Different Approaches To Handling Restricted Turning Radii
The Problem: Restricted Turning Messes Up Navigation
In video games and simulations involving character or vehicle movement, restricted turning radii can cause major issues during navigation. When a character or vehicle has a minimum turning radius, it is unable to make sharp 90 degree turns and instead has to move in a curved path. This can cause problems when navigating tight spaces or trying to avoid obstacles.
For example, if a vehicle with a large minimum turning radius attempts to turn down a narrow alleyway, it may be unable to complete the turn without colliding with buildings or obstacles. Similarly, a character with a restricted turning radius may have difficulty navigating tight indoor spaces like hallways and rooms.
Restricted turning interferes with pathfinding and navigation meshes that assume free movement in all directions. The inability to make sharp turns means characters and vehicles get stuck or collide with objects when following automatically generated paths. This results in immersion-breaking glitchy movement.
Solutions are needed to allow smooth, natural movement despite restricted turning radii. This article explores approaches like steering behaviors, nav meshes, animation blending, and more to overcome tight turning constraints.
Understanding Turning Radii
The turning radius determines the sharpest turn a character or vehicle can make at a given speed. At low speeds, the turning radius is lower, enabling tighter turns. At higher speeds, the turning radius increases, requiring wider turns.
The turning radius depends on factors like:
- Type of character or vehicle
- Velocity
- Turn rate
- Weight distribution
- Wheelbase length
For example, a large truck has a much larger minimum turning radius than a small compact car. And even the same vehicle, like a sedan, has a wider turning radius at 60 mph than at 30 mph.
When implementing restricted turning, the minimum radii for various velocities must be properly tuned for each character/vehicle type to achieve realistic movement. Then solutions can work within those physical constraints.
Solving With Steering Behaviors
Steering behaviors are AI techniques that guide characters and vehicles towards goals while avoiding obstacles. They generate movement by applying forces, unlike waypoint and node graph based approaches.
Steering behaviors are a great fit for overcoming limited turning radii since they use continuous collision detection and avoid hard direction changes. Rather than pre-planning rigid paths, they create organic movement reacting to the moment.
Seeking
The seeking behavior moves a character directly towards a target position, accelerating until within a small distance. This basic goal-directed behavior must be combined with others to handle restrictions.
Arrival
Arrival extends seeking by ensuring the character decelerates as it nears the target to come to a stop. This prevents overshooting due to momentum and enables precise positioning.
Obstacle Avoidance
This behavior steers characters around obstacles while still seeking goals, critical for restricted turning. It uses feelers to detect imminent collisions then alters trajectory to avoid while minimizing diversion.
Wandering
Wandering creates lifelike, non-uniform motion by randomly changing headings. Using a wander radius that fits within the minimum turning radius prevents winding paths.
Path Following
Path following makes characters track smooth spline paths, ensuring turns stay within radius limits. Dynamic path generation methods create curves accounting for the current velocity’s turning constraints.
Pursuit
Pursuit moves a character towards the predicted future position of a moving target. Leading the target and using arrival prevents overshoot when reacting to sharp turns.
Evade
Evade does the opposite, steering away from a pursuer’s expected future position. Restricted turning can make dodging challenging, so large evasion radii tuned to limits work best.
Code Example
Here is C# code demonstrating a basic seek behavior with constraints:
public class SeekBehavior : SteeringBehavior {
public Vector3 target;
public float maxAcceleration;
public override Vector3 Calculate() {
Vector3 desiredVelocity = target - transform.position;
desiredVelocity.Normalize();
desiredVelocity *= maxAcceleration;
Vector3 steeringForce = desiredVelocity - agent.velocity;
steeringForce = Vector3.ClampMagnitude(steeringForce, maxForce);
return steeringForce;
}
public void SetTarget(Vector3 target) {
this.target = target;
}
}
The key is to clamp steering forces to gradual rates and leverage layering behaviors to create emergent constrained motion.
Using Nav Meshes
Building the Nav Mesh
A navigation mesh or navmesh is a mesh representation of walkable areas for path planning. Navmesh generation begins by marking unwalkable geometry as obstacles then polygonalizing the free space.
To handle restricted turning, the navmesh must be built with wider clearance around obstacles. The agent radius parameter should match the turning radius to provide adequate spacing.
Convex polygons work better than concave for smooth turning movements. Long thin triangles or other awkward shapes should be decomposed into cleaner polygons.
Pathfinding on the Nav Mesh
Given an accurate navmesh, pathfinding calculates a route from A to B. The simplest algorithm is A* which finds the shortest collision-free path.
For restricted turning, waypoint distances must be small enough to meet radius constraints. Adaptive waypoint counts based on velocity and radii prevent paths with impossible sharp turns.
More advanced nonlinear path optimization methods take into account kinematic constraints, generating smooth spline paths guaranteed to keep within turning limits.
Updating the Nav Mesh Dynamically
For games with a dynamic environment, the navmesh needs real-time updates to reflect scene changes. Obstacle expansion radii allow room to recalculate paths when new objects appear.
It’s also possible to forgo precomputation and generate paths on the fly using expensive continuous collision detection. This ensures fully up-to-date constraint compliance.
Manipulating Animations
Blending Animations
Animation blending crossfades between animations to achieve smooth transitions. This prevents abrupt stance changes during tight turns.
For example, subtly blending in sideways walking during a constrained turn avoids foot sliding. And transitioning from run to walk animations during a sharp deceleration prevents unrealistic motion.
Adding Transitional Animations
Besides blending, creating short transition animations also improves realism. Explicitly authored transitions like shuffle turns or lateral steps hide technical limitations.
Augmenting the base animation set with extra transitional clips tailored for constrained directions expands capabilities.
Applying Root Motion
Root motion derives character movement from animations rather than physics. This guarantees turns, stops, and starts respect animation constraints, at the cost of control.
Retargeting mocap data also brings real life movement limits built-in. But root motion requires carefully crafting acceleration, deceleration, and turn animations to give control back to gameplay code.
Choosing the Right Solution
In summary, overcoming tight turning constraints requires:
- Tuning acceleration and velocities for realistic minimum turning radii
- Layering steering behaviors to create emergent constrained paths
- Generating navmeshes with wider spacing around obstacles
- Shortening waypoint distances between mesh polygons
- Blending animations to hide technical limitations
- Adding transitional animations for critical poses
- Leveraging root motion for pre-baked turn realism
The best approach depends on the specific game and situation. Steering behaviors work well for online multiplayer with dynamic environments and few scripted interactions. Precomputed navmeshes shine in open world singleplayer games where performance matters more than fully dynamic updates. And animation layering brings responsive visual smoothness on top of path planning and obstacles avoidance systems.
By intelligently combining and tuning these methods, realistic restricted turning motion emerges, keeping players immersed in the experience.
