What does wipe style 'alpha' do in the color page?

Hi all,
I'm digging in the features I've never used. Impossible to find the answer in the manual.
Anybody?

Seems to show the other image through the alpha of the first, rather than side by side.

Seems to show the other image through the alpha of the first, rather than side by side.

Yes, I wonder what is the logic behind this. Is it usefull in any way?

It never has been for me.

Seems to show the other image through the alpha of the first, rather than side by side.

Yes, I wonder what is the logic behind this. Is it usefull in any way?

TLDR: It could be useful for those keying in Color, or for someone who wants to essentially create their own wipe. You can create your own wipe using Power Windows or Qualifiers, getting effects that the other wipes can't do. Probably not a huge win for most, but it's probably what BMD is using under-the-hood to implement the other wipes, so it's essentially free for them to expose to users.

I've played with it and I think that the Alpha Wipe is actually the basic wipe that underlies all of the others. For example, the Box wipe creates a rectangular alpha channel with hard edges that's centered in the window and you can scale it interactively.

You can do a more flexible version by: 1) add a Correction node to the end of your tree, just before the output, 2) create an alpha output for your clip and connect the alpha of this last node to it, and 3) create a rectangular Power Window in that node. Now bring up whatever still/clip you want to wipe with as you would normally and then select Alpha Wipe. You can take your Power Window and resize it, move it, rotate it, and soft-edge it. If you want total control over wiping, this is it.

I imagine they're taking their basic FX Transisions and using them to power an alpha channel and applying that to an internal Alpha Wipe mode and that's what is creating all of the other wipes. So it would be trivial to create a swirl wipe or a page-turn wipe or whatever. Since it's already all coded, it's basically adding a button to let users use it directly via Qualifiers and Power Windows.

So it could theoretically be helpful for someone keying in Color, to put their keyed clip over arbitrary other clips for checking. Or for someone who really, really wants a circular wipe. Or a qualifier-based wipe which need not be geometrical at all.