Tween a body - Need b2.Body:getX/Y(), b2.Body:setX/Y()?

Mells · April 2013

Hi,

I wanted to play with bodies but noticed that b2.Body:getX/Y(), b2.Body:setX/Y() are not available.
I thought this would be handy.

Specifically in the following case :

local tween = GTween.new(myBody, 1, {x = myBody:getX()}, {repeatCount = 0, setLoop = true, reflect = true})

Does it make sense and do you know of an alternative?

ar2rsawseen · April 2013

@Mells yes I think you are right, since sprites are usually bind to bodies it would make sense to use tweening on body and not sprite.

On the other hand it might not make sense to tween the bodies at all, as in, they are there for a physics purpose, and tweening might only disrupt it. But of course there can always be specific cases.

But it's actually not getX/Y needed for tweening, but just general get/set methods

I'll talk about it with @atilim, but in the mean time, you can probably do something like this:

function b2.Body:set(param, val)
    local x, y = self:getPosition()
    if param == "x" then
        x = val
        self:setPosition(x, y)
    elseif param == "y" then
        y = val
        self:setPosition(x, y)
    elseif param == "rotation" then
        self:setAngle(math.rad(val))
    end
end
 
function b2.Body:get(param)
    local x, y = self:getPosition()
    if param == "x" then
        return x
    elseif param == "y" then
        return y
    elseif param == "rotation" then
        return math.deg(self:getAngle())
    end
end
 
function b2.Body:getX()
    return self:get("x")
end
 
function b2.Body:getY()
    return self:get("y")
end
 
function b2.Body:setX(x)
    self:set("x", x)
end
 
function b2.Body:setY(y)
    self:set("y", y)
end

atilim · April 2013

@ar2rsawseen's solution is totally correct

On the other hand, changing the position of a physical body at each frame is not physically correct

Mells · April 2013

@ar2rsawseen
thank you!

By the way what is the correct way to require the libs in init.lua?

 
-- -----------------------
-- REQUIRE
-- -----------------------
require "box2d" --Required in Box2dEasy?
require "Box2dEasy"
require "GiderosCodingEasy"
 
-- -----------------------
-- SET BOX2D WORLD
-- -----------------------
world = b2.World.new(0, 9.8, true)
 
function createWorld()    	
	local worldb2Draw = world:getDebug() -- ######## attempt to call method 'getDebug' (a nil value) ########
	return worldb2Draw
end
worldb2Draw = createWorld()

@atilim I was looking for a way to move my bodies on loop (ex moving platforms that go up and down, gravityScale = 0, etc), maybe this method to tween anything could be used to tween the linear velocity?

I will have to search more.

Edit : I required "Box2dEasy" before "box2d" and it worked. Should I remove the require("box2d") line?

Mells · April 2013

So it works :

 
function b2.Body:set(param, val)
		local x, y = self:getPosition()
                local lvx, lvy = self:getLinearVelocity()
		if param == "x" then
			x = val
			self:setPosition(x, y)
		elseif param == "y" then
			y = val
			self:setPosition(x, y)
		elseif param == "rotation" then
			self:setAngle(math.rad(val))		
 
                -- #####
		elseif param == "linearVelocity" then
			self:setLinearVelocity(val)
		elseif param == "linearVelocityX" then			
			self:setLinearVelocity(val, lvy)
		elseif param == "linearVelocityY" then
			self:setLinearVelocity(lvx, val)
                elseif param == "gravityScale" then
			self:setGravityScale(val)
                -- #####
 
		end
	end
 
	function b2.Body:get(param)
		local x, y = self:getPosition()
                local lvx, lvy = self:getLinearVelocity()
		if param == "x" then
			return x
		elseif param == "y" then
			return y
		elseif param == "rotation" then
			return math.deg(self:getAngle())		
 
                -- #####
		elseif param == "linearVelocity" then
			return lvx, lvy
		elseif param == "linearVelocityX" then
			return lvx
		elseif param == "linearVelocityY" then
			return lvy
                elseif param == "gravityScale" then
			return self:getGravityScale()
                -- #####
 
		end
	end

If you want a body to move/loop from left to right without messinging up physics :

       -- How to use
       local value = 10
       myBody:set("linearVelocityX", -value)
       local tween = GTween.new(myBody, .8, {linearVelocityX = myBody:get("linearVelocityX") + value * 2}, {reflect = true, setLoop = true, repeatCount = 0})

thanks to @bowerandy for creating this discussion in the first place.

Btw @atilim I see in the docs :

Sprite:set(param, value)

Sets the specified property of this sprite instance by its name. These names are supported:
"x"

"y"

"rotation"

"scaleX"

"scaleY"

"scale"

"alpha"

"redMultiplier"

"greenMultiplier"

"blueMultiplier"

"alphaMultiplier"

I see that we have a getWidth/setWidth method available.
Maybe it's worth adding "width" and "height" to the list of properties above?

