Original subtitles

So far we've just been talking about how to get the

timing of the clip that we want.

We haven't been paying much attention to the quality of the results,

but now that we have the particular frames we want to work with we

can start looking at how they relate to each other.

If we are just moving a clip along the timeline or trimming the

length of it without changing the speed of it,

then we don't have to worry about any of this.

Those operations simply move the frames of the clip together,

they don't change their relationship to each other,

but the moment we start changing the speed of a clip we are

changing the relationship between the frames,

and this can lead to visual artifacts that need fixing.

If we speed up a shot we are removing frames,

and we're going to see stroppy motion caused by the lack of motion blur.

If we slow down a shot we are spreading the existing frames out

and need to create new frames to fill the gaps.

The first thing we're going to look at is motion blur.

Motion blur is found in most live action footage.

It is caused when objects move relative to the camera

while each frame is being capture,

and is related to the shutter speed of the camera.

Shutter speed and frame rate are closely related.

The frame rate defines how many frames of action we record every second.

The shutter speed defines how long each frame is exposed for.

In film cameras the shutter was a spinning disk that had an opening

that could be adjusted to change the exposure time.

If the disk was half way open it was called a 180 degree shutter.

This meant that the shutter speed was half of the frame rate.

Each frame would be exposed for half the amount of time

that the frame was being captured,

so at 24 frames a second each frame would be exposed for 1/48th of a second.

That's a pretty long time.

Anything moving in a frame can move a long way in 1/48th of a second,

and anything that moves while the shutter is open will

create a motion blur effect in the frame.

This 180 degree shutter became the standard for most films,

and as viewers we have become used to the look of this amount of motion blur.

If we shoot at higher frame rates we will have shorter shutter speeds.

The basketball at 96 frames per second will have a shutter

speed of nearly 1/200ths of a second,

which is going to have much less noticeable motion blur.

We can see that if we look on some of the frames

there's almost no motion blur at all.

I've rendered out two versions of this clip to QuickTime files.

The first one is the original clip, set to play at 96 frames a second.

The second is the clip sped up by 4 and rendered at 24 frames a second.

If I load these two clips into Nuke and view them in the Dope Sheet

you can see that the 96 frame per second clip has 4 times as many

frames as the 24 frame a second clip.

Nuke ignores the frame rate set in the QuickTime file and just matches

the frames in the file to the frames in the timeline,

and plays back at the frame rate set in the project settings.

If we load these two clips into Premier,

however, you'll see that it deals with them differently.

Instead of frames Premier thinks in time,

and these two clips are basically the same length in time,

3.

5 seconds.

If we create a new sequence with the 96 frame a second clip and play it

you'll see that this clip doesn't play back in slow motion.

It plays back at normal speed with this very smooth look.

You might not see this effect fully because the screen

capture is only 30 frames a second,

but I'll put an example QuickTime in with the project files.

This smooth fluid look is created by very high frame

rate with very little motion blur.

This is closer to how we perceive real life,

but it's very different to the images we've gotten used to seeing in movies.

There was a strong negative reaction to this effect by many viewers when

Peter Jackson shot the Hobbit movies at 48 frames a second.

If we drop the 24 fps clip into the track above we see

that it plays back at the same speed, but it's much less smooth.

When we display the 2 clips side by side you can see that

for every 1 frame in the 24 frames a second clip there were

4 frames in the 96 fps version.

We can use these extra frames from the 96 frames per second version to

recreate the missing motion blur when we speed up the shot.

So here we are back in Nuke.

I have a TimeWarp node set up the same as in the previous video,

and a Retime node with the speed set to four.

In the previous videos I was changing this filter

setting from the default of box to none.

If we set it back to box you can see that it is already

creating a rough motion blur effect.

This motion blur is created by blending together several of the source

frames to create a new frame with a sense of motion.

The shutter slider allows us to change how much motion blur we are creating.

The default of 0.

5 matches a 180 degree shutter in a camera.

