Tutorial 31 - Codea Collision Detection

Main.lua

--# Main
-- Asteroids
--
-- Simple app to demonstrate collision detection using
-- the built in Box2D functionality.

-- Use this function to perform your initial setup

function setup()

   physics.gravity(0.0, 0.0)
   physicsDraw = PhysicsDraw()
   ship = createShip(WIDTH/2, HEIGHT/2)
   physicsDraw:addBody(ship)
   asteroidTimer = 1

end

function createShip(x, y, w, h)

   -- (x, y) are the centre co-ordinates of the ship.
   -- (w, h) define the width and height of the box containing the ship,
   -- if not specified they are 50 and 35 respectively.

   local width = w or 50
   local height = h or 35
   local shipBody = physics.body(POLYGON, vec2(0, height), vec2(width, height/2), 
                                       vec2(0, 0), vec2(height/2, height/2), vec2(0, height))

   shipBody.x = x
   shipBody.y = y
   shipBody.type = KINEMATIC
   shipBody.sleepingAllowed = false
   shipBody.info = "ship"
   shipBody.interpolate = true

   return shipBody

end

function createRandPoly(x, y, s1, s2)

   -- s1 and s2 define the range of length for the poly sides
   -- count defines the number of sides of the poly

   local minLength = s1 or 1
   local maxLength = s2 or 5
   local count = math.random(5,10)
   local r = math.random(minLength, maxLength)
   local a = 0
   local d = 2 * math.pi / count
   local points = {}

   for i = 1,count do
       local v = vec2(r,0):rotate(a) + vec2(math.random(-10,10), math.random(-10,10))
       a = a + d
       table.insert(points, v)
   end

   local poly = physics.body(POLYGON, unpack(points))

   poly.x = x
   poly.y = y
   poly.type = DYNAMIC
   poly.sleepingAllowed = false
   poly.restitution = 0.5
   poly.info = "poly"
   physicsDraw:addBody(poly)

   return poly

end

function createAsteroid(x, y)

   return createRandPoly(x, y, 25, 65)

end

function placeRandomAsteroid()

   -- Generate (x, y) co-ordinates which are
   -- initially off the screen.

   local x = math.random(WIDTH)

   if x < WIDTH/2 then
       x = x - WIDTH
   else
       x = x + WIDTH
   end

   local y = math.random(HEIGHT)

   if y < HEIGHT/2 then
       y = y - HEIGHT
   else
       y = y + HEIGHT
   end

   -- Create an asteroid at the new co-ordinates

   local asteroid = createAsteroid(x, y)

   -- Give the asteroid a linear velocity
   -- towards the screen (and our ship)

   local dX, dY = math.random(50, 100), math.random(50, 100)

   if x > WIDTH then
       dX = -dX
   end

   if y > HEIGHT then
       dY = -dY
   end

   asteroid.linearVelocity = vec2(dX, dY)

end

function createExplosionAt(x, y)

   -- Creates 6 small polygon moving out from (x, y)
   -- to simulate our ship exploding.
   --
   -- the body.info field is set to debris so that we
   -- can render it the same colour as our ship (i.e. to
   -- distinguish them from the asteroid polygons.

   randPoly1 = createRandPoly(x, y)
   randPoly1:applyForce(vec2(50,50))
   randPoly1.info = "debris"
   randPoly2 = createRandPoly(x, y)
   randPoly2:applyForce(vec2(50,50))
   randPoly2.info = "debris"
   randPoly3 = createRandPoly(x, y)
   randPoly3:applyForce(vec2(50,50))
   randPoly3.info = "debris"
   randPoly4 = createRandPoly(x, y)
   randPoly4:applyForce(vec2(-50,50))
   randPoly4.info = "debris"
   randPoly5 = createRandPoly(x, y)
   randPoly5:applyForce(vec2(-50,50))
   randPoly5.info = "debris"
   randPoly6 = createRandPoly(x, y)
   randPoly6:applyForce(vec2(-50,50))     
   randPoly6.info = "debris"   

end

-- This function gets called once every frame

function draw()

   -- This sets a dark background color 

   background(40, 40, 50)

   -- Generate a random asteroid every second

   asteroidTimer = asteroidTimer + DeltaTime

   if asteroidTimer > 1.0 then
       placeRandomAsteroid()
       asteroidTimer = 0
   end

   -- Do your drawing here

   physicsDraw:draw()

end

-- Collision Detection
--
-- This is simply a matter of checking whether one of the two
-- colliding bodies are our ship.

function collide(contact)

   if contact.bodyA == ship or contact.bodyB == ship then
       createExplosionAt(ship.x, ship.y)
       ship.info = "destroyed"
       ship = nil
   end

end

PhysicsDraw.lua

--# PhysicsDraw
PhysicsDraw = class()

-- Simplified Physics Debug Draw class from the Physics Lab
-- example in Codea.

function PhysicsDraw:init()
   self.bodies = {}
   self.contacts = {}
end

function PhysicsDraw:addBody(body)
   table.insert(self.bodies,body)
end

function PhysicsDraw:draw()

   pushStyle()
   smooth()
   noFill()

   for i,body in ipairs(self.bodies) do

       pushMatrix()
       translate(body.x, body.y)
       rotate(body.angle)

       if body.type == STATIC then
           stroke(255,255,255,255)
       elseif body.type == DYNAMIC and body.info ~= "debris" then
           stroke(150,255,150,255)
       else
           stroke(150,150,255,255)
       end

       if body.shapeType == POLYGON and body.info ~= "destroyed" then
           strokeWidth(3.0)
           local points = body.points
           for j = 1,#points do
               a = points[j]
               b = points[(j % #points)+1]
               line(a.x, a.y, b.x, b.y)
           end
       elseif body.shapeType == CHAIN or body.shapeType == EDGE then
           strokeWidth(3.0)
           local points = body.points
           for j = 1,#points-1 do
               a = points[j]
               b = points[j+1]
               line(a.x, a.y, b.x, b.y)
           end      
       elseif body.shapeType == CIRCLE then
           strokeWidth(3.0)
           line(0,0,body.radius-3,0)            
           ellipse(0,0,body.radius*2)
       end

       popMatrix()
   end 

   stroke(255, 0, 0, 255)
   fill(255, 0, 0, 255)

   for k,v in pairs(self.contacts) do
       for m,n in ipairs(v.points) do
           ellipse(n.x, n.y, 10, 10)
       end
   end

   popStyle()
end

function PhysicsDraw:collide(contact)
   if contact.state == BEGAN then
       self.contacts[contact.id] = contact
       sound(SOUND_HIT, 2643)
   elseif contact.state == MOVING then
       self.contacts[contact.id] = contact
   elseif contact.state == ENDED then
       self.contacts[contact.id] = nil
   end
end