"width"
"height"

@ar2rsawseen
yes if everything (gravityScale, linearDamping, localCenter, mass) was accessible through body:set(param, value) we could for example tween the gravityScale for all objects on screen for a nice slow motion effect, etc...

ar2rsawseen · April 2013

@Mells yes tweening velocity probably makes much more sense

BTW there is no setWidth method, only getWidth as the width of Sprite is determined by its contents. Thus you can't adjust nor tween it. Same for height.
But adjusting/tweening scale should provide similar effect.

And yes to your last comment, I'm adding this to GCE

Also I will fix so Box2dEasy would work if required after box2d. So it won't matter where you require box2d completely

Mells · April 2013

@ar2rsawseen

And yes to your last comment, I'm adding this to GCE

Great, about that I have a few questions :

Is here : https://github.com/ar2rsawseen/GiderosCodingEasy the right place to look for updates about GiderosCodingEasy?
Is there an "official" link somewhere on the website?

ar2rsawseen · May 2013

@Mells yes basically for now there is only this repo, but I had no chance to update it yet, cause I also need to test it

ar2rsawseen · May 2013

I think I've found a way to provide any setter/getter (current or future) method through set/get function dynamically. And it even seems to work, I've updated the repo.

BTW @Mells your provided tween gives an awesome dancing box effect

Mells · May 2013

@ar2rsawseen wow that was fast and it works great, thank you

What do you think about the following :

elseif param == "worldVector" then
     local x, y = -- 0, 0?
     return self:getWorldVector(x, y)
elseif param == "localPoint" then
     local x, y = -- 0, 0?
     return self:getLocalPoint(x, y)
elseif param == "localVector" then
     local x, y = -- 0, 0?
     return self:getLocalVector(x, y)

I don't see how useful it would be (I've never used it and have no idea of a specific case where it is relevant) but that would make everything accessible from the get/set methods?

For those who want to see how to use it :

print ("->", 	"position", body:get("position"))																	
print ("->", 	"angle", body:get("angle"))						
print ("->", 	"linearVelocity", body:get("linearVelocity"))
print ("->", 	"angularVelocity", body:get("angularVelocity"))
print ("->", 	"angularDamping", body:get("angularDamping"))
print ("->", 	"worldCenter", body:get("worldCenter"))
print ("->", 	"localCenter", body:get("localCenter"))
print ("->", 	"mass", body:get("mass"))
print ("->", 	"inertia", body:get("inertia"))
print ("->", 	"gravityScale", body:get("gravityScale"))

Now you can tween all those values if you want.

A modified version of the physics example provided with Gideros.

--[[
 
This code is MIT licensed, see <a href="http://www.opensource.org/licenses/mit-license.php" rel="nofollow">http://www.opensource.org/licenses/mit-license.php</a>
(C) 2010 - 2011 Gideros Mobile 
 
]]
 
require "box2d"
require "Box2dEasy"
require "GiderosCodingEasy"
require "easing"
 
b2.setScale(20)
 
local sky = Bitmap.new(Texture.new("sky.png"))
stage:addChild(sky)
 
local grass = Bitmap.new(Texture.new("grass.png"))
grass:setY(400)
stage:addChild(grass)
 
-- this table holds the dynamic bodies and their sprites
local actors = {}
 
-- create world
local world = b2.World.new(0, 9.8)
 
-- this function creates a 80x80 physical box and adds it to the stage
local function createBox(x, y, name)
	local body = world:createBody{type = b2.DYNAMIC_BODY, position = {x = x, y = y}}
 
	body.name = name
 
	local shape = b2.PolygonShape.new()
	shape:setAsBox(40, 40)
	body:createFixture{shape = shape, density = 1, restitution = 0.2, friction = 0.3}
 
	local sprite = Bitmap.new(Texture.new("box.png"))
	sprite:setAnchorPoint(0.5, 0.5)
	stage:addChild(sprite)
 
	actors[body] = sprite
 
	-- ------------------------------
	-- Tween Body Properties
	-- ------------------------------
	local tween1 = GTween.new(body, 1, {gravityScale = 0}, {autoPlay = false})
	local tween2 = GTween.new(body, 1, {linearVelocityY = 0}, {ease = easing.outQuintic, autoPlay = false})
	local tween3 = GTween.new(body, .3, {gravityScale = 1}, {ease = easing.inExponential, autoPlay = false})
	tween1.nextTween = tween2
	tween2.nextTween = tween3
	tween3.nextTween = tween1	
	tween1:setPaused(false)
 
end
 
-- create ground body
local ground = world:createBody({})
 
ground.name = "ground"
 
local shape = b2.EdgeShape.new(-200, 400, 520, 400)
ground:createFixture({shape = shape, density = 0})
 
