Difference between revisions of "Tutorial:Physics"

m (The main.lua: ... preferably without introducing bugs)
 
(21 intermediate revisions by 16 users not shown)
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In this [[example]] we will create a red ball that rolls around on a green ground.
+
In this example we will create a red ball that rolls around on a green ground.
  
We'll start in the [[love.load]] function.
+
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() ===
 
=== love.load() ===
  
First we need to set up a world for the physics bodies to exist in.
+
First we need to set up a [[love.physics.newWorld|world]] for the physics bodies to exist in.
 
<source lang="lua">
 
<source lang="lua">
 
function love.load()
 
function love.load()
   world = love.physics.newWorld(650, 650) --create a world for the bodies to exist in with width and height of 650
+
  love.physics.setMeter(64) --the height of a meter our worlds will be 64px
  world:setGravity(0, 700) --the x component of the gravity will be 0, and the y component of the gravity will be 700
+
   world = love.physics.newWorld(0, 9.81*64, true) --create a world for the bodies to exist in with horizontal gravity of 0 and vertical gravity of 9.81
  world:setMeter(64) --the height of a meter in this world will be 64px
 
 
</source>
 
</source>
  
Now that a [[world]] has been created, we can add [[bodies]] to it.
+
Now that a [[love.physics.newWorld|world]] has been created, we can add [[Body|bodies]], [[Shape|shapes]], and [[Fixture|fixture]] to it.
  
 
<source lang="lua">
 
<source lang="lua">
  bodies = {} --create tables for the bodies and shapes so that the garbage collector doesn't delete them
+
  objects = {} -- table to hold all our physical objects
  shapes = {}
 
 
    
 
    
 
   --let's create the ground
 
   --let's create the ground
   --we need to give the ground a mass of zero so that the ground wont move
+
   objects.ground = {}
   bodies[0] = love.physics.newBody(world, 650/2, 625, 0, 0) --remember, the body anchors from the center of the shape
+
   objects.ground.body = love.physics.newBody(world, 650/2, 650-50/2) --remember, the shape (the rectangle we create next) anchors to the body from its center, so we have to move it to (650/2, 650-50/2)
   shapes[0] = love.physics.newRectangleShape(bodies[0], 0, 0, 650, 50, 0) --anchor the shape to the body, and make it a width of 650 and a height of 50
+
   objects.ground.shape = love.physics.newRectangleShape(650, 50) --make a rectangle with a width of 650 and a height of 50
 +
  objects.ground.fixture = love.physics.newFixture(objects.ground.body, objects.ground.shape) --attach shape to body
 
    
 
    
 
   --let's create a ball
 
   --let's create a ball
   bodies[1] = 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 = {}
   shapes[1] = love.physics.newCircleShape(bodies[1], 0, 0, 20) --the ball has a radius of 20
+
  objects.ball.body = love.physics.newBody(world, 650/2, 650/2, "dynamic") --place the body in the center of the world and make it dynamic, so it can move around
 +
   objects.ball.shape = love.physics.newCircleShape( 20) --the ball's shape has a radius of 20
 +
  objects.ball.fixture = love.physics.newFixture(objects.ball.body, objects.ball.shape, 1) -- Attach fixture to body and give it a density of 1.
 +
  objects.ball.fixture:setRestitution(0.9) --let the ball bounce
 +
 
 +
  --let's create a couple blocks to play around with
 +
  objects.block1 = {}
 +
  objects.block1.body = love.physics.newBody(world, 200, 550, "dynamic")
 +
  objects.block1.shape = love.physics.newRectangleShape(0, 0, 50, 100)
 +
  objects.block1.fixture = love.physics.newFixture(objects.block1.body, objects.block1.shape, 5) -- A higher density gives it more mass.
 +
 
 +
  objects.block2 = {}
 +
  objects.block2.body = love.physics.newBody(world, 200, 400, "dynamic")
 +
  objects.block2.shape = love.physics.newRectangleShape(0, 0, 100, 50)
 +
  objects.block2.fixture = love.physics.newFixture(objects.block2.body, objects.block2.shape, 2)
 
</source>
 
</source>
  
Now to wrap up the love.load function, let's set up the screen size and background color.
+
Now to wrap up the [[love.load]]() function, let's set up the screen size and background color.
  
