Learn about time warping with optical flow.
- [Instructor] Alright we've come back to our lovely ocean to demonstrate another type of warping, which is time warping. Time warping allows you to change the speed inherent in the piece of footage. You can make something look faster or slower, and also therefor, affect the duration, of the piece of footage in terms of how long it lasts in the time-line. Let's give that a try, this project is saved as 3_3. I'll go ahead and select the ocean layer, and I will go to Effect, and instead of going to Warp or I have Image Warping, I'll go to Time, where I have Optical Flow.
Optical Flow is probably the most advanced time-warping technique. Now before I apply it, or adjust it, I'm going to play it back and just see what it looks like at normal speed, so I can gauge it as a benchmark. So there's the surf, medium speed, not too fast, not too slow. Alright, there's two directions I can go with this, I can make it faster or slower. First thing I'll do though is make sure that my Input Frame Rate, is the same as my Footage Frame Rate.
So I need 24 here. In order to change the speed, I change the velocity property. If I go above 100 it's faster, if I go beloW 100 it's slower. Let's go faster first, it's easier for the effect. So now this will be twice as fast at 200, and half the duration. In fact, once it gets to the halfway mark, it disappears because it's run out of frames. Speeding it up essentially throws away every other frame in this situation. Now if you didn't want the black, you could just change the composition length.
Like 48 frames. And now it's sped up and it'll have missing footage. Alright, that's pretty easy for Optical Flow, let's make it more difficult. Now in anticipation of slowing this down, I'm going to go back to Composition Setting, and make my timeline twice as long, so that's 192 frames. Twice as long as it was originally. Of course it runs out.
Now I'm going to warp it, by going below 100. Let's try 50, twice as slow, twice the original length. Now, it doesn't actually change the duration of the footage bar. This is a limitation of this particular effect, but there's ways to offset the time. So let me go ahead and return the Composition Setting to, 96 duration, so we don't have that empty space at the end. And then we can go ahead and play it back, so now it's half the speed of the original.
Let's zoom in and see what the quality is like. It's slower, it looks pretty nice, difficult to tell that it's warping it, considering it has to make brand new frames that weren't there, it's doing a pretty good job. Now if you really want to push your luck, and make it difficult for the effect, you can slow it way down. For example 10, will make it 10% of the original speed.
It's still doing a pretty good job. The surf is definitely slow, there's a little bit of strobing down here at this bush, but not bad. Now before we get to that in terms of how to improve it, let's talk about the offset. Because right now, I've slowed down the footage, and it's going off the end of the timeline but I can't get to it. That's why they include a Start/Offset Frame. What I can do is move the footage so I start later into it and I lose less at the end. For example, if I enter 10 into this property, it jumps forward so it starts at frame 10 and I'm using more of the center of the footage right now.
Still the same speed, it's just using a different section. Here it is, nice and slow. Now in terms of quality, there are some things you can try. There's a Blending mode, which is the math it's using. By default it's using Bi-Directional Mix, which means it goes to a future frame and a past frame, and tries to use those to generate a new in between frame. There's also Nearest Mix, that goes for the nearest existing frame to try to create a new frame.
And depending on your footage, it might be better or worse, you just have to experiment and see what looks good. In this case, Nearest Mix doesn't work quite as well, there's going to be some popping right here, so not quite so good. Another thing you can do is to improve the Optical Flow Resolution, this is the density of a grid it lays on top of the image to track patterns, so the higher the resolution, the more motion vectors it has to work with and the more accurate. So, you start on Medium, but there's also High and Best.
Best will take longer, but you should get more accurate results. So I don't think it's going to help Nearest Mix that much, but I can go back to Bi-Directional. So Bi-Directional with Best, is about as good as I can get. So it looks pretty nice, very few artifacts. 10% of the original speed, and there we go. So again, even though there are effects and work flows built into the after effects, that allow you to time warp, like through time stretch or through frame blending.
It's nice to have a standalone effect, like this Optical Flow, that gives you access to the most advanced features of that technique.
VFX expert Lee Lanier begins by exploring shared Boris Continuum controls, and then shows how to apply stylistic effects. He explains how to work with the PixelChooser and Boris Lights, and discusses how to color grade and warp footage. He also takes you through using the Boris Chroma Key Studio and working with Mocha Pro for motion tracking, as well as how to add particles and work with 3D text.
- Overview of Boris Continuum
- After Effects preferences
- Applying stylistic effects
- Relighting with Boris Lights
- Adjusting colors
- Changing the time of day
- Warping footage
- Keying green screen
- Motion tracking
- Adding particles
- Working with 3D text