Collision Detection

Let's improve our collision detection to handle all sides of platforms, not just the top.

AABB Collision

We're using AABB (Axis-Aligned Bounding Box) collision - checking if two rectangles overlap:

lua
function checkCollision(a, b)
    return a.x < b.x + b.width and
           a.x + a.width > b.x and
           a.y < b.y + b.height and
           a.y + a.height > b.y
end

Resolving Collisions

When a collision happens, we need to figure out which side was hit and push the player out:

lua
function love.load()
    player = {
        x = 50,
        y = 100,
        width = 30,
        height = 40,
        speed = 200,
        velocityY = 0,
        gravity = 800,
        jumpForce = -350,
        onGround = false
    }

    platforms = {
        {x = 0, y = 250, width = 400, height = 50},
        {x = 100, y = 180, width = 80, height = 15},
        {x = 220, y = 130, width = 100, height = 15},
        {x = 50, y = 80, width = 70, height = 15},
    }
end

function love.update(dt)
    -- Store previous position
    local prevX = player.x
    local prevY = player.y

    -- Horizontal movement
    if love.keyboard.isDown("left") or love.keyboard.isDown("a") then
        player.x = player.x - player.speed * dt
    end
    if love.keyboard.isDown("right") or love.keyboard.isDown("d") then
        player.x = player.x + player.speed * dt
    end

    -- Apply gravity
    player.velocityY = player.velocityY + player.gravity * dt
    player.y = player.y + player.velocityY * dt

    player.onGround = false

    -- Check collisions with platforms
    for i, plat in ipairs(platforms) do
        if checkCollision(player, plat) then
            -- Determine collision direction
            local overlapLeft = (player.x + player.width) - plat.x
            local overlapRight = (plat.x + plat.width) - player.x
            local overlapTop = (player.y + player.height) - plat.y
            local overlapBottom = (plat.y + plat.height) - player.y

            -- Find smallest overlap
            local minOverlap = math.min(overlapLeft, overlapRight, overlapTop, overlapBottom)

            if minOverlap == overlapTop and player.velocityY > 0 then
                -- Landing on top
                player.y = plat.y - player.height
                player.velocityY = 0
                player.onGround = true
            elseif minOverlap == overlapBottom and player.velocityY < 0 then
                -- Hit head on bottom
                player.y = plat.y + plat.height
                player.velocityY = 0
            elseif minOverlap == overlapLeft then
                -- Hit left side
                player.x = plat.x - player.width
            elseif minOverlap == overlapRight then
                -- Hit right side
                player.x = plat.x + plat.width
            end
        end
    end

    -- Screen boundaries
    if player.x < 0 then player.x = 0 end
    if player.x + player.width > 400 then player.x = 400 - player.width end
    if player.y < 0 then player.y = 0; player.velocityY = 0 end
end

function checkCollision(a, b)
    return a.x < b.x + b.width and
           a.x + a.width > b.x and
           a.y < b.y + b.height and
           a.y + a.height > b.y
end

function love.keypressed(key)
    if (key == "space" or key == "w" or key == "up") and player.onGround then
        player.velocityY = player.jumpForce
    end
end

function love.draw()
    love.graphics.setColor(0.4, 0.7, 1)
    love.graphics.rectangle("fill", 0, 0, 400, 300)

    love.graphics.setColor(0.4, 0.8, 0.4)
    for i, plat in ipairs(platforms) do
        love.graphics.rectangle("fill", plat.x, plat.y, plat.width, plat.height)
    end

    love.graphics.setColor(1, 0.3, 0.3)
    love.graphics.rectangle("fill", player.x, player.y, player.width, player.height)

    love.graphics.setColor(1, 1, 1)
    love.graphics.circle("fill", player.x + 10, player.y + 12, 5)
    love.graphics.circle("fill", player.x + 22, player.y + 12, 5)
    love.graphics.setColor(0, 0, 0)
    love.graphics.circle("fill", player.x + 12, player.y + 12, 2)
    love.graphics.circle("fill", player.x + 24, player.y + 12, 2)

    love.graphics.setColor(1, 1, 1)
    love.graphics.print("Full collision detection!", 10, 10)
end

Try it! Notice how you can't pass through platforms from any direction now.