 
<source lang="lua">
 
<source lang="lua">
 
   --initial graphics setup
 
   --initial graphics setup
   love.graphics.setBackgroundColor(104, 136, 248) --set the background color to a nice blue
+
   love.graphics.setBackgroundColor(0.41, 0.53, 0.97) --set the background color to a nice blue
   love.graphics.setMode(650, 650, false, true, 0) --set the window dimensions to 650 by 650
+
   love.window.setMode(650, 650) --set the window dimensions to 650 by 650 with no fullscreen, vsync on, and no antialiasing
 
end
 
end
 
</source>
 
</source>
  
 
Okay, that's enough for the initial set up of the physics engine.
 
Okay, that's enough for the initial set up of the physics engine.
Now we need do edit the [[love.update]]() function.
+
Now we need to edit the [[love.update]]() function.  
  
 
=== love.update() ===
 
=== love.update() ===
Line 49: Line 64:
 
   --here we are going to create some keyboard events
 
   --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
 
   if love.keyboard.isDown("right") then --press the right arrow key to push the ball to the right
     bodies[1]:applyForce(400, 0)
+
     objects.ball.body:applyForce(400, 0)
 
   elseif love.keyboard.isDown("left") then --press the left arrow key to push the ball to the left
 
   elseif love.keyboard.isDown("left") then --press the left arrow key to push the ball to the left
     bodies[1]:applyForce(-400, 0)
+
     objects.ball.body:applyForce(-400, 0)
 
   elseif love.keyboard.isDown("up") then --press the up arrow key to set the ball in the air
 
   elseif love.keyboard.isDown("up") then --press the up arrow key to set the ball in the air
     bodies[1]:setY(650/2)
+
     objects.ball.body:setPosition(650/2, 650/2)
 +
    objects.ball.body:setLinearVelocity(0, 0) --we must set the velocity to zero to prevent a potentially large velocity generated by the change in position
 
   end
 
   end
 
end
 
end
Line 61: Line 77:
  
 
=== love.draw() ===
 
=== love.draw() ===
 +
 +
First, the ground.
  
 
<source lang="lua">
 
<source lang="lua">
 
function love.draw()
 
function love.draw()
   local x1, y1, x2, y2, x3, y3, x4, y4 = shapes[0]:getBoundingBox() --get the x,y coordinates of all 4 corners of the box.
+
   love.graphics.setColor(0.28, 0.63, 0.05) -- set the drawing color to green for the ground
 +
  love.graphics.polygon("fill", objects.ground.body:getWorldPoints(objects.ground.shape:getPoints())) -- draw a "filled in" polygon using the ground's coordinates
 
</source>
 
</source>
  
Here we grab the coordinates of the four corners of the box that represents the ground to be able to draw the ground. In the segment above...
+
And finally, we can draw the circle that represents the ball and the blocks.
  x1, y1 represents the bottom left corner of the bounding box
 
  x2, y2 represents the top left corner of the bounding box
 
  x3, y3 represents the top right corner of the bounding box
 
  x4, y4 represents the bottom right corner of the boudning box
 
  
Now we will do a little math to calculate the height and width of the box that represents the ground.
+
<source lang="lua">
 +
  love.graphics.setColor(0.76, 0.18, 0.05) --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())
  
<source lang="lua">
+
   love.graphics.setColor(0.20, 0.20, 0.20) -- set the drawing color to grey for the blocks
   local boxwidth = x3 - x2 --calculate the width of the box
+
  love.graphics.polygon("fill", objects.block1.body:getWorldPoints(objects.block1.shape:getPoints()))
   local boxheight = y2 - y1 --calculate the height of the box
+
   love.graphics.polygon("fill", objects.block2.body:getWorldPoints(objects.block2.shape:getPoints()))
 +
end
 
</source>
 
</source>
  
Pretty simple. Now that we are ready to draw the box for the ground, we must remember that [[love.graphics.rectangle]]() draws from the upper-left hand corner, and that the coordinates [[love.physics.Body]]() is anchored at the center.
+
There you have it! Put this file in a [[zip|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.
  
