Collision Detection Basics

# CHAPTER 5

Collision Detection Basics #

1. Introduction #

2. Learning Objectives #

• Understand the necessity of simplified Collision Shapes (Colliders).

• Explain Axis-Aligned Bounding Box (AABB) collision math.

• Explain Circle/Sphere collision math using radius and distance.

• Differentiate between solid Collisions and ghost Triggers.

• Utilize Collision Layers to optimize game performance.

3. The Collider (Simplifying Reality) #

• Box Collider: A simple cube. Extremely fast to calculate. Used for crates, walls, and floors.

• Sphere/Circle Collider: A perfect ball. Even faster to calculate! Used for projectiles, wheels, and area-of-effect spells.

• Capsule Collider: A cylinder with rounded ends. The industry standard for humanoid players, because the rounded bottom glides smoothly up stairs and ramps.

4. Circle Collision Math (The Pythagorean Check) #

1. 1. Find the distance between the center of Circle A and Circle B.

1. 2. Add the Radius of Circle A to the Radius of Circle B.

1. 3. If the Distance is LESS than the combined Radii, they are colliding!

// Conceptual Math for Circle Collision
float distance = Vector3.Distance(circleA.position, circleB.position);
float combinedRadii = circleA.radius + circleB.radius;

if (distance < combinedRadii) {
    Console.WriteLine("CRASH!");
}

5. Box Collision Math (AABB) #

• To check if two AABBs are colliding, the CPU simply checks if the Min and Max boundaries of the boxes overlap on all three axes simultaneously. Because it's just a series of < and > checks, it is incredibly fast.

6. Solid Collisions vs. Triggers #

1. 1. Collision (Solid): The physics engine detects the overlap, panics, and mathematically pushes the two objects apart so they don't intersect. (A car hitting a brick wall).

1. 2. Trigger (Ghost): The engine detects the overlap, but does *nothing* to push them apart. It simply sends a C# message to the programmer saying, "Hey, Object A is inside Object B." (A player walking into a toxic gas cloud or collecting a coin).

7. Collision Matrix (Optimization) #

• Collision Layers: You assign objects to layers (e.g., "Player", "Enemy", "Bullet").

• The Matrix: In the engine settings, you tell the physics engine: "Bullets should never check for collisions against other Bullets." This simple matrix tick-box saves immense CPU power.

8. Visual Learning: Circle Collision #

   (Circle A)                 (Circle B)
    Radius: 2                  Radius: 3
       .-------------(5m)------------.
       
Distance (5) is EXACTLY EQUAL TO combined Radii (2+3).
They are perfectly touching! If distance drops to 4.9, they overlap!

9. Best Practices #

• Never Use Mesh Colliders for Moving Objects: A "Mesh Collider" wraps a custom 3D model perfectly. They are incredibly expensive to calculate. You can use them for static, unmoving terrain (like a complex mountain), but *never* use them for a moving car or character. Always approximate moving objects with a combination of primitive boxes and spheres.

10. Common Mistakes #

• Fast-Moving Projectiles (The Bullet Through Paper Problem): A bullet is moving at 1000m/s. In Frame 1, it is in front of a thin wall. In Frame 2, it is behind the wall. The engine *never* detected an overlap, so the bullet completely ignores the wall! To fix this, high-speed objects must use Continuous Collision Detection (CCD) or Raycasts instead of standard colliders.

11. Mini Project: Raycast Hit Detection #

class SniperRifle
{
    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            // Shoot an invisible mathematical laser forward
            RaycastHit hitInfo;
            
            // Physics.Raycast(StartPos, Direction, OutputData, MaxDistance)
            if (Physics.Raycast(transform.position, transform.forward, out hitInfo, 1000f))
            {
                Console.WriteLine("The laser hit: " + hitInfo.collider.name);
                
                // Spawn a bullet hole exactly where the laser hit the wall!
                Instantiate(bulletHoleDecal, hitInfo.point, Quaternion.identity);
            }
        }
    }
}

12. Practice Exercises #

1. 1. Why is a Capsule Collider the industry standard shape for a humanoid character?

1. 2. What does AABB stand for, and what restriction is placed on this type of bounding box?

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the mathematical logic behind detecting a collision between two 2D Circle Colliders. Why is this significantly faster for a CPU to calculate than polygon intersection?

• Q: Contrast a physical Collision with a Trigger event. Provide a gameplay example for each.

• Q: Explain how a Collision Matrix optimizes game performance. Why is it computationally dangerous to leave all physics layers interacting with each other?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #