One way to control which pixels in an image will be transparent or opaque is to use the contents of another clip as a reference.
The Track Matte Key effect can use an Alpha channel or the luminance of pixels in one clip to set the level of opacity for pixels in another clip.
Let's see how this works.
I'm using the Effects workspace.
I'm working on an opening title sequence and in the Timeline, you can see my sequence is set up for compositing with clips trimmed and stacked neatly on top of each other.
Starting top down in the first stack I've got a title on V5.
This title is not needed for the Track Matte Key I'm about to create, but I want to design and adjust the effects settings around the title placement, so I've locked that track.
Below this is a group of clips stacked on video tracks V3, V2 and V1 and I'll use these 3 clips to create a Track Matte Key effect.
So, let's examine each of these clips.
There's a reason I work top down.
The clip on V3 is what I like to call the cookie cutter.
I'll double click the clip on V3 to load it into the Source Monitor.
You can see it's an animating black and white graphic.
Now I'll double click the clip on V2.
This sweet unicorn is the cookie dough.
Finally, I'll double click the clip on V1.
When working with Track Mattes the bottom clip is the background shot, so I think of this clip as the cutting board.
From the Effects panel under Video Effects, Keying I'll drag the Track Matte Key and I'll drop it onto the cookie dough clip the clip on V2.
With the clip on V2 selected, I'll go to the Effect Controls panel and setup the Matte.
The cookie dough clip on V2 is affected by the cookie cutter on V3.
So, from the Matte drop down menu I'll choose Video 3 since that's the cookie cutter clip, I want to apply to the cookie dough.
Because this clip is an opaque black and white graphic, I'll choose to composite using Matte Luma.
If I was using a title or a graphic with an alpha channel as the cookie cutter, then I choose Alpha.
But in this case Luma will make the white pixels transparent.
The white areas from the clip on V3 reveal the clip on V2.
So, now we see the cookie.
The black areas from the clip on V3 reveal the clip on V1.
So, the clip on V1 becomes the background for this new composite image.
In my analogy I've taken away the excess dough from around the cookie and can now see the cutting board.
Let's reposition this Track Matte.
I'll position the playhead so I can see the title fading on.
I'll make sure the clip on V3 is selected.
In the Effect Controls panel, I'll select the word Motion.
I'll adjust the size of the circle by typing 120 for the Scale value.
In the Program Monitor I'll drag to reposition the Track Matte, so it circles her face making sure the entire circle stays within the video frame.
Now I'll select the clip on V2 - the cookie - and again select the word Motion in the Effect Controls panel.
In the Program Monitor I'll reposition the exploding bubble graphic, so it lines up to reveal the title.
This looks pretty good, but towards the end of the animation I wish I could see the girl's face a little longer.
Let's reposition the image showing through the bubble graphic.
After selecting the V3 clip in the Timeline, I'll go back to the Effect Controls panel and drag the playhead through the clip.
It works well until around 2 seconds in.
So, I'll stop the playhead here at 01:00:02:04.
I'll click the stopwatch next to Position to set a key frame.
I'll drag the playhead forward, another 2 seconds.
I'll select the word Motion , then reposition the bubble, also known as the cookie cutter in the Program Monitor.
When I let go a second key frame will be created and the Track Matte will animate when I play the sequence.
The Track Matte Key will use any clip on the designated track.
So, if the V2 clip has a Track Matte applied, and its Matte drop down menu is pointing to the clip on V3 a new black and white animation can be placed on V3 and it will instantly become the new cookie cutter clip.
Practice creating a few Track Matte Keys using some of your own footage.
