Take a look at FilmFX64 (http://www.satoripaint.com). It's object oriented and resolution independant. It offers 64bit color, rotoscoping and luma/chroma keying as well.
Basically, you are able to edit/delete/move to a different layer/change z-order of every object from paint strokes to filters at any time without disturbing the rest of the image. It isn't a replacement for Photoshop per se. It lacks many features that Photoshop has. But then the reverse is true as well. It does beat Photoshop out in the brush department and is exceptionally good with large images (I can paint in real-time on images that would bring Photoshop to it's knees). If you are looking for an app for painting textures, masking, rotoscoping, matte painting etc... Satori is great. And it's far cheaper than it used to be. They used to charge in the range of $1200 for it. I think it's $100 right now. Don't let the price fool you. It is very powerful.
V4 is due out in the new year and should bring some significant improvements based on what I have been told.
I have a website devoted to the program:
http://satoricentral.pair.com/ (Don't mind the lame design. It was the first site I designed a couple of years ago and I have never bothered to redesign it)
SatoriGFX.