Gravity & Jumping

Platformers need gravity to pull the player down and jumping to move up. Let's implement both!

How Gravity Works

Gravity is an acceleration - it continuously increases the player's downward velocity:

lua
velocity.y = velocity.y + gravity * dt
position.y = position.y + velocity.y * dt

Implementing Jump

When the player presses jump, we set an upward velocity. Gravity then pulls them back down:

lua
if jump_pressed and on_ground then
    velocity.y = -jump_force
end

The Complete Code

lua
function love.load()
    player = {
        x = 50,
        y = 200,
        width = 30,
        height = 40,
        speed = 200,
        velocityY = 0,
        gravity = 800,
        jumpForce = -400,
        onGround = false
    }

    ground = {
        y = 250
    }
end

function love.update(dt)
    -- 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

    -- Update vertical position
    player.y = player.y + player.velocityY * dt

    -- Check ground collision
    if player.y + player.height >= ground.y then
        player.y = ground.y - player.height
        player.velocityY = 0
        player.onGround = true
    else
        player.onGround = false
    end

    -- Keep player on screen horizontally
    if player.x < 0 then player.x = 0 end
    if player.x + player.width > 400 then
        player.x = 400 - player.width
    end
end

function love.keypressed(key)
    -- Jump when space or W is pressed
    if (key == "space" or key == "w" or key == "up") and player.onGround then
        player.velocityY = player.jumpForce
        player.onGround = false
    end
end

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

    -- Ground
    love.graphics.setColor(0.4, 0.8, 0.4)
    love.graphics.rectangle("fill", 0, ground.y, 400, 50)

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

    -- Eyes
    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)

    -- Pupils
    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)

    -- Instructions
    love.graphics.setColor(1, 1, 1)
    love.graphics.print("SPACE/W/UP to jump, LEFT/RIGHT to move", 10, 10)
end

New Concepts:

love.keypressed(key) is called once when a key is pressed (not held)
velocityY tracks vertical speed (negative = up, positive = down)
onGround prevents double-jumping

Try it! Press SPACE to jump!