[Knoll Unmult After Effects Cs6 29
Eliora Shopbell
Third-party unmult plug-ins for After Effects are used to create a composite with an alpha channel of a layer based on luminance or brightness. They're often used instead of composite modes like Screen that don't create an alpha channel. Alpha channels were discussed in a companion post recently, Alpha channels: premultiplied vs straight.
You can also use Xmult from Fandev, which is also free, and 32-bit capable for Mac and Windows. It still works, and comes with a Blend with Original slider. Also, Video Copilot Color Vibrance, a free new plug-in, does the same and lets you adjust color and alpha at the same time. Here's video backgrounders on Red Giant Unmult and Video Copilot Color Vibrance.
Mark Christiansen has a good succinct advice and discussion of alpha channels in his book Adobe After Effects CC Visual Effects and Compositing Studio Techniques. An excerpt of an earlier edition is still available as Alpha Channels and Premultiplication, part of a chapter on Selections: The Key to Compositing in Adobe After Effects 7.0. It's updated and more readable in the current version of the book, now for Creative Cloud. That chapter is just one reason to have the book handy.
Another excellent resource is the Channel filter section of After Effects: Insight into Effects with Chris Meyer and Trish Meyer on Lynda.com. There you get an entire context of video tutorials for Calculations, Channel Combiner, transparency & mattes, Remove Color Matting, and more. There's even movies on how to use Channel Combiner (an 8-bpc effect) to process filters in YUV (and back after the effect), or convert Straight alpha to Premultiplied for filters that crunch some alpha channels. With both processes, Channel Combiner instances sandwich the process in question (to convert back to straight alpha, check the Invert box on the bottom duplicate).
Chris and Trish also have discussion in Creating Motion Graphics, and more directly at Artbeats in Managing Transparency Part 1 (simple techniques to get rid of ugly black fringes) and Managing Transparency Part 2 (dealing with backgrounds of different colors, plus edges. You'll get great bang for your buck by spending time with these two PDFs.
For a brief period in the CS5 era, Knoll Unmult was unavailable even on the Mac, and users gravitated to a 32-bpc Adobe solution, 'Alpha from Max Color', a Pixel Bender filter from Adobe. Pixel Bender shipped only with CS3-CS5. If you can find Pixel Benders and really want to use them now, Pixel Bender Accelerator from CROSSPHERE.Koizum runs PB filters natively using the GPU of your video card.
One recipe and preset you might try is Removing black or white from layer/footage, deMult/unMult at Motion Graphics Exchange. There you can download a free AE preset that does this, staying in 32-bpc color:
Todd Kopriva posted a similar approach from Steve Holmes that describes the use of the Remove Color Matting and Shift Channels effects to remove black backgrounds from stock footage of fire, smoke, rain, etc. In his handy summary of Holmes' 15 wide-ranging Artbeats podcast episodes, Todd explained Steve's approach:
There were several nice video tutorials in the recent post on alpha channels in AE. If you haven't tried it already and want to a good workout, check out Lens Flare in AE and Premultiplication Explained by Ben Rollason. It's pretty clear, slightly different, and pays off more for those who keep focus on the material as it advances.
Would it possibly go Application - After Effects - Plug-In (and then i could create the 7.0/MediaCore files? I'm not going to try it as i don't want to mess anything up, but that seems to be were my after effects plug-in's live....
sampleImage() is a powerful new addition to AE's expressions arsenal, and it opens the doors to a lot of new possibilities. One is idea is to use an "obscuration" layer to modulate the intensity of the light source on another layer. As a specific example, let's look at how we can use sampleImage() to cause a cloud layer to affect the intensity of a lens flare.
First we need to create a cloud layer. We can use AE's Fractal Noise effect and then apply the Knoll Unmult effect (free at Red Giant Software) to create an alpha channel (so any underlying layers will be visible). Then we can apply a lens flare to a layer below the cloud layer. Then we can add a sampleImage() expression to the lens flare's Flare Brightness parameter. As a sample point, we'll use the lens flare's Flare Center parameter (but applied to the cloud layer). We'll specify a 7 pixel by 7 pixel sample area (which seems to strike a good balance between a representative sample size and render time).
Note that sampleImage() will take up to four arguments, but in this case we only need to specify the first two because the default values for the third and fourth arguments are already what we need. If we needed the sampling to occur before masks or effects are applied, we could set the third argument to false, but since we need the sampling to occur after the effects (because the clouds and alpha channel are generated by effects), we'll just accept the default value of true.
Once we've retrieved the sample data from the cloud layer, we can isolate the alpha value from the returned [r,g,b,a] array by using an index of three. Since the color and alpha values returned are numbers between 0.0 and 1.0, we'll use our old friend linear() to translate the alpha value to an appropriate corresponding value of Flare Brightness.
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