The Center Of Gravity Must Stay Fairly Centered In 3d Animation

It is important to notice that even though a figure of a man jumping is squatting in a somewhat awkward stance, the center of gravity stays fairly centered over the feet. As part of this shifting of balance, the back takes on an arch, the chest leans forward of the center of gravity, and the buttocks lean back. Also notice that the head is tilted back so that the character is always sure to know where he is going. The next keyframe will be only 10 frames from where you are currently situated.

This is because the next motion is the liftoff, and thus is a quick motion of the arms swinging forward and upward and the legs uncoiling to release the energy stored up in the squatting action. At this point, the feet have not yet left the ground, but the body is at its full length pointing in the direction it is headed. There is actually quite a bit that happens at the ultimate crest of the hop. It is at this important moment that the body changes overall shape, and the speed and direction of the body in flight is altered. Since it is such a complex time in the hop process, it needs to be broken down into at least two keyframes.

The first of these three crest keyframes is the next keyframe. The keyframe that occurs at the beginning of the hop occurs within rapid succession of the keyframe before it. The up-ward travel of the jumping character is fast, accompanying the release of power in the legs. A new keyframe needs to be applied 8-10 frames after the keyframe defining the outstretching that defines the recoiling of the body as its momentum slows and it begins to prepare for the downward travel. At this point, the legs are beginning to coil beneath the body as they begin to swing around to catch the body as it falls.

To counterbalance this shift of lower weight, the arms bend and the back arches. The head remains upright to help in the balancing act. The pose at this point in the hop should appear something like. The second part of the hop crest should be about 12 frames from the first. A large amount of time actually transpires during the crest as the upward momentum gives up its strength and gravity kicks in. It is a good thing, as the body must make a lot of changes and get prepared for the downward flight.

The legs are swung around in anticipation of landing. The chest is forward to counterbalance this dramatic change in weight of the swinging legs. The arms are pointed down, helping in the balancing act, and the head is looking toward where it is going. The primary focus of the entire body from here on out is damage control as it comes down, making sure it does not hurt itself on impact.

Now that the shifting of weight has occurred and the body is resigned to the downward fall, the speed at which it is traveling dramatically increases. Gravity is a powerful thing, and once it has a hold of the falling character, it is going to win. Figure 11.20 shows the point at which the body is bracing itself for the fall while attempting to keep its balance within air and keep a good control over the center of gravity. Notice that as the legs kick forward in anticipation of the landing, the arms fall back. This keyframe should be only about 7-10 frames after the end of the crest keyframes.

The next noteworthy keyframe is very soon after the last (3-4 frames), and is simply a rotation of the feet as they strike the ground. There is a slight shift in the body as it continues its forward momentum, but there is a moment of impact when only the feet hit the floor. The next keyframe should be about 10 frames later. This keyframe shows the effect of the floor stopping the body’s downward momentum while the forward momentum continues.

As the weight lands on the floor, there is a forward rocking of the center of gravity as the body attempts to arrest its forward momentum. The arms (that ever-present indicator of secondary movement) are continuing in the forward momentum and balancing out the buttocks as the body comes to rest. By this time, the back is curved as it absorbs the power of the fall, and the head is looking up to see the situation around it.

Now that the danger of the situation has lessened, the body moves a little slower in its recovery. The next keyframe that shows the dummy beginning to stand up can be 15 frames from the last. When the human form is in the squat, the center of gravity is fairly well balanced by the head/shoulder/arm combination and the buttocks. When the body stands up, it has to make adjustments to keep from toppling over. A common tool is to hold the arms out in front of the body as it lifts its own buttocks higher.

The last keyframe (another 15 frames) is simply a return to the neutral position. The arms come down, the head is straight up, the back is in a normal arc, the knees are slightly bent, and the elbows are slightly bent off the side of the form. In true Muybridge fashion, if all has gone well, you should have a final animation.

Make sure to take plenty of time to tweak the animation. You may find that some of the time between frames is too far apart for your liking, or that you need to add extra frames to ensure that the arms or legs swing in an arc.

Make sure that as you are tweaking, you render test renders, or renders done in wireframe or some other feedback-oriented display. Once you have the timing just right, a full render will finish your masterpiece.

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