Love2D Collision Detection: When AABB Meets Performance in Real Games

The Problem: Sprites Passing Through Each Other

In a 2D game with dozens of moving sprites—enemies, bullets, pickups—detecting when they touch is fundamental. Love2D's built-in collide module handles this with AABB (Axis-Aligned Bounding Box) collision, but developers often hit two walls: objects clipping through each other at high speeds, or frame rates tanking when checking collisions between hundreds of objects.

Why AABB Works for Most 2D Games

AABB treats each object as a rectangle defined by its top-left corner and dimensions. The collision check is simply comparing min/max coordinates on both axes—a few arithmetic operations that run in constant time. This makes it ideal for real-time games where you need to check collisions every frame without slowing down.

Love2D's collide.solve() function takes two objects and returns collision information. Here's the core pattern:

local collState = love.graphics.newBoundingBox(x, y, width, height)
local result = collide.solve(object1, object2)
if result.state == "overlap" then
    -- Handle collision
end

Worked Example: A Simple Pong Clone

Let's implement collision between a ball and paddle. We'll use AABB since both are rectangular:

function checkBallPaddleCollision(ball, paddle)
    local ballBox = { 
        x = ball.x - ball.radius, 
        y = ball.y - ball.radius, 
        width = ball.radius * 2, 
        height = ball.radius * 2 
    }
    
    local paddleBox = { 
        x = paddle.x, 
        y = paddle.y, 
        width = paddle.width, 
        height = paddle.height 
    }
    
    local result = collide.solve(ballBox, paddleBox)
    return result.state == "overlap"
end

function love.update(dt)
    if checkBallPaddleCollision(ball, playerPaddle) then
        ball.dx = -ball.dx  -- Simple bounce
    end
end

This runs in love.update for every frame. With two objects, performance impact is negligible. But scale this to 100 balls and 50 paddles, and you need collide.setEpsilon to handle floating-point precision or spatial partitioning to reduce checks.

Trade-offs: Accuracy vs Performance

AABB's limitation is clear: it treats all objects as rectangles, even if they're circular or have irregular shapes. A bullet might appear to miss a sprite visually while still triggering collision because their bounding boxes overlap.

For pixel-perfect collision, you'd need to implement your own system using image masks or distance formulas. But this costs significantly more CPU per check. The engineering decision comes down to:

• Fast-paced action games: AABB is sufficient. Players expect immediate feedback, not pixel precision.
• Puzzle games with tight-fitting sprites: Consider circle-based collision or custom polygons for better accuracy.
• Large object counts: Implement a simple spatial grid to only check nearby objects.

Practical Verification

To confirm collision detection works in your game:

1. Create two rectangles that definitely overlap
2. Use collide.solve() and verify result.state returns "overlap"
3. Move one rectangle slowly and check that collision stops when they separate
4. Add debug drawing: love.graphics.rectangle("line", x, y, w, h) to visualize bounds

If you need to handle many objects, profile with love.timer.getFPS() before and after adding collision checks. A drop of more than 10% suggests you need spatial optimization.

The Takeaway

Love2D's AABB collision is a pragmatic choice for most 2D games. It's not perfect—objects can clip at high speeds, and bounding boxes don't match irregular shapes—but it's fast enough to check every frame without complex setup. Start with AABB, verify it works for your use case, then optimize only if performance demands it.