Using Unmult on Whites

For years and years Unmult has been one of the best free (and essential) plug-ins out there. If you don’t already use it, Unmult (though certainly offering a more technical role… but I’ll leave that to Stu and Mark) is often used to simply “knock out the black” from a layer in After Effects. For example, say I have a sexy lens-flare that I want to have alpha everywhere the flare is not… sure, I could put the layer in “screen” or “add” mode, but I don’t want to fuss with transfer modes and how it will affect my composite. So in steps Unmult - apply to your layer, and BAM, alpha everywhere you want and not where you don’t.

Ok great, so that works fine for black backgrounds. WTF, the headline says unmult on whites - what are you on about? What if I have a layer with a white background that I want to knock out?

The trick comes from a combination of Unmult, and our old friend Invert.

So you’ve got an image. For the example I will use an image from the free stock image site stock.xchng:

You want to kill all the white out of the background so you can animate this baby all over the place while still seeing through the lens and keeping the shadows, etc. Photoshop? I don’t think so. Good luck pen-tooling those soft shadows.

Let’s Bring this in to After Effects and get started - import your image and place it on the timeline.. Make sure you have Unmult installed:

Download Unmult here

Magic Step #1:

Select your layer and apply the Invert effect (Effect->Channel->Invert):

If it isn’t obvious enough, we just made our whites black and our blacks white. A great first step.

Magic Step #2:

Now we get to use our Unmult. It most likely installed to (Effect->Knoll->Unmult):

Woah, pretty gangster. And look at all those nice alpha checkers. The final step is pretty logical, and really finishes the trick off.

Magic Step #3:

Apply Invert again… BAM!

Absolute beauty. Look at those sexy soft shadows.
Now you can either re-render this as a still and use it everywhere on it’s own, or you can just pre-compose it and use that inside your project. Either way this technique is super fast and really easy. Save it out as an Effect Favorite and you’ve got an instant classic.

Cheers to the wicked-smaht Efrain Montanez at King & Country for enlightening me to this technique.

Enjoy!