Is there a very easy physics tutorial?

Tom2012 · September 2012

I'm trying to work my way through the examples and I've seen a couple of tutorials but they're way over my head. Is there a super-easy one for newbs?

For example - how do you add a box that falls down the screen? And then add a surface to stop it's movement?

I looked at the sleeping bodies example but it was a bit complex for me.

Thanks

Tom

ar2rsawseen · September 2012

Hello, have you check these tutorials?

http://giderosmobile.com/forum/discussion/1387/box2d-tutorial#Item_6

Tom2012 · September 2012

Thanks, yep saw your tutorials but for me they're still a bit complex.

Is there a simple class to quickly set up physics and add bodies?

It's perhaps hard for beginners to grasp how physics works.

ar2rsawseen · September 2012

Here is an example class, created from other SDK port
http://www.giderosmobile.com/forum/discussion/373/display-object-physics-helper-library/p1

But there is also an idea to create abstraction layer as talked about here:
http://www.giderosmobile.com/forum/discussion/1780/extending-gideros

Tom2012 · September 2012

Hi thanks for the help.

I'll look into those.

I'm talking mega basic. Like I didn't even know you had to run an onEventEnterframe to update the box2D objects each frame. :-)

Getting the hang of it now.

One thing I really don't understand is:

-- edit the step values if required. These are good defaults!
self.world:step(1/60, 8, 3)

What do those 3 values do?

Thank you!

petec · September 2012

There's a reasonably understandable explanation of the step parameters here http://www.iforce2d.net/b2dtut/worlds
The step bit is in the second half. Hope that helps make some sense of them (I didn't know what they did until I read that just now!).

Tom2012 · September 2012

Thanks Pete - much appreciated!!

ar2rsawseen · September 2012

Well it's really hard to describe if you understand how it all works, on the other hand its hard to describe if you are learning and don't know much. Thus it's hard to create great tutorial.

But to improve my skills, I'd like to get feedback. For example, in http://appcodingeasy.com/Gideros-Mobile/Gideros-Box2D-basics

There is a paragraph which describes that you need to move Gideros Sprites along with box2d objects and there is an example function, which is onEnterFrame event handler.

So, in your opinion, how could I improve it, so the next one that is trying to learn something, could understand it?

Tom2012 · September 2012

Hey I love your tutorials - I'm just a bit slower than most :-)

Something for me that would have helped would have been a small paragraph at the start.

Physics in Gideros consists of 4 steps:

1) Creating the physics world (setting gravity etc)
2) Creating physics bodies and attaching them to sprites
3) Using a function to update objects each frame
4) Running the world

As a newb, it would really have helped to have a tutorial before this one on just adding a sprite and adding a physics body to it. Perhaps how to add a circle and how to add a box.

I played about with your code this afternoon and was able to make myself a simple project with a falling box with a body. (code below).

This has really helped me out.

Thanks again and keep up the good work.

level1 = Core.class(Sprite)

function level1:init()

-- Setup box2D world
self.world = b2.World.new(0, 10, true)

-- Create image

local happyBox = Bitmap.new(Texture.new("happy-box.png"))
happyBox:setAnchorPoint(0.5,0.5)
happyBox:setPosition(50,50)
self:addChild(happyBox);

-- Create box2D body

local body = self.world:createBody{type = b2.DYNAMIC_BODY}

body:setPosition(happyBox:getX(), happyBox:getY());

body:setAngle(happyBox:getRotation() * math.pi/180);

local poly = b2.PolygonShape.new()

poly:setAsBox(happyBox:getWidth()/2, happyBox:getHeight()/2)

local fixture = body:createFixture{shape = poly, density = 1.0, friction = 0.1, restitution = 0.2}

happyBox.body = body

happyBox.body.type = "happyBox";

--set up debug drawing
-- has to go at end

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

-- Function to update box2D world each frame

local function onEnterFrame()

-- edit the step values if required. These are good defaults!
self.world:step(1/60, 8, 3)

--iterate through all child sprites
for i = 1, self:getNumChildren() do

--get specific sprite
local sprite = self:getChildAt(i)

-- check if sprite HAS a body (ie, physical object reference we added)
if sprite.body then

--update position to match box2d world object's position
--get physical body reference
local body = sprite.body

--get body coordinates
local bodyX, bodyY = body:getPosition()

--apply coordinates to sprite
sprite:setPosition(bodyX, bodyY)

--apply rotation to sprite
sprite:setRotation(body:getAngle() * 180 / math.pi)

end
end
end

--run world
self:addEventListener(Event.ENTER_FRAME, onEnterFrame)

ar2rsawseen · September 2012

Hmm, I guess I got it. Maybe I put too much sense on code, where I could emphasize things in text more. For example, this same Box2d basic tutorial includes how to create physics object both from Shape and Bitmap, using both circle and rectangle. But I guess it's not mentioned in text.

