The Cascadeur team shared their research on movements in zero gravity and experiments with character animation that allowed them to achieve believable results in the software.
Movements in Zero Gravity
Suppose there are no objects nearby that the astronauts could push off from. Can they turn in the direction they need using only the movements of their own limbs? Let's say an astronaut wants to turn 180 degrees to see what’s behind him. If he stood with his feet on the ground, it wouldn’t be difficult. But in space, the movements of his upper and lower body begin to compensate each other: he turns his chest in one direction, and his legs immediately turn in the opposite direction. As a result, he cannot turn around.
Experiments in Cascadeur
Cascadeur is a physics-based animation program. Rig characters consist of many rigid bodies that have mass and tensor of inertia, used by the program to calculate the physical behavior of the characters.
An important point in conditions of zero gravity is that the speed of the center of mass cannot change without an outside force. If the character’s center of mass was initially static, it will remain in the same place in space regardless of the movements performed. The characters can only revolve around their centers of mass!
In the original animation (first), the character is looking behind his back, turning his upper body considerably.
After using physical tools to apply zero gravity to this animation (second), the movement looks a little different. The character doesn’t have the same range of movement and his hips also begin to turn, compensating for the movement of the upper body. At the same time, the character’s poses in each frame remain exactly the same as in the original animation!
So how do you turn around in zero gravity using only your own movements? We’ve checked videos of real astronauts and analyzed their movements to find out.
Above is the video that we took as a reference for our animation. Our first impression was that the astronaut turns around mostly because of his arms movement. So we made a draft animation with all the poses from the video:
But once we applied our physical tools, the result was disappointing. The animation was nothing like the reference!
The conclusion was that in order for the character to move as in the reference, it is fundamentally important to correctly do the first part of his movement – a turnover to his back. So we had to investigate what and how affects this turnover and recreate these movements in the animation.
To research this, we experimented by fixing and moving various body parts, while checking the effect with our physical tools. Finally, we discovered that we could get the needed result if the character twisted his legs.
Here is an animation in which the upper part of the character remains fixed, while the legs are pulled to the stomach making a circle.
This proves that circular movements of the limbs work well for turning around, but you need to use different types of movement, depending on the axis on which you need to turn.
For rotating around the transversal axis you have to rotate your limbs in a sagittal plane: