For the film, "Legally Blonde 2" we were tasked with creating the "Million Dog March".
We were provided with 6 passes of greenscreen footage. Problems with this included the fact that all of the actors at both sides of the frame eventually walked outside the bounds of the greenscreen. This resulted in extensive rotoscoping of every man, woman, child, and dog that crossed out of bounds. This was further compounded by the length of the shot being hundreds of frames long, and all 6 greenscreen crowds suffering from the same problem. This resulted in weeks of rotoscoping thousands of frames. Not only did the people at the edge of frame require rotoscoping, but every flag and sign that had any amount of greeen or yellow in it, also had to be rotoscoped.
Along with the six passes of marching picketers, we were given footage of one of the lead actors filmed at a sound-stage . Due to a scheduling problem, Luke Wilson was unable to be there on the day when the rest of the actors shot the scene. So he was shot separately using a stopwatch to match the pace set by his co-stars at the previous shoot . Further complicating matters was the fact that Luke had removed his shoes to walk on the greenscreen and was filmed in only his socks. His feet had to be rotoscoped and darkened to give the illusion of shoes for the final scene.
We were also provided footage of the National Mall to be used as the background for the scene. Production wanted the walkway to be flanked by trees on both sides of crowd and for the construction vehicles, and fencing to be removed. Trees were "borrowed" and re-arranged slightly from one side of the frame and transposed over to the other to create the illusion.