Stock images often come on white backgrounds. While I need this for print, it got me thinking if there was any clever way to build an alpha channel in AE. I haven't found a really fool proof way other than "Blend if" sliders. But that requires that I have a background to composite onto. It doesn't actually generate an alpha.
I don't wanna touch luma key and edge feather/choke. It's really important that I keep the detail. Its like I need Keylight for White.
I have keyed dozens of those in PS by using first Quick Selection for the rough matte, then (don't panic) Color Range, which is completely improved in CS4 by using "clusters" - it not only takes into account similarity, but also proximity to build clusters of similar colors (in the past, clicking a color here selected all similar pixels everywhere). The way I use all this is with two layers - one for the hard matte from Quick Select, then just the edges from Color Range. All with non-destructive layer masks, to retouch if needed.
In AE, Linear Color Key works very, very well for plain color backgrounds. But white is awful because highlights on the subject/object are keyed as well.
The Extract effect is as close as you can get to Keylight for luminance. But you're still going to have issues with highlights. You need something that doesn't just select based on luminance values, but also takes regions into account. That's what Quick Select/new Color Range in PS does for me in a couple of minutes.