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The Art of Rotoscoping with Multiple Splines
Weve all had one. The blue screen shot from hell that wasnt lit right, or being asked to perform a miracle and composite something with no key whatsoever. In this tip Ill explain how using multiple splines can give a smoother result and save extra work in the long run.
The theory is this. If we have one spline to roto a person walking, we end up with trying to match the movement of different parts of the body all at once. Not only is this difficult at the best of times, its also a pain to go back and change. And unless you are a roto god, you will have to go back and change something. The other problem this tends to create, is a boiling edge, which is generally caused by too many key frames which results in little tweening taking place. By using multiple overlapping splines, we can key frame just one part of the body, and minimize the impact of extra key frames on the rest of the subject. By doing this, we achieve a smoother roto job, and make it much easier to tweak afterward.
One of the things I like to do to, to make my life easier, is track a moving subject. Ideally we want to track something close to the center of gravity, like a belt buckle, but if there isnt one try using the head. Arms and Legs move too much to be of any use. (If all else fails, and there is nothing suitable to track, create a path.) The advantage to this, is I can attach the polygons to the tracker, and get a quick and dirty roto. This also minimizes the amount of movement to the points during the key frame stage.
Sometimes I will grab a scrap sheet of paper and create a number of columns. Label them like so: L-Leg, R-Leg, torso, head etc. Now, concentrating on one body part at a time, I look for the start and stop of action and write down the frames at which they occur. By doing this, it allows me to enter precise key frames, which in turn allows Digital Fusion to interpolate (tween) the action. I try to never have more than 1 key every 2 frames (or fields).
Often I find that if I first rough out the shape, and then keyframe it, Im moving fewer points around the scene during the keyframe phase. Any points I add later during the refining stage, are added to all key frames, and just need to be moved a little bit.
After the key frame phase, I go back and check the tweening by splitting time between each key frame. Often the motion is not linear, and so needs to be adjusted. Sometimes requiring an extra keyframe, sometimes just adjusting the curve.
The final step, is obviously just going back over your key frames, and tweaking until you get the desired result. Be sure to set your motion blur settings before you get too detailed, as they can impact the quality of the roto greatly.
Keep on Fusioning
Eyeon Software Inc.
Related Keywords:compositing, special effects, rotoscoping
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