 +
[[Image:Tutorial-PhysicsBall.jpg|thumb|Screenshot of the mostly finished product.]]
  
This is and example to help you understand!
 
[[Image:Love ex physics explaination.png|450px]]
 
 
 
So we need to compensate so that the two will match up.
 
  
 +
== The main.lua ==
 
<source lang="lua">
 
<source lang="lua">
  love.graphics.setColor(72, 160, 14) --set the drawing color to green for the ground
+
function love.load()
   love.graphics.rectangle("fill", bodies[0]:getX() - boxwidth/2, bodies[0]:getY() - boxheight/2, boxwidth, boxheight)
+
  -- the height of a meter our worlds will be 64px
</source>
+
   love.physics.setMeter(64)
 +
  -- create a world for the bodies to exist in with horizontal gravity
 +
  -- of 0 and vertical gravity of 9.81
 +
  world = love.physics.newWorld(0, 9.81*64, true)
  
And finally, we can draw the circle that represents the ball. Since [[love.graphics.circle]]() has coordinates that are anchored at the center of the circle, we do not need to compensate here like we did for the rectangle.
+
  objects = {} -- table to hold all our physical objects
  
<source lang="lua">
+
  -- let's create the ground
   love.graphics.setColor(193, 47, 14) --set the drawing color to red for the ball
+
  objects.ground = {}
   love.graphics.circle("fill", bodies[1]:getX(), bodies[1]:getY(), shapes[1]:getRadius(), 20)
+
  -- remember, the shape (the rectangle we create next) anchors to the
end
+
  -- body from its center, so we have to move it to (650/2, 650-50/2)
</source>
+
   objects.ground.body = love.physics.newBody(world, 650/2, 650-50/2)
 +
  -- make a rectangle with a width of 650 and a height of 50
 +
   objects.ground.shape = love.physics.newRectangleShape(650, 50)
 +
  -- attach shape to body
 +
  objects.ground.fixture = love.physics.newFixture(objects.ground.body,
 +
                                                  objects.ground.shape)
  
There you have it! Put this file in a [[zip|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.
+
  -- let's create a ball
 +
  objects.ball = {}
 +
  -- place the body in the center of the world and make it dynamic, so
 +
  -- it can move around
 +
  objects.ball.body = love.physics.newBody(world, 650/2, 650/2, "dynamic")
 +
  -- the ball's shape has a radius of 20
 +
  objects.ball.shape = love.physics.newCircleShape(20)
 +
  -- Attach fixture to body and give it a density of 1.
 +
  objects.ball.fixture = love.physics.newFixture(objects.ball.body,
 +
                                                objects.ball.shape, 1)
 +
  objects.ball.fixture:setRestitution(0.9) -- let the ball bounce
  
[[Image:Tutorial-PhysicsBall.jpg|thumb|Screenshot of the finished product.]]
+
  -- let's create a couple blocks to play around with
 +
  objects.block1 = {}
 +
  objects.block1.body = love.physics.newBody(world, 200, 550, "dynamic")
 +
  objects.block1.shape = love.physics.newRectangleShape(0, 0, 50, 100)
 +
  -- A higher density gives it more mass.
 +
  objects.block1.fixture = love.physics.newFixture(objects.block1.body,
 +
                                                  objects.block1.shape, 5)
  
 +
  objects.block2 = {}
 +
  objects.block2.body = love.physics.newBody(world, 200, 400, "dynamic")
 +
  objects.block2.shape = love.physics.newRectangleShape(0, 0, 100, 50)
 +
  objects.block2.fixture = love.physics.newFixture(objects.block2.body,
 +
                                                  objects.block2.shape, 2)
  
