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Bifurcation means to split into two. The idea here is to plant keyframes far apart, then go halfway between them to add a new keyframe, then add another one halfway between the first one and the middle keyframes and so on until the roto is locked to target. I am using the Interlaced Video clip from Lesson_01_Media and I've already set my deinterlacing so when you load the clip make sure that you set the Interlaced for Lower field first. Go back to the clip and move to frame 1.
If you would like to follow along, you can use the bifurcation.mocha script in the Project Media folder. I've already divided my shark into three separate shapes. I am going to turn off the lower shape and the fin and concentrate on demonstrating the bifurcation keyframing using just the upper frame only. So the idea is -- let me zoom in a little bit here, I want to turn off the zoom windows, get a little closer, okay.
Okay, like we said at the beginning bifurcation is to plant keyframes at the ends and then subdivide, subdivide, subdivide. I am going to select all of the control points on the spline in order to show the keyframe here at frame 1. I'll jump to the last frame in the clip and I am going to pick up the entire spline, reposition it and move these keyframes over here.
So I am establishing the last keyframe at the end of the clip. Okay and I'll move this here and tighten up that there. This point wants to be right here on the corner of the mouth. Remember, our temporal coherency, the control point must stay in the same position identified with the same piece of body over the length of the shot. Okay, I've got keyframes at the beginning and the end, bifurcation is to cut them in half.
So I'll go halfway between beginning of the end like say here, reposition my shape and the idea is I'm actually reusing the work that I invested in the first and last frames. Because the motion is gradual, I only have to move things a little bit, so I am really close to my target and again the whole concept is to be reusing the work on that first and last frame.
Okay, I've a keyframe in the middle, the beginning and the end, so I'll go halfway between the beginning and the middle let's say, big on my shape, reposition it, touch up any points that needed. Okay and then I'll go down here halfway between the middle and the end, reposition my shape, under there a little rotation, okay, touch up my control points here, okay so I now have split the middle and end in half.
Now I could go here halfway between the middle and the one quarter point and if I need to, looks like I need to, reposition this frame like so. Okay there we go and then I could go halfway between these two frames. Actually, that looks okay. Halfway here, that looks okay, halfway there, that looks okay. All right! So, let's call that good.
So the entire idea of this process is because the motion is gradual, we can put in a keyframe at the beginning and the end and then cut that in half and then halfway between those and then halfway between those and so on and so forth, constantly reutilizing the previous work we've already done. I'll turn on the Mattes, then deselect this spline to turn the Matte green. Turn off the spline and preview the animation.
Okay, we'll stop that. Bifurcation keyframing is best used for steady, regular motions like this short clip where the shape outlines don't change too radically.
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