I'm myself rarely read whats written on website, and usually jump to code snippets and try to understand what there is, and that's probably reflects in my tutorials

You on the other hand are more of a reading type. I will take note of that and try to improve in further blog posts. Thank you so much for feedback

BJG · January 2013

Hi Tom2012 -

As a complete novice I was interested to try your code. I tried pasting it into main.lua, but when I run it I get:

main.lua:76: 'end' expected (to close 'function' at line 3) near ''
main.lua:76: 'end' expected (to close 'function' at line 3) near ''
main.lua:76: 'end' expected (to close 'function' at line 3) near ''
main.lua:76: 'end' expected (to close 'function' at line 3) near ''

I'm obviously doing something dumb. :-/

hgvyas123 · January 2013

@BJG

as you are complete novice it is recommended that you should start with something which is fully running

start from this tutorials for physics

http://appcodingeasy.com/Gideros-Mobile/Debug-drawing-for-physics-engine-in-Gideros-Mobile

http://appcodingeasy.com/Gideros-Mobile/Gideros-Box2D-basics

http://appcodingeasy.com/Gideros-Mobile/Gideros-Box2d-gravity-switch

also try to search tutorials from same site there are some others also

and you can also try to learn from example already shipped with Gideros installation

hgvyas123 · January 2013

ouch this thread lists tons of tutorial

go with this

http://giderosmobile.com/forum/discussion/1387/box2d-tutoria#Item_7

Tom2012 · January 2013

Hi Tom2012 -

As a complete novice I was interested to try your code. I tried pasting it into main.lua, but when I run it I get:

main.lua:76: 'end' expected (to close 'function' at line 3) near ''
main.lua:76: 'end' expected (to close 'function' at line 3) near ''
main.lua:76: 'end' expected (to close 'function' at line 3) near ''
main.lua:76: 'end' expected (to close 'function' at line 3) near ''

I'm obviously doing something dumb. :-/

Hello

Apologies for the time taken to get back to you...

If you've not found how to do it yet then here's some code that should work...

Let me know if you get stuck

--include box2d library
require "box2d"
 
-- Setup box2D world
stage.world = b2.World.new(0, 10, true)
 
-- Create image
 
local testImage = Bitmap.new(Texture.new("block.png"))
testImage:setAnchorPoint(0.5,0.5)
testImage:setPosition(50,50)
stage:addChild(testImage);
 
-- Set up enterframe loop
 
local function onEnterFrame()
 
	-- edit the step values if required. These are good defaults!
	stage.world:step(1/60, 8, 3)
 
	--iterate through all child sprites
	for i = 1, stage:getNumChildren() do
 
	--get specific sprite
	local sprite = stage:getChildAt(i)
 
	-- check if sprite HAS a body (ie, physical object reference we added)
		if sprite.body then
 
			--update position to match box2d world object's position
			--get physical body reference
 
			local body = sprite.body
 
			--get body coordinates
			local bodyX, bodyY = body:getPosition()
 
			--apply coordinates to sprite
			sprite:setPosition(bodyX, bodyY)
 
			--apply rotation to sprite
			sprite:setRotation(body:getAngle() * 180 / math.pi)
 
		end
	end
end
 
 
-- Create box2D body
 
local body = stage.world:createBody{type = b2.DYNAMIC_BODY}
 
body:setPosition(testImage:getX(), testImage:getY());
 
body:setAngle(testImage:getRotation() * math.pi/180);
 
local poly = b2.PolygonShape.new()
 
poly:setAsBox(testImage:getWidth()/2, testImage:getHeight()/2)
 
local fixture = body:createFixture{shape = poly, density = 1.0, friction = 0.1, restitution = 0.2}
 
testImage.body = body
 
--set up debug drawing
-- has to go at end
 
local debugDraw = b2.DebugDraw.new()
stage.world:setDebugDraw(debugDraw)
stage:addChild(debugDraw)
 
-- Function to update box2D world each frame
 
--run world
stage:addEventListener(Event.ENTER_FRAME, onEnterFrame)

BJG · January 2013

(Thanks guys...)

Javi · January 2013

Next month will be published "Introduction to Game Physics with Box2D" by Ian Parberry. I have most of the books of this author and they are all really great, I'm sure it'll be a good reference text if you need it.

http://www.amazon.com/Introduction-Game-Physics-Box2D-Parberry/dp/1466565764

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