Tutorial 11 - Physics 101

Main.lua

supportedOrientations(ANY)

function setup()

   -- Physics Demo Metadata

   version = 1.0

   saveProjectInfo("Description", "Physics v"..version)
   saveProjectInfo("Author", "Reefwing Software")
   saveProjectInfo("Date", "30th July 2012")
   saveProjectInfo("Version", version)

   print("Physics Demo v"..version.."\n")

   -- Load the sprite you want to attach to your physics object
   -- you want the sprite and the object to be roughly the same
   -- size and shape.

   img = readImage("Dropbox:Mine_64x64")

   -- Create an instance of PhysicsDebugDraw(). You will need to copy
   -- this tab from the Physics Lab project or duplicate the project
   -- and make it a dependency to your project. We have modified the 
   -- code to allow you to make the ground invisible and attach a 
   -- sprite. A copy of the modified class is provided below.

   physicsDraw = PhysicsDebugDraw()
   physicsDraw.spriteImage = img
   physicsDraw.staticVisible = false

   -- These functions are from the Physics Lab example which we have
   -- extracted and provided in the Physics file below. As our sprite
   -- is circular we attach it to the circle sprite.

   createGround()
   createBox(WIDTH/2, 100, 30, 30)
   createCircle(WIDTH/2 + 50, HEIGHT, 32)
   createRandPoly(WIDTH/2 + 150, 120)

end

function draw()

   -- This sets the background color to black

   background(blackColor)

   -- Draw Demo Physics objects

   physicsDraw:draw()

end

function collide(contact)

   -- Leave this function out if you don't want to show the red dots 
   -- which indicate the collision points.

   physicsDraw:collide(contact)

end

function touched(touch)

   -- Use this method to demonstrate the pointInRect() collision detection function

   if touch.state == ENDED then
       
       -- Create a random polygon wherever you tap the screen.

       createRandPoly(touch.x, touch.y, 25, 25)

   end

end

Physics.lua

--# Physics
--
-- Functions from the Physics Lab example project
-- courtesy of Two Lives Left

function createCircle(x,y,r)
   local circle = physics.body(CIRCLE, r)
   -- enable smooth motion
   circle.interpolate = true
   circle.x = x
   circle.y = y
   circle.restitution = 0.25
   circle.sleepingAllowed = false
   physicsDraw:addBody(circle)
   return circle
end

function createBox(x,y,w,h)
   -- polygons are defined by a series of points in counter-clockwise order
   local box = physics.body(POLYGON, vec2(-w/2,h/2), vec2(-w/2,-h/2), vec2(w/2,-h/2), vec2(w/2,h/2))
   box.interpolate = true
   box.x = x
   box.y = y
   box.restitutions = 0.25
   box.sleepingAllowed = false
   physicsDraw:addBody(box)
   return box
end

function createGround()
   local ground = physics.body(POLYGON, vec2(0,20), vec2(0,0), vec2(WIDTH,0), vec2(WIDTH,20))
   ground.type = STATIC
   physicsDraw:addBody(ground)
   return ground
end

function createRandPoly(x,y)
   local count = math.random(3,10)
   local r = math.random(25,75)
   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.sleepingAllowed = false
   poly.restitution = 0.25
   physicsDraw:addBody(poly)
   return poly
end

function cleanup()
   clearOutput()
   physicsDraw:clear()
end

PhysicsDebugDraw.lua

--# PhysicsDebugDraw
--
-- Class extracted from the Physics Lab project.
-- courtesy of Two Lives Left
--
-- Modified by Reefwing Software
-- Version 1.0

PhysicsDebugDraw = class()

function PhysicsDebugDraw:init()
   self.bodies = {}
   self.joints = {}
   self.touchMap = {}
   self.contacts = {}
   self.spriteImage = nil     -- spriteImage parameter added
   self.staticVisible = true  -- used to make ground invisible if required.
end

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

function PhysicsDebugDraw:addJoint(joint)
   table.insert(self.joints,joint)
end

function PhysicsDebugDraw:clear()
   -- deactivate all bodies

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

   for i,joint in ipairs(self.joints) do
       joint:destroy()
   end

   self.bodies = {}
   self.joints = {}
   self.contacts = {}
   self.touchMap = {}
end

function PhysicsDebugDraw:draw()

   pushStyle()
   smooth()
   strokeWidth(5)
   stroke(128,0,128)

   local gain = 2.0
   local damp = 0.5
   for k,v in pairs(self.touchMap) do
       local worldAnchor = v.body:getWorldPoint(v.anchor)
       local touchPoint = v.tp
       local diff = touchPoint - worldAnchor
       local vel = v.body:getLinearVelocityFromWorldPoint(worldAnchor)
       v.body:applyForce( (1/1) * diff * gain - vel * damp, worldAnchor)

       line(touchPoint.x, touchPoint.y, worldAnchor.x, worldAnchor.y)
   end

   stroke(0,255,0,255)
   strokeWidth(5)
   for k,joint in pairs(self.joints) do
       local a = joint.anchorA
       local b = joint.anchorB
       line(a.x,a.y,b.x,b.y)
   end

   stroke(255,255,255,255)
   noFill()


   for i,body in ipairs(self.bodies) do
       pushMatrix()
       translate(body.x, body.y)
       rotate(body.angle)

       if body.type == STATIC then
           if self.staticVisible then
               stroke(255,255,255,255)
           else
               stroke(0,0,0,0)
           end
       elseif body.type == DYNAMIC then
           stroke(150,255,150,255)
       elseif body.type == KINEMATIC then
           stroke(150,150,255,255)
       end

       if body.shapeType == POLYGON then
           strokeWidth(5.0)
           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(5.0)
           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
           if self.spriteImage == nil then
               strokeWidth(5.0)
               line(0,0,body.radius-3,0)
               strokeWidth(2.5)
               ellipse(0,0,body.radius*2)
           else
               sprite(self.spriteImage, 0, 0, body.radius*2)
           end
       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 PhysicsDebugDraw:touched(touch)
   local touchPoint = vec2(touch.x, touch.y)
   if touch.state == BEGAN then
       for i,body in ipairs(self.bodies) do
           if body.type == DYNAMIC and body:testPoint(touchPoint) then
               self.touchMap[touch.id] = {tp = touchPoint, body = body, anchor = body:getLocalPoint(touchPoint)}
               return true
           end
       end
   elseif touch.state == MOVING and self.touchMap[touch.id] then
       self.touchMap[touch.id].tp = touchPoint
       return true
   elseif touch.state == ENDED and self.touchMap[touch.id] then
       self.touchMap[touch.id] = nil
       return true;
   end
   return false
end

function PhysicsDebugDraw: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