== The main.lua ==
+
   -- initial graphics setup
<source lang="lua">
+
   -- set the background color to a nice blue
function love.load()
+
   love.graphics.setBackgroundColor(0.41, 0.53, 0.97)
   world = love.physics.newWorld(650, 650) --create a world for the bodies to exist in with width and height of 650
+
   love.window.setMode(650, 650) -- set the window dimensions to 650 by 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
 
 
 
  bodies = {} --create tables for the bodies and shapes so that the garbage collector doesn't delete them
 
  shapes = {}
 
 
 
  --let's create the ground
 
  --we need to give the ground a mass of zero so that the ground wont move
 
   bodies[0] = love.physics.newBody(world, 650/2, 625, 0, 0) --remember, the body anchors from the center of the shape
 
  shapes[0] = love.physics.newRectangleShape(bodies[0], 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
 
  bodies[1] = love.physics.newBody(world, 650/2, 650/2, 15, 0) --place the body in the center of the world, with a mass of 15
 
  shapes[1] = love.physics.newCircleShape(bodies[1], 0, 0, 20) --the ball 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
 
end
  
  
 
function love.update(dt)
 
function love.update(dt)
   world:update(dt) --this puts the world into motion
+
   world:update(dt) -- this puts the world into motion
 
+
 
   --here we are going to create some keyboard events
+
   -- 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
+
   -- press the right arrow key to push the ball to the right
    bodies[1]:applyForce(400, 0)
+
   if love.keyboard.isDown("right") then
   elseif love.keyboard.isDown("left") then --press the left arrow key to push the ball to the left
+
    objects.ball.body:applyForce(400, 0)
     bodies[1]:applyForce(-400, 0)
+
  -- press the left arrow key to push the ball to the left
   elseif love.keyboard.isDown("up") then --press the up arrow key to set the ball in the air
+
  elseif love.keyboard.isDown("left") then
     bodies[1]:setY(650/2)
+
     objects.ball.body:applyForce(-400, 0)
 +
  -- press the up arrow key to set the ball in the air
 +
   elseif love.keyboard.isDown("up") then
 +
    objects.ball.body:setPosition(650/2, 650/2)
 +
    -- we must set the velocity to zero to prevent a potentially large
 +
    -- velocity generated by the change in position
 +
     objects.ball.body:setLinearVelocity(0, 0)
 
   end
 
   end
 
end
 
end
  
 
function love.draw()
 
function love.draw()
   local x1, y1, x2, y2, x3, y3, x4, y4 = shapes[0]:getBoundingBox() --get the x,y coordinates of all 4 corners of the box.
+
   -- set the drawing color to green for the ground
   --x1, y1 represent the bottom left corner of the bounding box
+
   love.graphics.setColor(0.28, 0.63, 0.05)
   --x2, y2 represent the top left corner of the bounding box
+
   -- draw a "filled in" polygon using the ground's coordinates
   --x3, y3 represent the top right corner of the bounding box
+
   love.graphics.polygon("fill", objects.ground.body:getWorldPoints(
  --x4, y4 represent the top right corner of the boudning box
+
                          objects.ground.shape:getPoints()))
  local boxwidth = x3 - x2 --calculate the width of the box
+
 
   local boxheight = y2 - y1 --calculate the height of the box
+
   -- set the drawing color to red for the ball
   love.graphics.setColor(72, 160, 14) --set the drawing color to green for the ground
+
   love.graphics.setColor(0.76, 0.18, 0.05)
  --the rectangle is drawing from the top left corner
+
   love.graphics.circle("fill", objects.ball.body:getX(),
  --so we need to compensate for that
+
                      objects.ball.body:getY(), objects.ball.shape:getRadius())
   love.graphics.rectangle("fill", bodies[0]:getX() - boxwidth/2, bodies[0]:getY() - boxheight/2, boxwidth, boxheight)
+
 