If we increase the shutter value Nuke will sample more

frames to increase the amount of motion blur.

A vale of 1 is equivalent to a 360 degree shutter.

We can produce the same effect with the TimeWarp node,

but it's labeled slightly differently.

Here instead of shutter there is something called input length.

This tells the node how many frames to sample to create the motion blur.

If we start increasing the number you can see the amounts of motion

blur increases as it blends more frames together.

If we click the button labeled,

set df/dt Nuke will use the slope of the Retime curve on each

frame to calculate the correct amount of motion blur based on

the shutter angle we specify.

This means that if our retime varies throughout the shot the

number of frames being sampled will also change.

In this case, with a constant speed of four,

Nuke is going to sample two frames for each new frame.

If we compare this to the result of the Retime node with a shutter of 0.

5 you'll see that we have exactly the same result.

If we play back one of these nodes you'll see that the motion blur

definitely helps to create smoother playback,

but it still isn't great, and when we pause we see two very distinct images,

not the smooth motion blur that we would expect.

This is because this method of blending frames together is

quite a blunt way of creating the motion blur,

and is limited by the number of frames that we have to work with.

The node has no real knowledge of how these objects are moving in the frame.

Optical flow technology tracks the movement of pixels from frame to frame

to create motion vectors for each region of the image.

These vectors allow Nuke to know which areas or

objects are moving and which aren't.

With this information Nuke can create much more accurate speed effects,

including motion blur.

The two nodes in the time menu that use optical flow are OFlow and Kronos.

They produce very similar results and have very similar controls.

Kronos has few extra features,

and in the user guide the Foundry recommend using Kronos over OFlow,

but Kronos is only available with Nuke X,

so for now I'll show you OFlow.

If a node is selected when you load OFlow,

the input range will be automatically filled in based on the input,

but if you bring OFlow in without connecting it to another node you'll

have to click the reset button once you connect it.

If your frame range changes or you connect it to a different

input you might have to click the reset again.

OFlow and Kronos allow you to specify the retime

either as speed or source frame.

In this example let me use speed, and set it to four just like the Retime node.

Make sure that the method is set to Motion.

This enables optical flow filtering.

Further down there is a section of settings dedicated to motion blur.

Let's check the Automatic Shutter Time checkbox.

This automatically sets the shutter based on the speed changes.

We won't see anything until we increase the shutter samples.

The more samples we use the smoother the results,

but the slower the calculations.

Let's increase the samples until we don't see any

discreet lines in the motion blur.

A value of 10 seems good for this shot.

Now if we play the results we get a much smoother,

more cinematic motion in the shot.

Sometimes you might get given footage with incorrect motion blur.

Perhaps the shutter was set incorrectly on the camera,

perhaps it is a 3D render that didn't have motion blur switched on.

Optical flow can be used to add motion blur to a shot

without having to apply retime.

Let's load in the 24 frames a second QuickTime.

This is the original footage sped up without any motion blur.

If we connect a new OFlow to this Read node,

and set the speed to one, we won't affect the speed of the clip,

but if we increase the shutter samples and the shutter

time you can see the OFlow is analyzing the motion of the

clip and adding motion blur.

If you look closely,

you'll see that the results of this are not as good as when we did

the speed up in the OFlow node at the same time.

In this case, it only has access to the final result at the retime,

which is every fourth frame of the original.

We can tweak some of the motion vector controls to try and improve the result,

but we will look at that in another video.

There's actually a separate MotionBlur node in the filter menu.

This uses the same optical flow controls to generate motion blur,

but without the respeed controls.

We don't just have to create motion blur to make the footage look correct.

If we add an OFlow node to the original slow motion footage and turn

up the shutter samples and shutter time we can create a motion blur

where we wouldn't expect to see it.

This creates a kind of dreamy, unreal look to the shot.

So now we can speed up a shot and create good looking motion blur for it.

In the next video we will look at using optical flow to slow down footage.