Collision Detection

Overview

Did the bullet hit? Did the player land? Collision detection answered gaming’s fundamental questions—when objects occupy the same space. Simple boxes worked for most cases, but fighting games needed precision. Each method traded accuracy against CPU cost, and clever programmers chose wisely based on game requirements.

Fast facts

Purpose: Detect object intersection.
Trade-off: Accuracy vs performance.
Methods: Bounding box, circle, pixel-perfect.
Application: Every action game.

Bounding box collision

Step	Process
1	Define rectangle around object
2	Check if rectangles overlap
3	Overlap = collision

if (a.x < b.x + b.width &&
    a.x + a.width > b.x &&
    a.y < b.y + b.height &&
    a.y + a.height > b.y)
    // Collision!

Circle collision

Step	Process
1	Define circle (centre + radius)
2	Calculate distance between centres
3	Distance < combined radii = collision

Good for: Balls, spherical objects, approximate checks.

Pixel-perfect collision

Step	Process
1	Check bounding box first
2	If overlap, check overlapping pixels
3	Both non-transparent = collision

Expensive but precise.

Hardware-assisted (C64)

Register	Function
$D01E	Sprite-sprite collision
$D01F	Sprite-background collision
Usage	Read to check collisions

Tile-based collision

Advantage	Detail
Grid lookup	Check tile at position
Tile properties	Solid, empty, hazard
Performance	Very fast
Limitation	Grid-aligned only

Optimisation techniques

Technique	Purpose
Broad phase	Quick rejection test
Spatial partitioning	Only check nearby objects
Bitmasks	Fast boolean operations
Frame skipping	Check every N frames

Common issues

Problem	Cause
Tunnelling	Fast objects skip through
False positives	Bounding box too large
False negatives	Check too infrequent

When objects meet