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Posted: **Tue Jan 25, 2011 4:50 am**

I've been having a problem with the Box2D system in Love. Most everything works properly, except what you might call rotational falls. Falls like when a box has passed its center of mass over a ledge, but doesn't fall. I have explicitly setFixedRotation(false) on all objects. I have even tried starting out one of the blocks with an initial angle of somewhere around 120 degrees, but it still remains constant. Any ideas? Also the "theta-meter" at the top is getAngle-ing the top most brick just in case it was a rendering glitch, which it isn't.

Posted: **Tue Jan 25, 2011 5:09 am**

Are you perhaps accidentally using setAngle somewhere? That could create the behavior I think you described.

The easiest thing to do would be to post the code you have so far so we can look at it and see exactly whats happening.

Posted: **Tue Jan 25, 2011 5:12 am**

Right, I knew I forgot to post something.

Code: Select all

-- Object layout:
-- id = stringy-name, body = physics-body, shape = physics-shape, sprite = string-index-into-sprite-table

function love.load()
   world = love.physics.newWorld(0, 0, 1024, 600)
   world:setGravity(0, 700)
   world:setMeter(64)
   
   -- Open up a bottle of sprite(s)
   sprites = {}
   sprites["floor"] = love.graphics.newImage("floor.png")
   sprites["man"] = love.graphics.newImage("man.png")
   sprites["brick"] = love.graphics.newImage("brick.png")

   -- Manufacture some delicious objects
   objects = {}
   objects[0] = createRectangleObject("floor", 1024/2, 575, 1024, 50, 0, world, "floor")
   objects[1] = createCircleObject("man", 1024/2, 600/2, 32, 15, world, "man")
   objects[2] = createRectangleObject("brick1", 600, 500, 30, 16, 4, world, "brick")
   objects[3] = createRectangleObject("brick2", 600, 435, 30, 16, 4, world, "brick")
   objects[4] = createRectangleObject("brick3", 600, 484, 30, 16, 4, world, "brick")

   objects[2]["body"]:setFixedRotation(false)
   objects[3]["body"]:setFixedRotation(false)
   objects[4]["body"]:setFixedRotation(false)

   for id = 1, 4 do
      objects[id]["shape"]:setFriction(0.25)
   end
   objects[1]["shape"]:setFriction(0.17)

   local f = love.graphics.newFont(12)
   love.graphics.setFont(f)
   
   love.graphics.setBackgroundColor(131,192,240)
   love.graphics.setMode(1024, 600, false, true, 0)
   
end

function love.update(dt)
   world:update(dt)
   
   if love.keyboard.isDown("up") then
      objects[1]["body"]:applyForce(0, -1000)
   end
   

   if love.keyboard.isDown("right") then
      objects[1]["body"]:applyForce(300,0)
   elseif love.keyboard.isDown("left") then
      objects[1]["body"]:applyForce(-300,0)
   elseif love.keyboard.isDown("escape") then
      love.event.push('q')
   end
end

function love.draw()
   local boxwidth, boxheight = getBoxDims(objects[0]["shape"])
   love.graphics.print("theta", 10, 10)
   love.graphics.print(objects[3]["body"]:getAngle(), 50, 50)
   love.graphics.draw(sprites[objects[0]["sprite"]], objects[0]["body"]:getX() - boxwidth / 2, objects[0]["body"]:getY() - boxheight / 2)
   renderCircleObject(1,0)
   renderRectanlgeObject(2)
   renderRectanlgeObject(3)
   renderRectanlgeObject(4)
end

function getBoxDims(shape)
   local x1, y1, x2, y2, x3, y3, x4, y4 = shape:getBoundingBox()
   local boxwidth = x3 - x2 --calculate the width of the box
   local boxheight = (-1) * (y2 - y1) --calculate the height of the box
   return boxwidth, boxheight
end

