one of the huge reasons that the mattes and cg and real life characters all looked so damn good in the movie was from the digital grade process no matter how hard you try theres virtualy no way to perfectly match the colors tones and saturations when combineng the three but when you use digital grade you take all three and apply the same tones values etc so you get the perfect blend as if you lite them all in the same render machine really cool stuff as far as mattes go teyon hit on what you said and dusso actauly uses some 3 d copenets in his pics as well alot of time the matte is only on the screen for a inbetween shot where it pops up on the screen for about 3 seconds so no animation is needed just a dust layer etc maybe some birds in a layer over top etc some times 3d is used to lay down the foundation for the matte as well or the matte is used to cover just a portion of the background