Cascadeur: Why 12 Disney Principles Are Not Enough

To understand how Cascadeur works , we need to look at the history of animation and the approach to creating it. In this article, Evgeny Khapugin, a leading animator at Banzai.Games , will try to explain how going beyond Walt Disney’s fundamental principles helps create realistic animations and how to use Cascadeur’s physical tools .

Sometimes, when we watch movies or play games, we notice scenes that look unnatural. Usually we cannot explain what is wrong with them. For example, we may not see the cables attached to the actor, but we feel that he does not move as he should. And suspicion does not deceive us. Our brain easily recognizes unrealistic movements. In other words, we always notice that the animation is physically incorrect .

Modern animation can be very complex, its creation takes a lot of time and requires the use of real references and capture movements. And yet we encounter such scenes again and again. Why so?

What happens behind the scenes

Oddly enough, one of the reasons may be the well-known 12 principles of Disney animation. They work well and explain well how to create animations. But they don’t say anything about realism. These principles appeared in the 1930s, and animators rely on them to this day.

In 2002, Jeremy Cantor, a Sony Animation supervisor, supplemented 12 Disney principles with 12 of his own. They help to improve the quality of animation, introduce the concept of forces applied to the character. Forces can be internal, such as weight or muscle movement, or can be external, such as gravity or interaction with air, water, or other characters.

But the animator must apply these forces manually, so that they allow you to make the animation more expressive, but no more believable. No, a truly realistic animation should take into account the real physical forces applied to the character, and where these forces come from. Here motion capture can help - it is done in reality, which means that the movements must be real.

But, alas, the possibilities of motion capture are also not unlimited. To take a simple scene with a horse, you will need to rent a large area, pay for food and horse care, cleaning, etc.

And all this despite the fact that the horse itself is at least real. If we need a dragon or a superhero, a mocap is unlikely to help.

Physically correct animation

Every year, more and more tools appear for realistic rendering, creating simulations and shaders. And, strangely enough, only the trend does not apply to animation.

When Eugene Dyabin, the founder of Banzai.Games and Cascadeur, worked on the first Shadow Fight prototype, he was very surprised that the center of mass - absolutely necessary for creating realistic acrobatics - is missing in all well-known animation programs. It turns out that animators are now creating realistic movements by hand, relying on video references.

Shadow Fight, our series of fighting games, places great emphasis on animation. This project turned out to be very successful, and many realistic reactions deserved just the realistic animation of the blows. We could not do such an animation using only references and motion capture. To work on it, we created Cascadeur - a system for creating physically correct animations.

CascadeurIt has been under development for almost ten years, and during this time we found out that in order to create a realistic movement it is enough to correctly calculate its main parameters: center of mass, moment of inertia and force at the fulcrum.

Center of mass

The center of mass is one of the character’s most important points. It determines whether the character can maintain balance in a given position.

Take a look at an example from the beloved Mortal Kombat. The punch that Kitana makes looks rather strange and unnatural. Given her posture and how her center of mass moves, she would have to tumble forward and fall.

This is how a similar movement might actually look. This man turns a little during the strike, but, unlike Kitana, he does not try to hit anyone, and therefore maintains balance.

There can be several centers of mass in a scene. For example, when this girl throws her glove, they begin to move away from each other, but their common center of mass remains in place.

The center of mass is a very inert point. It cannot slow down or accelerate quickly, its trajectory must always remain smooth, although the trajectories of individual parts of the body can be very sharp.

Ballistics

Another important point: when the character is in the air, the trajectory of his center of mass should always be parabolic. Whatever actions the character performs in the air, he must move along a ballistic trajectory.

In Cascadeur have a tool for the creation of such trajectories. It is enough for the animator to create a curve, set the start and end poses and, if necessary, add an intermediate animation. The height of the curve can be adjusted manually, it determines how long the character is in the air.

Once everything is ready, Cascadeur calculates the trajectory and can only attach to it the center of mass of the character. As a result, we get a plausible, physically correct jump path.

Angular momentum

The angular momentum determines the rotation of the character in a given position. It depends on the moment of inertia and the speed of rotation of the character.

In the image below, the moment of inertia is presented in the form of two circles of gray color. The radius of these circles is determined by the character’s pose. The more open the pose, the greater the moment of inertia.

The arrow represents the angular momentum. The greater the moment of inertia, the slower the character rotates, since the arrow needs more time to describe a full circle. Conversely, the smaller the circle, the faster the rotation.

This skater, for example, rotates more slowly when she takes her leg to the side than when her body is assembled.

Like the center of mass, the angular momentum is a very inert quantity. It cannot suddenly appear or disappear and must change smoothly. It also remains in flight when there are no fulcrum. Energy at the start of rotation should not change until the end of the flight.

Because of all these nuances, realistic rotation in a jump is almost impossible to animate manually: turns in intermediate frames should be so accurate. But Cascadeur has a tool for that.

All that he needs is a rough animation: the initial and final poses and, possibly, some intermediate positions. Our algorithm calculates the exact values ​​of rotation and corrects the character's position in all frames.

Forces at the fulcrum

Pivot points also play an important role in the movement of the character, so physical tools must take them into account. When the force at the fulcrum and the force applied to the character’s center of mass lie on the same line, they do not affect the rotation. When this is not so, the character begins to spin.

Cascadeur is able to visualize these forces and take them into account when calculating the character’s movement.

For example, we want a character to land on a pillar in a specific pose. Our program will independently calculate all the necessary rotations, taking into account the pivot points, so that the character will land in the exact position that we need and will not fall.

In addition to physical laws, when creating combat animations, we use two more principles. We call them separation and compensation, and they successfully complement the laws of physics.

Separation

Separation is the accumulation of energy in one place and its transfer to another place. This principle applies when animating powerful punches. Here, for example, the character translates all the energy of his jump into a blow with a sword.

Another example: this taekwondo player. He twists very much, then his body practically stops, and only the leg continues to move. As a result, all the rotation energy is transferred to the leg.

Compensation

Compensation is the movement of one or more parts of the character’s body in the direction opposite to the general direction of movement. This is necessary to maintain equilibrium and smooth the trajectory of the center of mass.

Take a look at this flail. During the strike, the character holds a flail over his head. But the flail is heavy and can pull it back, causing it to lose balance.

To compensate for this movement, the character takes his body forward and turns around a bit. Their common center of mass moves up and then down, but the center of mass of the character himself always moves forward, compensating for this movement.

Conclusion

Realism of animation is often destroyed by small details: the center of mass, which does not move as it should, or the angular momentum, which accelerates at the wrong time. Together, these little things become noticeable and spoil the overall impression.

12 principles are a good basis for animation, but they alone are not enough when realistic movements are required. We believe that these principles should be supplemented with our physical tools. They could take on the issues of physical realism and let the animators concentrate on the artistic component.

Learn more about Cascadeur:

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Banzai Games Senior Unity Developer. .