-- create two boxes
createBox(140, -30, "box1")
createBox(180, 300, "box2")
 
local function onBeginContact(event)
	-- you can get the fixtures and bodies in this contact like:
	local fixtureA = event.fixtureA
	local fixtureB = event.fixtureB
	local bodyA = fixtureA:getBody()
	local bodyB = fixtureB:getBody()
 
	print("begin contact: "..bodyA.name.."<->"..bodyB.name)
 
	if 	bodyA.name == "box1" and bodyB.name == "box2" or
		bodyB.name == "box1" and bodyA.name == "box2" then
			GTween.pauseAll = true
			bodyA:set("gravityScale", 1)
			bodyB:set("gravityScale", 1)
			print ("All tweens paused")
	end
end
 
local function onEndContact(event)
	-- you can get the fixtures and bodies in this contact like:
	local fixtureA = event.fixtureA
	local fixtureB = event.fixtureB
	local bodyA = fixtureA:getBody()
	local bodyB = fixtureB:getBody()
 
	--print("end contact: "..bodyA.name.."<->"..bodyB.name)
end
 
local function onPreSolve(event)
	-- you can get the fixtures and bodies in this contact like:
	local fixtureA = event.fixtureA
	local fixtureB = event.fixtureB
	local bodyA = fixtureA:getBody()
	local bodyB = fixtureB:getBody()
 
	--print("pre solve: "..bodyA.name.."<->"..bodyB.name)
end
 
local function onPostSolve(event)
	-- you can get the fixtures and bodies in this contact like:
	local fixtureA = event.fixtureA
	local fixtureB = event.fixtureB
	local bodyA = fixtureA:getBody()
	local bodyB = fixtureB:getBody()
 
	--print("post solve: "..bodyA.name.."<->"..bodyB.name)
end
 
 
-- register 4 physics events with the world object
world:addEventListener(Event.BEGIN_CONTACT, onBeginContact)
world:addEventListener(Event.END_CONTACT, onEndContact)
world:addEventListener(Event.PRE_SOLVE, onPreSolve)
world:addEventListener(Event.POST_SOLVE, onPostSolve)
 
-- step the world and then update the position and rotation of sprites
local function onEnterFrame()
	world:step(1/60, 8, 3)
 
	for body,sprite in pairs(actors) do
		sprite:setPosition(body:getPosition())
		sprite:setRotation(body:getAngle() * 180 / math.pi)
	end
end
 
stage:addEventListener(Event.ENTER_FRAME, onEnterFrame)

ar2rsawseen · May 2013

Those actually quite awesome effects.

About the other options, well both getter and setter is needed to tween something, and I actually don't know why most of them has only getters, maybe its a box2d limitation.

Mells · May 2013

@ar2rsawseen

Those actually quite awesome effects.

And it's even more powerful than I thought.

Btw how to scale a body? I have found that fixtures need to be destroyed and recreated.
Are there alternatives? I would like to tween the scaling.

For example in the collision sample provided with Gideros (and modified above), how would you scale both the crate and it's body?
That sounds trivial but I have no idea how to do it.

ar2rsawseen · May 2013

Well I don't know how efficient that is, but I've implemented a scaleBody property.
Unfortunately you can't really scale x or y separate in most cases, thus there is only one scaleBody property.

Updated the repo

Cliff76 · May 2013

I tried running @Mells example code above in a new GIderos project. I copied in the image assets from CollisionDetection. I downloaded GCE just now and added all of it's lua files to my new project. I then copied the code from above into a new main.lua file and ran. I get a stack overflow on Texture.__new(...). After looking at @ar2rsawseen code (line 1161 in GidereosCodingEasy.lua) I don't understand the intention. I totally understand the S/O error but I'm not follwoing why the recursive call to Texture.__new(...). Is it intended to call into a different module or namespace of some sort? (I'm aware that this could totally be a configuration error on my part but brand new to GCE and Gideros so bear with me.)

Cliff76 · May 2013

Ah wait, I see now. It's not a recursive call as there is only 1 "_" in the name caling to the original implementation I guess. I still don't understand why I get a S/O...

ar2rsawseen · May 2013

@Cliff76 you need to exclude GCE and B2DE from execution and require them in init.lua

Cliff76 · May 2013

@ar2rsawseen Wha? Ok, I think I see it now. I'm bouncing between ZeroBrane and Gideros Studio and I didn't see anyway to change project properties in ZB but I am able to open the gproj file directly and add excludeFromExecution="1" to the file tag for GCE. I guess this is the way to do what you said. Here goes nothing...

Cliff76 · May 2013

@ar2rsawseen Yup that worked! I'll need to revisit the Gideros docs and see if I missed the part that describes excluding files from execution.

Mells · May 2013

Well I don't know how efficient that is, but I've implemented a scaleBody property.

@ar2rsawseen It works like a charm. Thanks a lot

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