Build it and they will come

At last. Final Cut Pro 7 is now happily installed and earning money.

But oddly enough, I started a new project this week and, entirely unbidden, we’re suddenly deep into the world of Alpha Transitions: a little animation or visual element that swoops over the screen, hiding the outgoing clip and revealing a new clip.

Although its presented as a new feature in FCP7, it’s not exactly that new to users of Final Cut Studio, as Alpha Transitions were used in DVD Studio Pro 3, and they’re a sort of ‘apprentice piece’ technique in most FCP editors’ education when learning how to use travelling (moving) mattes.

What FCP7 has done is to make the whole process easier by providing a simple interface: here’s the transition, there’s your animated video element, and here’s a separate matte that defines where to wipe in the new video (hidden under the element above).

Even so, it’s not the first Alpha Transition plug-in. I’ve owned the SupaWipe plug-in published by Idustrial Revolution for some time now, and as usual for me, I used it once and it has since languished in my Effects folder.

So, fast forward to today, getting stuck into a new project, and Russell is hard at work in After Effects cooking up an Alpha Transition or two to help the programme get between historic and cultural references to ‘right here, right now’ voxpops.

Okay, so there’s 720 MB of  Apple-supplied transitions available, which boil down to a dozen or so examples – a couple of which are very useable! But one should really ‘roll your own’. I’ve had a quick browse of the online tutorials (including a great set from Ripple Training) and I’m sure new ones will crop up that are FCP specific.

But when planning an alpha (or ‘object’) transition there’s one major step that I haven’t seen made obvious enough, and that’s the transition matte or ‘Wipe Matte’.

For an alpha transition to work, think about it like this: it’s a standard wipe where you hide the edges behind a moving object. At some point, the object has to extend over the entire width or height of the screen. This may sound blindingly obvious, but in the many examples I’ve seen, it hasn’t been mentioned. So a pirouetting dancer is scaled to fit so her head touches the top and feet touch the bottom of the screen. The Apple ‘leaves’ transition ensure that at one key point, the leaves overlap the width of the screen. A seagull swooping over the screen has its wings extend off the screen edges. And so on. So not every element is ready for Alpha Transitions.

How do you start making one?

Motion is excellent for this. Having isolated your video element (through rotoscoping, chromakey, or more commonly using a rendered object’s alpha channel), create a new layer above it to generate the Wipe channel. Keyframe the points of a black bezier shape over your video element ensuring that it’s a full edge to edge wipe against a white background. Export this as a separate movie and drop into your Wipe matte channel.
Ripple Training have an excellent series of tutorials on using Alpha Transitions.

Should I get the time, I’ll make a ‘how to make your own’ movie too.

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