Tutorial:Physics

In this example we will create a red ball that rolls around on a green ground.

The finished example is at the end of this page. All of these functions may be placed in one file: main.lua

We'll start in the love.load function.

love.load()

First we need to set up a world for the physics bodies to exist in.

function love.load()
  world = love.physics.newWorld(0, 0, 650, 650) --create a world for the bodies to exist in with width and height of 650
  world:setGravity(0, 700) --the x component of the gravity will be 0, and the y component of the gravity will be 700
  world:setMeter(64) --the height of a meter in this world will be 64px

Now that a world has been created, we can add bodies to it.

  objects = {} -- table to hold all our physical objects
  
  --let's create the ground
  objects.ground = {}
  --we need to give the ground a mass of zero so that the ground wont move
  objects.ground.body = love.physics.newBody(world, 650/2, 625, 0, 0) --remember, the shape anchors to the body from its center
  objects.ground.shape = love.physics.newRectangleShape(objects.ground.body, 0, 0, 650, 50, 0) --anchor the shape to the body, and make it a width of 650 and a height of 50
  
  --let's create a ball
  objects.ball = {}
  objects.ball.body = love.physics.newBody(world, 650/2, 650/2, 15, 0) --place the body in the center of the world, with a mass of 15
  objects.ball.shape = love.physics.newCircleShape(objects.ball.body, 0, 0, 20) --the ball's shape has no offset from it's body and has a radius of 20

Now to wrap up the love.load function, let's set up the screen size and background color.

  --initial graphics setup
  love.graphics.setBackgroundColor(104, 136, 248) --set the background color to a nice blue
  love.graphics.setMode(650, 650, false, true, 0) --set the window dimensions to 650 by 650 with no fullscreen, vsync on, and no antialiasing
end

Okay, that's enough for the initial set up of the physics engine. Now we need do edit the love.update() function.

love.update()

function love.update(dt)
  world:update(dt) --this puts the world into motion
  
  --here we are going to create some keyboard events
  if love.keyboard.isDown("right") then --press the right arrow key to push the ball to the right
    objects.ball.body:applyForce(400, 0)
  elseif love.keyboard.isDown("left") then --press the left arrow key to push the ball to the left
    objects.ball.body:applyForce(-400, 0)
  elseif love.keyboard.isDown("up") then --press the up arrow key to set the ball in the air
    objects.ball.body:setY(650/2)
  end
end

Now that the world is updating, we can draw the ground and ball.

love.draw()

First, the ground.

function love.draw()
  love.graphics.setColor(72, 160, 14) -- set the drawing color to green for the ground
  love.graphics.polygon("fill", objects.ground.shape:getPoints()) -- draw a "filled in" polygon using the ground's coordinates

And finally, we can draw the circle that represents the ball.

  love.graphics.setColor(193, 47, 14) --set the drawing color to red for the ball
  love.graphics.circle("fill", objects.ball.body:getX(), objects.ball.body:getY(), objects.ball.shape:getRadius(), 20) -- we want 20 line segments to form the "circle"
end

There you have it! Put this file in a zip file, rename it to physics.love (or whatever), run it. And you'll have a ball rolling around in a lush green environment like I promised.

Screenshot of the finished product.

The main.lua

function love.load()
  world = love.physics.newWorld(0, 0, 650, 650) --create a world for the bodies to exist in with width and height of 650
  world:setGravity(0, 700) --the x component of the gravity will be 0, and the y component of the gravity will be 700
  world:setMeter(64) --the height of a meter in this world will be 64px
  
  objects = {} -- table to hold all our physical objects
  
  --let's create the ground
  objects.ground = {}
  --we need to give the ground a mass of zero so that the ground wont move
  objects.ground.body = love.physics.newBody(world, 650/2, 625, 0, 0) --remember, the body anchors from the center of the shape
  objects.ground.shape = love.physics.newRectangleShape(objects.ground.body, 0, 0, 650, 50, 0) --anchor the shape to the body, and make it a width of 650 and a height of 50
  
  --let's create a ball
  objects.ball = {}
  objects.ball.body = love.physics.newBody(world, 650/2, 650/2, 15, 0) --place the body in the center of the world, with a mass of 15
  objects.ball.shape = love.physics.newCircleShape(objects.ball.body, 0, 0, 20) --the ball's shape has no offset from it's body and has a radius of 20

  --initial graphics setup
  love.graphics.setBackgroundColor(104, 136, 248) --set the background color to a nice blue
  love.graphics.setMode(650, 650, false, true, 0) --set the window dimensions to 650 by 650
end


function love.update(dt)
  world:update(dt) --this puts the world into motion
  
  --here we are going to create some keyboard events
  if love.keyboard.isDown("right") then --press the right arrow key to push the ball to the right
    objects.ball.body:applyForce(400, 0)
  elseif love.keyboard.isDown("left") then --press the left arrow key to push the ball to the left
    objects.ball.body:applyForce(-400, 0)
  elseif love.keyboard.isDown("up") then --press the up arrow key to set the ball in the air
    objects.ball.body:setY(650/2)
  end
end

function love.draw()
  love.graphics.setColor(72, 160, 14) -- set the drawing color to green for the ground
  love.graphics.polygon("fill", objects.ground.shape:getPoints()) -- draw a "filled in" polygon using the ground's coordinates

  love.graphics.setColor(193, 47, 14) --set the drawing color to red for the ball
  love.graphics.circle("fill", objects.ball.body:getX(), objects.ball.body:getY(), objects.ball.shape:getRadius(), 20) -- we want 20 line segments to form the "circle"
end

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