised.

So again we have this halo.

And we need to get rid of it to do our final composite.

So how can we get rid of that?

Well often when we pull the keys,

we create Mattes separately from the Despill and then the

Premultiplied image is left with a gray halo,

so such as we see in here.

Now the more transparent that the edge is,

such as with Motion Blur, the more apparent the gray halo is.

The gray halo is introduced from the neutralized or

Despilled backing region that is in your image.

So let's take a look one more time.

So here is a key.

There's our Despill and when we Premultiply it we get this halo along our edges,

particularly where the Motion Blur is.

So we have rid of it.

So now one way to rid of this is sometimes during the Despill process and we

will be discussing that in more detail in our next module.

However another great method is to do what some call the

edge smear or edge extend workflow.

Now this workflow is created using the strengths of the

Unpremultiply and Premultiply workflow.

Now if this method is new to you,

be sure to check out the Advanced Clean Plate Techniques in

NUKE and look for the clip that discusses Discovering

Premults versus Unpremult for Paint.

And that can be found on the Pluralsight website.

So now let's take a look at just a quick before and after

and we'll get into the details of how we can refine those

edges and the workflow for it.

So here's our switch.

So this is the Despilled image that does not have any edge treatments.

If I toggle to the other one, look what just happened.

So it disappeared on our edges.

Our alpha is the same as it was previously.

We've just pushed out those gray pixels.

So let's look at that one more time.

So there's before, that is after, so again we've replaced those pixels.

So what exactly are we doing?

Well we're taking the last blue pixel here and we are duplicating it

outwards so that we can re-Premultiply it after and basically have

blue in his edge and not the gray halo.

So we're pushing out the gray halo.

So let's take a look at how we can do that in our shot.

So here is our blue screen image.

It has been Despilled.

We have alpha from our previous clip and the first thing that

we want to do is we want to add a gray tool.

So make sure this gray tool is only set to the alpha.

And we're going to go look at our alpha and we want to rid of any gray values.

So anything in here and in here which represents transparency.

We want to create a Hard Matte.

Now there are other tools to do this,

but we're going to work with the Grade tool for this example.

So here's our Grade tool and you will dial back that blackpoint,

so somewhere around there, so that again it's mostly a Hard Matte.

And then we will Premultiply our image.

So notice how there's no more gray edges in here,

particularly on her shoulder and on part of the dash and so forth.

Now if I didn't hook that up, this is what it would look like before.

So it doesn't look so bad on a black backdrop,

however, other backdrops it might not hold up so well.

So let's look at our next step.

So we have our Premultiply image with our Hard Matte.

We're going to take a duplicate of the Hard Matte and do a slight erode,

blurring of the alpha, that's very, very small in there,

so that's our blur and invert our image.

So what's going to happen?

And we're going to take the blue value from here and smear it out.

However, it's very important not to change the RGB image from here inward.

So now what we're going to do is we have that Hard Matte,

we've inverted the Matte here and we're going to take the Hard Matte

image that has been Premultiplied and add a blur,

so a very, very small blur, in fact a blur of three.

Two or three should do the trick.

Now look what's happening here.

So here's my original image,

here's my blurred image and it's not changing in here.

That's very important.

If I didn't mask it out by the inverted Matte, look what happens.

It will actually change my image.

So there is before, there is after.

It is blurring it ever so slightly.

Just take that off, it's not quite working there.

So there's before, there's after and it's actually destroying my image.

So make sure the mask input is hooked up and then

continually start to stack those blurs.

So blur and blur and it's going to slowly push this color outwards.

Now when you Unpremutliply this image, it pushes out this color,

which is exactly what we want.

Now the larger the Motion Blur the more you want this color to be pushed out.

Next what are we going to do?

We're going to look at a blur that has not been stacked and

that's just going to be a blur of nine.

So this is three, three, and three, so nine in total,

versus a image where it's a single blur.

So this is the one where it is stacked and this is

the one where it has not stacked.

And it's hard to see here,

however if we take a look at over top of our head rest,

so this is before with the stacked.

Nice details,

it has a nice smooth transition in here versus nonstacked and

it looks just like a majorly blurred image.

So it will be a little bit harder to basically fool the audience that

this is a continuation of our seat to fix the edge.

So this is bad, stay away from it.

Better to do small increments if need be.

Now for some reasons,

this still including some of the edge detail that we don't want.

You can repeat this process all over again.

So again, take this again.

Oops, let's go over there.

So take this,

erode away or color correct that alpha so that it is eroded a little

bit more and then go back in and resmear it outwards.

So it can be duplicated multiple times just so that you

can get the details that you need.

So now what are we going to do?

We're going to go in and add a little bit more detail.

So this part is optional.

Doesn't do a whole lot for us in this particular example, but let's take a look.

So we take our Unpremultipled image and our image that was not

Premultiplied before and we're going to compare the two.

So we're going to compare the two.

So here's our divide.

Let's see what the difference is.

We're trying to add details in here.

And change the saturation so we're not adding any color back.

So that was before, this is after and multiply it.

So in some cases it will add nice detail back to that edge work.

So again, this is before, this is after, which may or may not be of use to us.

So a great little trick in here, works really well with wispy hair detail.

That may not make such a big difference with us in this example.

So take this final image,

copy over the final alpha that has all the nice gray values in it,

Premultiply and let's take a look at those edges.

So these are the edges that are looking pretty good.

It's hard to tell the difference,

so we're going to take a look at them over top of a new background.

So here's our new background.

We're going to toggle it back to the checkers.

So let's go back to the checker board.

So we'll at the color bars in this example and let's

take a look at what it's looking like.

So here we have the Premultiplied image without edge treatments.

Look at the black halo, the dark halo that's happening over here.

And we're going to switch it to with edge treatment.

Look at the difference it's making down here.

So again, without and with.

Now in some cases it's going to do more harm than good,

so feel free to isolate just that Motion Blur or wherever it's needed.

Now keep in mind that you may have to animate this mask on and off,

pull separate Motion Blur keys throughout the shot depending how

much information your initial key is picking up.

So no just one Fringe Matte and one Core Matte will generally do the job.

You have to pull multiple little Mattes in order to get those

fine details to take your final image to final.

So let's take a look at different backdrops again.

So here we are.

So this is a Premultiplied imaged with edge treatment over a colored bars.

This went up pretty good.

Little bit of Despill that we can fix and details we can

fix later in future modules and clips.

Here we have a gray backdrop, we have a checkerboard, and so forth.

So it definitely adds some of that detail in our edge work, it helps us.

You can see that in both of these examples,

but it may need to be masked off to just the areas that you need it.