Rotoscoping In After Effects
Rotoscoping is a visual effects technique that involves separating a subject from its background in a video clip, so that a different background can be used in its place. It's very similar in concept to green screen chroma key effects, but the method for obtaining it is very different. A chroma key effect involves shooting the subject in front of a solid color background, and then that color is made transparent in post production so that another background can been seen under it.
Rotoscoping involves literally cutting the subject out of the video frame. It was a very tedious process that had to be done frame by frame, until recently when Adobe After Effects CS5 was released.
Rotoscoping before CS5
After Effects CS5 was released in 2010 and makes rotoscoping very easy thanks to a new tool called the 'Rotobrush'. If you're working with an earlier version of the program, then you're going to have to manually perform your rotoscoping. You do this with the 'Pen Tool'. Drag the marker in the timeline to the first frame of your video. Click on the layer you are going to work with, and then click on the 'Pen Tool' in the tool bar. Click and drag the 'Pen Tool' around your subject to cut it out. You want to shrink and feather the mask to make sure that you didn't cut out any of the background with the subject.
Now here comes the unfortunate part. You have to go the next frame and make adjustments to it, because you're subject has moved and you need to compensate for this. You need to do this for every frame, which can be very time consuming. But, in the end, you get the effect you wanted.
Rotoscoping with CS5
Part of what makes Adobe such an innovative company is that they are very in tune with how their customers use their products, and they are always striving to make their experiences better. For CS5, they invented a new tool called the 'Rotobrush'. The 'Rotobrush' offers users the ability to just cut their subject out just once at the first frame. The program will then automatically do the rest of the work based on the shapes and colors in the frame. It's convenient and time saving, but it's not perfect. Keep a close eye on the screen when it is displaying and performing this function. You may need to pause it every once and awhile to make a tweak.
Once this is done, you may need to perform some blending to the image to smooth down any harsh lines around the subject.