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Next, we're going to discuss trimming layers, changing their internal In and Out points, which affects what portion of the original source material they are using in your composition. To practice along with me, have at least two movie files in a composition and drag them so that they overlap somewhat. You'll also notice that I have opened up the In, Out, Duration, and Stretch panels. There is a switch in the lower left corner of the Timeline panel which you can use to quickly open or close them. Also remember that you can right-click on any of these column headers and choose a specific item you'd like to see in the Timeline panel.
Trimming layers is very easy. Make sure that you have the Selection tool selected. Hover it over the start or end of any layer bar. You'll see this two-headed arrow and merely click and drag that portion of the layer bar. This is trimming what section of the layer you're using. Note that the Comp panel is not changing. I'm not sliding the layer relative to the composition. I am just trimming what portion of that layer source I am going to use in the composition. Of course, if I trim it past the Current Time Indicator, I'm going to alter what's visible in the Comp panel by revealing and obscuring different layers on the Timeline.
Obviously, you can only drag a layer out to the very end or very start of the source material. If you're up against the end, a little black triangle in the upper corner of that layer bar will indicate you are at the end or start of layer, and you can't go any further than that. You can also get the same result as dragging these In and Out points by scrubbing the In and Out values in the Timeline panel. You'll see again I am not moving the layer relative to the composition. I am trimming what portion of the layers are going to be used.
Same with the Out point. I can't go past the end source, but I can go before the end of the source like that. If you are using these columns, resist the urge to scrub the Duration value, because it will not retrim the layer. It will actually change the speed of the layer, play it back faster or slower. Most of the time you don't want to do that. So stay away from the Duration column. Notice there is a difference between grabbing and sliding a whole layer bar or just grabbing or sliding In and Out points. There is also difference between scrubbing the In and Out value in the Timeline panel and clicking and editing that value.
If you click and edit the value, it will move the layer rather than trim the layer. In other words, it will change the external In and Out points, not the internal In and Out points. The layers In and Out relative to its source. Here are some great shortcut keys to trim layers. Let's say you move your Current Time Indicator to exactly the time you'd like a layer to start, like maybe around this point right when the wheels touch the ground, right there. Have the layer selected. Hold down Option on Mac or Alt on Windows, then press the Left Bracket to trim the In point or-- I am going to undo here.
Hold down Option or Alt, and press the Right Bracket to trim the Out point. Notice is there a similar or different, so just pressing the left/right brackets to slide a layer. So, no modifier key. You're editing the external In and Out points. You are sliding the layer in time. I am going to undo here. Hold the Option key, move my Current Time Indicator and you are trimming the layers In and Out points. You're moving this internal In and Out points. This will quickly become intuitive, but if you're brand new to working with media in a program like After Effects, there is difference between your time in the overall composition versus your time inside a layer is a very important concept.
We'll be playing with that idea more in the next few movies. There is one other nice thing you can do with it that is related to trimming layers. Let's say that you want to split a layer in half. Let's say I want to split into two clips before and after this touchdown. Press Page Up to go one frame before, and Page Down to trim back to where I just see smoke. If I want to split this layer at this current time, I select it, then choose Edit > Split Layer. The shortcut is Command+Shift+D on Mac, Ctrl+Shift+D on Windows.
By doing so, I will now have two segments in the Timeline. Note again that they do not slide in position relative to the Timeline, and there will be no apparent difference when I just scrub the Current Time Indicator. But now I have two different layers that have been trimmed internally to use different segments of their source material. Once I have done that, I can go ahead and create some space, put another clip in between, and play around with my edit. Go to this clip. Then go back to the continuation of my earlier clip. Inside the clip, press I to go to its In point.
When you place the Current Time Indicator and split a clip, the frame that you see when you split will be the first frame of the later segment. You might also know that After Effects has sorted out these segments so that the later segment appears on top rather than underneath the earlier segment from the same clip. This is not about default, but if it is counter to the way you like to think, no worries. There is a preference. I go to After Effects > Preferences > General and look for Create Split Layers Above Original Layer. The default is on.
If you'd rather that split the segment, the later segment appear underneath the earlier segment, just turn that off. But again, I like the default. So I'll press Cancel. Trimming layers in the Timeline is easy and is one approach. But there may also be times when you'd prefer to trim layers in alternate panels and I'll go over that in the next two movies.
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