  love.graphics.setColor(193, 47, 14) --set the drawing color to red for the ball
+
  -- set the drawing color to grey for the blocks
   --the circle is drawing from the center
+
   love.graphics.setColor(0.20, 0.20, 0.20)
  --so we do not need to compensate
+
   love.graphics.polygon("fill", objects.block1.body:getWorldPoints(
   love.graphics.circle("fill", bodies[1]:getX(), bodies[1]:getY(), shapes[1]:getRadius(), 20)
+
                          objects.block1.shape:getPoints()))
 +
  love.graphics.polygon("fill", objects.block2.body:getWorldPoints(
 +
                          objects.block2.shape:getPoints()))
 
end
 
end
 
</source>
 
</source>
 
[[Category:Tutorials]]
 
[[Category:Tutorials]]
  
{{#set:LOVE Version=0.6.1}}
+
{{#set:LOVE Version=0.10.0}}
 
{{#set:Description=A tutorial for learning to use [[love.physics]].}}
 
{{#set:Description=A tutorial for learning to use [[love.physics]].}}
 +
 +
== Other languages ==
 +
{{i18n|Tutorial:Physics}}

Latest revision as of 12:19, 25 September 2019

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()
  love.physics.setMeter(64) --the height of a meter our worlds will be 64px
  world = love.physics.newWorld(0, 9.81*64, true) --create a world for the bodies to exist in with horizontal gravity of 0 and vertical gravity of 9.81

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

   objects = {} -- table to hold all our physical objects
  
  --let's create the ground
  objects.ground = {}
  objects.ground.body = love.physics.newBody(world, 650/2, 650-50/2) --remember, the shape (the rectangle we create next) anchors to the body from its center, so we have to move it to (650/2, 650-50/2)
  objects.ground.shape = love.physics.newRectangleShape(650, 50) --make a rectangle with a width of 650 and a height of 50
  objects.ground.fixture = love.physics.newFixture(objects.ground.body, objects.ground.shape) --attach shape to body
  
  --let's create a ball
  objects.ball = {}
  objects.ball.body = love.physics.newBody(world, 650/2, 650/2, "dynamic") --place the body in the center of the world and make it dynamic, so it can move around
  objects.ball.shape = love.physics.newCircleShape( 20) --the ball's shape has a radius of 20
  objects.ball.fixture = love.physics.newFixture(objects.ball.body, objects.ball.shape, 1) -- Attach fixture to body and give it a density of 1.
  objects.ball.fixture:setRestitution(0.9) --let the ball bounce

  --let's create a couple blocks to play around with
  objects.block1 = {}
  objects.block1.body = love.physics.newBody(world, 200, 550, "dynamic")
  objects.block1.shape = love.physics.newRectangleShape(0, 0, 50, 100)
  objects.block1.fixture = love.physics.newFixture(objects.block1.body, objects.block1.shape, 5) -- A higher density gives it more mass.

  objects.block2 = {}
  objects.block2.body = love.physics.newBody(world, 200, 400, "dynamic")
  objects.block2.shape = love.physics.newRectangleShape(0, 0, 100, 50)
  objects.block2.fixture = love.physics.newFixture(objects.block2.body, objects.block2.shape, 2)

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

  --initial graphics setup
  love.graphics.setBackgroundColor(0.41, 0.53, 0.97) --set the background color to a nice blue
  love.window.setMode(650, 650) --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 to 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:setPosition(650/2, 650/2)
    objects.ball.body:setLinearVelocity(0, 0) --we must set the velocity to zero to prevent a potentially large velocity generated by the change in position
  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(0.28, 0.63, 0.05) -- set the drawing color to green for the ground
  love.graphics.polygon("fill", objects.ground.body:getWorldPoints(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 and the blocks.

  love.graphics.setColor(0.76, 0.18, 0.05) --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())

  love.graphics.setColor(0.20, 0.20, 0.20) -- set the drawing color to grey for the blocks
  love.graphics.polygon("fill", objects.block1.body:getWorldPoints(objects.block1.shape:getPoints()))
  love.graphics.polygon("fill", objects.block2.body:getWorldPoints(objects.block2.shape:getPoints()))
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 mostly finished product.