function createRectangleObject(name, posx, posy, width, height, mass, worldT, spriteID)
   obj = {}
   obj["id"] = name
   obj["body"] = love.physics.newBody(worldT, posx, posy, mass, 0)
   obj["shape"] = love.physics.newRectangleShape(obj["body"], 0, 0, width, height, 0)
   obj["sprite"] = spriteID
   return obj
end

function createCircleObject(name, posx, posy, radius, mass, worldT, spriteID)
   obj = {}
   obj["id"] = name
   obj["body"] = love.physics.newBody(worldT, posx, posy, mass, 0)
   obj["shape"] = love.physics.newCircleShape(obj["body"], 0, 0, radius)
   obj["sprite"] = spriteID
   return obj
end

function renderRectanlgeObject(id)
   love.graphics.push()
   love.graphics.translate(objects[id]["body"]:getX(), objects[id]["body"]:getY())
   love.graphics.rotate(objects[id]["body"]:getAngle())
   local boxwidth, boxheight = getBoxDims(objects[id]["shape"])
   love.graphics.draw(sprites[objects[id]["sprite"]], -boxwidth / 2, -boxheight / 2)   
   love.graphics.pop()
end

function renderCircleObject(id, dOffset)
   if dOffset == nil then
      dOffset = 0
   end
   love.graphics.push()
   love.graphics.translate(objects[id]["body"]:getX(), objects[id]["body"]:getY())
   love.graphics.rotate(math.rad((objects[id]["body"]:getX() + dOffset) % 360))
   
   local radius = objects[id]["shape"]:getRadius()
   
   love.graphics.draw(sprites[objects[id]["sprite"]], -radius, -radius)
   love.graphics.pop()
end

No setAngle (although I did use setAngle to tip the box on its side to see if it would fall over; it didn't)

Posted: **Tue Jan 25, 2011 5:39 am**

The first thing I can think of is, are you setting inertia? Inertia makes objects behave a lot more natural (say a level of 20-50), and usually helps to stop problems like this. With a quick glance at your code, I don't think you are, as you are passing 0 as the last argument (inertia) for the creation of your bodies. Try bumping that up to a number of 20-50 and see what it does.

Posted: **Tue Jan 25, 2011 6:04 am**

Code: Select all

   objects[2]["body"]:setMassFromShapes()
   objects[3]["body"]:setMassFromShapes()
   objects[4]["body"]:setMassFromShapes()

Posted: **Tue Jan 25, 2011 11:08 am**

BlackBulletIV wrote:The first thing I can think of is, are you setting inertia? Inertia makes objects behave a lot more natural (say a level of 20-50), and usually helps to stop problems like this. With a quick glance at your code, I don't think you are, as you are passing 0 as the last argument (inertia) for the creation of your bodies. Try bumping that up to a number of 20-50 and see what it does.

That's it, I think.

TechnoCat wrote:

Code: Select all

   objects[2]["body"]:setMassFromShapes()
   objects[3]["body"]:setMassFromShapes()
   objects[4]["body"]:setMassFromShapes()

That would be even better, though not necessary to fix the problem (note that the mass argument passed to those functions has become useless)

Also, nat, FYI:

Code: Select all

object[1].body == object[1]["body"]

Posted: **Tue Jan 25, 2011 5:19 pm**

Thanks a bunch guys. It was the inertia thing. For whatever reason I always get inertia and momentum mixed up (perhaps from "moment" of inertia), and I wondered why you would want to set an initial momentum. Anyways, that fixed it for me.

And thanks for that object[id].body tip Robin, that should save me from a lot of tedious typing. I should probably go actually learn Lua instead of picking it up from the few examples I've seen.

Posted: **Tue Jan 25, 2011 8:31 pm**

nat1192 wrote:Thanks a bunch guys. It was the inertia thing. For whatever reason I always get inertia and momentum mixed up (perhaps from "moment" of inertia), and I wondered why you would want to set an initial momentum. Anyways, that fixed it for me.

No worries. I had the same issue happen, and I wondered why the physics system was so unrealistic; but then I remembered what inertia did, and there you go.

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