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We've imported an image sequence and we've changed the frame rate of that sequence to be slower, five frames per second, than the frame rate of the composition we put it into, which is at 29.97 frames per second. So how does After Effects make up the difference between those two frame rates? Well, it's simple. It just has to repeat or skip frames of the source in order to make it fit the frame rate of the composition. As I press Page Down and step one frame at a time through the composition, you'll see the frames of the source are being repeated.
That's because it's playing back at a slower speed than our comp is being sampled. As I do this you will even see we only get these green RAM preview cache bars where they're unique frames of source material to play. I am going to delete or fade up that we played around with earlier, stretch out the work area to take up more of the resource, and press 0 on the numeric keypad to RAM preview this. You see the result is fairly jerky motion rather than smooth motion. Well, After Effects gives us a couple tools to smooth this out and they're underneath the guise of Frame Blending.
Frame Blending is similar to Motion Blur and that you need to enable it in two places: for the layer and for the composition. For the layer, this little icon that looks like a couple of film frames on top of each other is the column for the Frame Blending switch. As you click once in the Frame Blending box for a given footage item, this means you will be in Frame Mix mode, and I will explain that later. Click again, you will be what's know as Pixel Motion mode, and I'll explain that as well. And click a third time, you're back to Frame Blending being turned off.
I'm going to go into Frame Mix mode for now. Turning on this switch only means that this layer will be Frame Blended when you do the final render. You still will not see frame blending in the Comp panel. The reason is Frame Blending can be somewhat computationally intensive. So to make your previews faster After Effects defaults to not displaying it in the Comp panel. To turn on the display in the Comp panel, you need to turn on this large frame blending switch for the composition.
This is how you preview Frame Blending for any layers that have it enabled. Now you'll see as I press Page Down and step through this footage that we're seeing a mix of frames before and after the most recent frame. This is the Frame Mix mode, which is automatically doing a crossfade between adjacent frames. That's what looks likes a frame at a time. Now I'll press 0 to RAM preview. It takes a moment to calculate, and you'll see this is a smoother motion that we had before.
It's not quite as jerky. Frame Mix mode works great on amorphous objects like clouds, out-of-focus backgrounds, etcetera, but it can be a bit obvious when you sharp edges like these legs. Well, what does the other mode, Pixel Motion, look like? Well, how it looks depends entirely on te source footage. To enable Pixel Motion I'll click one more time underneath the Frame Blending switch column and get this solid bar, which means Pixel Motion. What Pixel Motion tries to do is interpolate every pixel.
It says where was that pixel in the previous frame, where is that pixel in next frame, and try to create a brand-new intermediate positions, tracking the motion of each of those pixels. Sometimes this works, sometimes it doesn't. In the case of this particular shot where there is not a lot of visual information, you'll see it doesn't work very well as it tries to match up these grids and tries to match up the legs and things that. So this is a case where it's not successful. However, there are other types of footage where it works very well. I'm going to down to our Sources folder, select the Jet Landing footage, drag it onto this New Comp icon and I'm going to slow it down, basically give it a slower frame rate.
I'll open up the Stretch column, slow down to say 500%. Go somewhere later in time where we see more the plane. Now as I press Page Down, in the default mode you'll see again that many of the frames are being repeated. If I turn on Frame Blending, Frame Mix mode for layer and enable it for a composition, you'll see we have some ghosting or echoing going on in the details as we get these intermediate frames. So I will press Page Down again to step through this.
A little bit smoother, but you do see the sort of ghosting motion as the wheels get fat and thin again. Not ideal. So instead I'll click this one more time to go into Pixel Motion mode. Now you'll see the tires have gone skinny again. As I press Page Down, you'll see this actually re-creating the position of the wings of the airplanes, the tires, and everything as if those frames were in the original footage. Pretty darn good, I think. So I continue step through it. Pretty nice, but again it doesn't work well with all footage.
I'll select Jet Landing. Pick another piece of footage, like I will this Musical Instruments. I am going to press Command and Option on Mac, Ctrl and Alt on Windows then the forward slash key to replace that clip, and I am going to go a little bit later in time to where that stick is coming down. Just hit this cymbal and you'll see very strange things happen to the symbol surface. There is not enough information for Pixel Motion to guess what the intermediate frames should look like, because things are just moving too fast. This is a clip where I'd be better off going back to Frame Mix mode.
I'll back up to 16 seconds again, step through this, and at least you get a more gentle fading as opposed to strange warping of the cymbal. So in general Frame Blending is a great tool to use when you've got slowed down or even sped up footage in a Comp, but don't just blindly turn it on. Try the different modes, Frame Mix or Pixel Motion, and see which one works better for that particular piece of source footage.
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