The main.lua

function love.load()
  -- the height of a meter our worlds will be 64px
  love.physics.setMeter(64)
  -- create a world for the bodies to exist in with horizontal gravity
  -- of 0 and vertical gravity of 9.81
  world = love.physics.newWorld(0, 9.81*64, true)

  objects = {} -- table to hold all our physical objects

  -- let's create the ground
  objects.ground = {}
  -- remember, the shape (the rectangle we create next) anchors to the
  -- body from its center, so we have to move it to (650/2, 650-50/2)
  objects.ground.body = love.physics.newBody(world, 650/2, 650-50/2)
  -- make a rectangle with a width of 650 and a height of 50
  objects.ground.shape = love.physics.newRectangleShape(650, 50)
  -- attach shape to body
  objects.ground.fixture = love.physics.newFixture(objects.ground.body,
                                                   objects.ground.shape)

  -- let's create a ball
  objects.ball = {}
  -- place the body in the center of the world and make it dynamic, so
  -- it can move around
  objects.ball.body = love.physics.newBody(world, 650/2, 650/2, "dynamic")
  -- the ball's shape has a radius of 20
  objects.ball.shape = love.physics.newCircleShape(20)
  -- Attach fixture to body and give it a density of 1.
  objects.ball.fixture = love.physics.newFixture(objects.ball.body,
                                                 objects.ball.shape, 1)
  objects.ball.fixture:setRestitution(0.9) -- let the ball bounce

  -- let's create a couple blocks to play around with
  objects.block1 = {}
  objects.block1.body = love.physics.newBody(world, 200, 550, "dynamic")
  objects.block1.shape = love.physics.newRectangleShape(0, 0, 50, 100)
  -- A higher density gives it more mass.
  objects.block1.fixture = love.physics.newFixture(objects.block1.body,
                                                   objects.block1.shape, 5)

  objects.block2 = {}
  objects.block2.body = love.physics.newBody(world, 200, 400, "dynamic")
  objects.block2.shape = love.physics.newRectangleShape(0, 0, 100, 50)
  objects.block2.fixture = love.physics.newFixture(objects.block2.body,
                                                   objects.block2.shape, 2)

  -- initial graphics setup
  -- set the background color to a nice blue
  love.graphics.setBackgroundColor(0.41, 0.53, 0.97)
  love.window.setMode(650, 650) -- 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
  -- press the right arrow key to push the ball to the right
  if love.keyboard.isDown("right") then
    objects.ball.body:applyForce(400, 0)
  -- press the left arrow key to push the ball to the left
  elseif love.keyboard.isDown("left") then
    objects.ball.body:applyForce(-400, 0)
  -- press the up arrow key to set the ball in the air
  elseif love.keyboard.isDown("up") then
    objects.ball.body:setPosition(650/2, 650/2)
    -- we must set the velocity to zero to prevent a potentially large
    -- velocity generated by the change in position
    objects.ball.body:setLinearVelocity(0, 0)
  end
end

function love.draw()
  -- set the drawing color to green for the ground
  love.graphics.setColor(0.28, 0.63, 0.05)
  -- draw a "filled in" polygon using the ground's coordinates
  love.graphics.polygon("fill", objects.ground.body:getWorldPoints(
                           objects.ground.shape:getPoints()))

  -- set the drawing color to red for the ball
  love.graphics.setColor(0.76, 0.18, 0.05)
  love.graphics.circle("fill", objects.ball.body:getX(),
                       objects.ball.body:getY(), objects.ball.shape:getRadius())

  -- set the drawing color to grey for the blocks
  love.graphics.setColor(0.20, 0.20, 0.20)
  love.graphics.polygon("fill", objects.block1.body:getWorldPoints(
                           objects.block1.shape:getPoints()))
  love.graphics.polygon("fill", objects.block2.body:getWorldPoints(
                           objects.block2.shape:getPoints()))
end



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