How can I create a physics object?

Max · April 2014

Hello,
I'm new at programming with gideros, but I really like it

I'm reading the book Gideros Mobile Game Development and I really like it, but I don't understand how to create a physics object. How can I do this?
I hope you could answer my question

Thanks in advance for answers

ar2rsawseen · April 2014

Hello @Max

Sure
First you include box2d library:

--include box2d library
require "box2d"

Let's create the world:

local world = b2.World.new(0, 10, true)

Then you define the Graphical object you want to attach body to:

--create ball bitmap object from ball graphic
local ball = Bitmap.new(Texture.new("./ball.png"))
--reference center of the ball for positioning
ball:setAnchorPoint(0.5,0.5)
ball:setPosition(100,100)
--add to scene or stage
stage:addChild(ball)

Then you create a box2d body:

--get radius
local radius = ball:getWidth()/2
 
--create box2d physical object
local body = world:createBody{type = b2.DYNAMIC_BODY}
body:setPosition(ball:getX(), ball:getY())
body:setAngle(ball:getRotation() * math.pi/180)
local circle = b2.CircleShape.new(0, 0, radius)
local fixture = body:createFixture{shape = circle, density = 1.0, 
friction = 0.1, restitution = 0.2}
 
--add body reference to ball
 ball.body = body

Move the world and make ball move with physics body
on each frame

stage:addEventListener(Event.ENTER_FRAME, function()
    -- edit the step values if required. These are good defaults!
    world:step(1/60, 8, 3)
    --update position to match box2d world object's position
    --get physical body reference
    local body = ball.body
    --get body coordinates
    local bodyX, bodyY = body:getPosition()
    --apply coordinates to sprite
    ball:setPosition(bodyX, bodyY)
    --apply rotation to sprite
    ball:setRotation(math.deg(body:getAngle()))
end)

Then you would need to define world boundaries, for example using chain shape:

local body = world:createBody{type = b2.STATIC_BODY}
body:setPosition(0, 0)
local width = application:getContentWidth()
local height = application:getContentHeight()
--set surroungind box
local chain = b2.ChainShape.new()
chain:createLoop(
    0,0,
    0, height,
    width, height,
    width, 0
)
body:createFixture{shape = chain, density = 1.0, 
friction = 1, restitution = 0.3}

If something does not seem to work as expected, you can always use debug draw to draw where actually physics bodies are:

local debugDraw = b2.DebugDraw.new()
world:setDebugDraw(debugDraw)
stage:addChild(debugDraw)

That should be about it to get you started

Max · April 2014

Thanks for your fast answer and sorry for my Late one.

I typed in everything you wrote, but my image isn't moving

I don't get errors.
I hope you can help me again.
Thanks in advance

john26 · April 2014

Maybe try removing all "local" commands to begin with as this can be a source of confusion. Eg

local x=5

function foo()
local x
print (x) -- nil
end

Here the intent was to access the outer x set to 5 but because of the inner local x, this value is hidden. It is not an error to access x but its value is nil when printed.

ar2rsawseen · April 2014

Attached the example project with the code above (well little modification to restitution to make it jump infinitely)

Max · April 2014

Thanks to both

I'll try it later

I have another question.
I wanted to know if I can program that every 1 second the player sprite moves up a little bit (or something like this)?

Max · April 2014

@ar2rsawseen
It works now

Thanks for your help!

I have another question:
How can I make, that my player object is rotating around itself while you touch it?
If I do this with set Rotation, it rotates, but with every touch the sprite walks closer to the end of the screen and then outside of it.
I hope you can help me again

ar2rsawseen · April 2014

you could try

--on mouse down
ball.body:setAngularVelocity(3) --radians/second
 
--on mouse up
ball.body:setAngularVelocity(0) --radians/second

Max · April 2014

Thanks for your help, but I always get the error: Player.lua attempt to Index field 'body' (a Nil value)

Does anyone know why?

ar2rsawseen · April 2014

Did you add the reference to the ball?

--add body reference to ball
 ball.body = body

Max · April 2014

I think, that I did.
It worked after I used:
body:setAngularVelocity(3)

Thanks for your help

Is there a way to let the sprite only rotate, if you "draw" circles on the touchscreen with your finger? I tried it, but I can't figure out, how I have to do it.
Thanks in advance

ar2rsawseen · April 2014

You can use Gesture recognition lib:
http://appcodingeasy.com/Gideros-Mobile/Detecting-Gestures-in-Gideros

And when you detect circle, apply angular velocity

Max · April 2014

Thanks

I'll try it later.

My problem is the gideros player. It doesn't work. If I click play, I always get the message, that it doesn't react and asks me if I want to close it. It worked yesterday evening and it works fine with any other projects.

(
I hope you can help me

ar2rsawseen · April 2014

I assume you mean the Desktop player
Well it is possible you ran into some C++ mess associated with Box2d underneath.
So something you added to the code breaks it, you can either try backing back until you see it works or you can post your code/project for us to check

Max · April 2014

I found out what it was and now it works

Thanks ar2rsawseen for your great help.

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