Implementing AABB Collision Detection in SDL3

The AABB function

This is the main function for the AABB collision detection algrithm.

This function checks if two rectangles (represented as SDL_FRect) overlap in 2D space using Axis-Aligned Bounding Box (AABB) collision detection. The rectangles are axis-aligned, meaning they are not rotated — their edges are parallel to the X and Y axes.

bool AABB(const SDL_FRect& a, const SDL_FRect& b) {
    return a.x < b.x + b.w &&
           a.x + a.w > b.x &&
           a.y < b.y + b.h &&
           a.y + a.h > b.y;
}

Parameters of AABB function

a and b are two rectangles, each defined by:

• x, y: Top-left corner position
• w, h: Width and height

Return Value of AABB function

• Returns true if the rectangles intersect (i.e. they are colliding)
• Returns false otherwise

How It Works (Condition by Condition)

a.x < b.x + b.w

→ The left edge of a is to the left of the right edge of b

a.x + a.w > b.x

→ The right edge of a is to the right of the left edge of b

a.y < b.y + b.h

→ The top edge of a is above the bottom edge of b

a.y + a.h > b.y

→ The bottom edge of a is below the top edge of b

All four conditions must be true for the rectangles to overlap.

The main function

The goal is to write a program that move a green square and change colors to red when colliding with another square. For that we will use the AABB collision detection functioon previously defined.

Initializations

Window and renerer

SDL_Init(SDL_INIT_VIDEO);

SDL_Window* window = SDL_CreateWindow("AABB Collision - SDL3", 800, 600, SDL_WINDOW_RESIZABLE);
SDL_Renderer* renderer = SDL_CreateRenderer(window, nullptr);

if (!window || !renderer) {
    std::cerr << "Window/Renderer creation failed: " << SDL_GetError() << "\n";
    SDL_Quit();
    return 1;
}

Player and box

SDL_FRect player = { 100, 100, 100, 100 };
SDL_FRect box = { 400, 300, 100, 100 };

Speed and last time variable

float speed = 200.0f;
Uint64 lastTime = SDL_GetTicks();

The main loop

The time elapsed: delta time

To compute the framerate if you want more eexplanations you can follow the tutorial about spritesheet animations

Uint64 currentTime = SDL_GetTicks();
float delta = (currentTime - lastTime) / 1000.0f;
lastTime = currentTime;

The event loop

The event loop contains only the quit event.

while (SDL_PollEvent(&event)) {
    if (event.type == SDL_EVENT_QUIT)
        running = false;
}

Player inputs

const bool* keys = SDL_GetKeyboardState(nullptr);
if (keys[SDL_SCANCODE_LEFT])  player.x -= speed * delta;
if (keys[SDL_SCANCODE_RIGHT]) player.x += speed * delta;
if (keys[SDL_SCANCODE_UP])    player.y -= speed * delta;
if (keys[SDL_SCANCODE_DOWN])  player.y += speed * delta;

The AABB function call

bool collides = AABB(player, box);

The rendering

Clear the screen

SDL_SetRenderDrawColor(renderer, 30, 30, 30, 255);
SDL_RenderClear(renderer);

The box

SDL_SetRenderDrawColor(renderer, 100, 100, 255, 255); // static box
SDL_RenderFillRect(renderer, &box);

The player

The player change colors if it's colliding with the box:

if (collides)
    SDL_SetRenderDrawColor(renderer, 255, 50, 50, 255); // red on collision
else
    SDL_SetRenderDrawColor(renderer, 50, 255, 50, 255); // green otherwise

SDL_RenderFillRect(renderer, &player);

Download the full project : Implementing AABB Collision Detection in SDL3

• For macos: SDL3_AABBCollision_mac.7z
• For windows: SDL3_AABBCollision_win.7z
• For linux: SDL3_AABBCollision_linux.7z

Need another OS ? => Windows, Mac, Linux