@vozz That one suggest to render with background, which I don't want because I want to decrease render time. I'm sorta basically asking the same thing the original thread owner made, but none of what has been posted worked/made sense.
I'd like to bump this because I seriously could not get this to work. What I did so far is create a plane set it to composition in background. Enabled multi pass for shadow. I want to keep the shadows but the floor isn't being removed for the main render. I might also be doing things wrong in after effects. could someone help me layout the basic layers and this whole inverse luma matte thing? My project file : http://www.sendspace.com/file/2u59tz