Box2d tutorial

gaboritaly · July 2012

Hi,
Anyone know how can I start to learn box2d ?
Or how can I apply some examples find in other box2d tutorials for Gideros? (www.iforce2d.net/)
I watched the Gideros tutorial but i see in same case Gideros use in other solution.
Ad example I didn't find the B2Vec, and I see Gideros initialize the world and body in different mode.

Thanks a lot for any help.

ar2rsawseen · July 2012

atilim · July 2012

I'm always surprised when I see the amazing list of your tutorials. ar2rsawseen, you rock!

gaboritaly · July 2012

Thank you @ar2rsawseen for the fastest answer.
I read a little your articles, and I search it , also there how can I adapt C++ examples and found for b2vec , but without success.
If I understand well Gideros use only x , y , and not a table for b2vec.
Is it normal I found only 25 approximatelly articles (without duplicated articles), if I search box2d in the forum ?
Thank you

ar2rsawseen · July 2012

If I remember correctly, then b2vec is simply an object with two properties (probably x and y), so it's the same as simply passing two properties.

Well forum search is not the best search to try, but if you count only threads, not posts, then yeah, it could be about 20-25 threads about box2d. Mostly questions for support

gaboritaly · July 2012

Thank you yet @ar2rsawseen , I probe to start

Dikkesnoek · October 2012

Is there a simple tutorial about all the Box2D parts and there relation?
Like what is a body and what is the relation to a sprite, what is the role
of a fixture, shapes, etc. I think (but I am not a Box2D guru yet) that you
can explain it in an easy way graphically. I searched a lot but I couldn't
find a tutorial like that. Also this very important part is still missing in
the "Ultimate Guide". I think that this is more interesting than creating
plugins.

Regards,

Marc

MauMau · August 2013

Almost one year later, I am requesting the same -since the docs tell *nothing* about usage of physics in Gideros, is there a good box2D reference for Gideros anywhere?

I checked out the articles on appcodingeasy.com, but there is no explanation of a) the available physics methods and b) what their parameters mean. So you feel completely stuck :-(

ar2rsawseen · August 2013

While I agree with you, we have lots of other options to dedicate our resources, and box2d is not really a priority only because it has similar (if not the same API) with any other box2d port, thus any other box2d basics tutorial will explain all, like this one:

http://www.gamasutra.com/blogs/JuanFelipeBelonPerez/20130826/198897/How_to_create_2D_Physics_Games_with_Box2D_Library.php

Still I agree there should be more on physics in Gideros, and maybe even series of blog posts.

mysps · August 2013

that blog post is right on time! thanks

MauMau · August 2013

What I'd need to know is how to

1. Clean up or remove any physics bodies (from both, world and memory)?

2. Remove a created world (created with local World = b2.World.new(0, 9.8) )?

ar2rsawseen · August 2013

You can remove the bodies using b2.World:destroyBody method
And everything is removed automatically when the reference to object is lost.
So if you have everything in local variables, they will be available for garbagecollector after you are outside the scope
If everything is stored as Sprite object property, then they will be removed once both sprite and world get removed.
World gets removed again when there is no reference to it, so if world is scene property, it will be available to garbage collector, after you change the scene

MauMau · August 2013

Thanks, that makes sense :-)

What about scaling objects, by the way? If I apply a box2D polygon shape and a body to my sprite and want to scale the sprite afterwards, am I allowed to do so? Or do I have to re-create the physics shape then?

hgvyas123 · August 2013

yup you need to recreate physics shape

ar2rsawseen · August 2013

There is one Library inside GiderosCodingEasy called Box2D easy, and there lots of interesting features there, like one of them, scaling the fixture, or tweening the forces and velocities:

More info on that
http://www.giderosmobile.com/forum/discussion/comment/23980#Comment_23980

And library itself:
https://github.com/ar2rsawseen/GiderosCodingEasy

MauMau · August 2013

That's helping a lot, thanks!

While I am getting into this, I wonder if there is a way to simulate buoyancy to simulate bodies floating in liquids. How could this be achieved?

ar2rsawseen · August 2013

The simple idea is like this:

You draw a water line (a sensor body, for example) and if body intersects with it and collisions is in center (depending on the shape it can be easier or harder to determine), on each enter frame you apply force to it, which equals inverted world gravity, so body would float in the water.

The deeper the body in the water the bigger force should be applied, so you will need to find some kind of coefficient to multiply the inverted gravity force based on the distance of the body center under the water line, again on each enter frame.

If body is out of the water (upper part of the water line), then you don't apply anything there.

Here is more detailed info:
http://www.iforce2d.net/b2dtut/buoyancy

Cyberience · February 2014

A little struggle with Box2d. I have an object, in the b2 world, falling to the ground. similar to a rocket landing with no rocket, then I apply the rocket and I am using body:setGravityScale(sprite), problem is the effect is correct in terms of gravity and slowing down, and then overpowering the gravity, but what I need to do is create the effect of a bat, when it instantly, with a little lag, would defeat gravity and boost upward. the only way I can think to do this is create an object momentarily under the falling objects position, which would give an instant stop, and then boost. is there a simpler way to achieve this?
to give a simple understanding, i like to get the effect they have in Flappy Bird, but dont worry, I am not making a flappy bird clone. lol.

Regards

ar2rsawseen · February 2014

Maybe this would help:
http://www.giderosmobile.com/forum/discussion/4451/how-can-i-fly-with-setlinearvelocity/p1

Cyberience · February 2014

Wow ar2rsawseen You are just to Darn fast. lol

eezing · February 2014

I've been working a lot with box2d these past couple weeks. Word on the street is, box2d doesn't like when we directly set the linear velocity as it could cause funky behavior during collisions.

The recommended method is to apply linear impulse at every step. The result is the same as linear velocity, but also fully compatible with collisions.

The code below is the basis of what I'm using in my current project for both a jump and scroll behavior. The idea is that you insert the action function into the step only for as long as you need to move, then remove it.

Once inserted into step, your velocity will instantly change to desired rate and hold thereafter. Keep in mind, impulse does not constrain velocity, but instead applies the necessary forces upon the body in order to achieve the desired effect.

-- Define move parameters
local ratex = -- desired rate of move on x-axis
local ratey = -- desired rate of move on y-axis
local body  = -- body here
local world = -- world here
 
 
-- Need this to convert from gideros to box2d
local function toScale(n)
 
	n * (1 / b2.getScale())
end	
 
 
-- System stuff
local mass    = body:getMass()
local gx, gy  = world:getGravity()
local rx      = toScale(rateX)
local ry      = toScale(rateY)
local dx, dy  = 0, 0
 
 
-- Insert this into your step when you want to move
local function action()
 
	local x, y = body:getWorldCenter()
	local vx, vy = body:getLinearVelocity()
 
	dx = (rx - vx) * mass
	dy = (ry - vy) * mass
 
	body:applyLinearImpulse(dx, dy, x, y)
end

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