I’m trying to detect when 2 objects touch each other.
I have a physics game
one ball
and one rect
ball is falling and bouncing on many objects
the rect is on the bottom of the screen.
what kind of function do I need to tell the program
– if you ball, touches the rect
do this…
end
Please I need help on this
thanks
You must have listener functikn listening to collision of on obkect with another
Hi piotrz55
I did not really understand this –
You must have listener functikn listening to collision of on obkect with another
and Saerothir
I have been there many times, and still I don’t get it.
That page is for people that already knows how to use it
and they just need to refresh the code. it’s not for me
I read that, I don’t understand it. I copy the crate1 code
I get errors.
would it be to much to ask one of you guys, just to type a very simple code
for collision.
I mean, if it’s simple, if it is too complicated, I guess I would have to learn a lot more for this
Did you take a look at the ‘event.phase’ bit of documentation?
http://docs.coronalabs.com/api/event/collision/phase.html
The example code is very self-explanitory. read every line and you’ll easily be able to see what is happening. 
The logic for basic collision handling is very simple -
Write a collision function > add an event listener (or depending on the scenario, a Runtime listener ) to the object you want said function to apply to.
Hi guys
I came out with a way of doing it.
I don’t know if it’s the “correct” way or the “best” way
but I can detect when the ball gets to a certain place
like this
local function sound () if ball.y \> 500 then audio.play(win) end end Runtime:addEventListener("enterFrame", sound )
The problem is that once the ball reaches 500
the audio plays, and plays and plays and keeps playing forever
because of the Runtime
How do I solve that.
I just want the function to execute once.
I guess you may find it so easy to do…
once you already know.
but I’m just going crazy…
Honestly I do need someone to be so kind to actually tell me how
not just to send me to another link.
Please, anyone out there, I’m begging for help.
I try this function -- from corona labs local function onLocalCollision( self, event ) if ( event.phase == "began" ) then elseif ( event.phase == "ended" ) then audio.play(win) end end
And as soon as the ball touches “anything” it plays the audio
I want to be able to tell the “ball”
– play the audio, ONLY if you touch this “item” or object
I also made the ball like this
ball = display.newImage ("ball.png") group:insert ( ball ) ball.x = math.random(100, 945) ball.y = -200 ball:scale (.2, .2) ball.isVisible = false ball.myName = "ball" ball.collision = onLocalCollision ball:addEventListener( "collision", ball )
and I though I made the second “crate” another object
only the one, I need to be detected by the ball
local rect14 = display.newRect(0, 0, 5, 1680) group:insert ( rect14 ) rect14.x = 1025; rect14.y = 400 rect14:setFillColor(255) rect14.myName = "rect" physics.addBody( rect14, "static", { density = 1.0, friction = 0.9, bounce = 0.8 } ) rect14.collision = onLocalCollision rect14:addEventListener( "collision", rect14 )
but the problem is the ball is “detecting” every single object
any help will be greatly appreciate it.
That’s my point, I didn’t “already know how to do it”… nobody did, that’s why the docs are there so you can learn how to do it.
(I’ve learned quite a bit over the past 20ish months, however I’m not very good at “teaching” others So hopefully the following will make sense)
Basically when it comes to collisions there are three phases
Began
Moved
Ended
This is very helpful for deciding when you want something to happen during a collision either with a specific object or any object.
Now, you say “play the audio, ONLY if you touch this “item” or object”
Okay so let’s think about this…
We have two objects - ‘ball’ and ‘rect14’ - and we want something to happen when the ball touches/collides with ‘rect14’.
Now in your own code you give both objects a .myName value, I’m assuming you know why and how it can be used.
Since we’ll want to check for collisions on our ‘ball’ we should add a few things to it -
ball.collision = onballCollision -- this is a reference to what will be the name of our collision function. ball:addEventListener( "collision", ball ) -- this adds a collision listener to the ball object
So we’ve got our objects, each with their own unique identifier and a listener for collisions.
Now we need a collision function.
This is what you have so far -
First thing is to change the name of the function to match the name we created earlier
-- I assume you know what 'self' and 'event' in the parenthesis is for local function onballCollision( self, event ) if ( event.phase == "began" ) then elseif ( event.phase == "ended" ) then audio.play(win) end end
You want this function to apply only when the ‘ball’ collides with ‘rect14’
So think about it… how can we get the name of each object? Well we gave each object their own identifier, so let’s use that!..
Oh, but where do we put it? Well if we want something to happen on the ended phase then it would make sense to add something to that ‘if statement’… but again, how?
Back to the docs!
local function onCollision( event ) if ( event.phase == "began" ) then print( "began: " .. event.object1.myName .. " & " .. event.object2.myName ) elseif ( event.phase == "ended" ) then print( "ended: " .. event.object1.myName .. " & " .. event.object2.myName ) end end
Well this is printing something in the terminal when the event begins and ends…
Back to the docs again (read event.object1 and event.object2 )
Oh so it’s printing the name of the first and second object involved in the collision!.. maybe I can reference what object I want to be the first and second, in addition to whatever event.phase I’m checking for.
(Notice they are using .myName for identifying each object… maybe you could do the same )
Now I don’t want to just type the code you need so you can capo&paste it, no, I gave you some information so you can understand how it works so later on when you want to do more complex collision detection you’ll understand some of the basics of what’s going on.
-Saer
Thanks Saer
I changed the code. I added the .myName
print( "began: " .. event.ball.myName .. " & " .. event.rect14.myName )
it gives me error…
I know that you want to help me learn.
thank you for that.
But if I see the code…
I copy and paste in my app.
and it works…
then I know that — that code works
then I can study and read, line by line
and think about it a lot
until it make sense to me brain.
the little bit of code I know, that’s how I learned it.
but if I just try to figure that out
honestly it is very difficult…
I might get it— after several months
but if you help me
and write the code that works
I will learn faster.
I know that you have no obligation to help me
you don’t even know me.
But in the name of my family, and me
I would really be very honored if you share your knowledge with me.
I’m using it to make nice apps for kids, and teach a little bit to them.
hoping that they will be better in the future, by learning something positive.
Thanks for understanding
and I hope you do help me solve this problem.
Could you post the error?
Also, you do know that line is nothing more than a print statement right?
Well it’s one thing to have some code that you can past in and figure out “okay if I use this line here it will do this” etc… etc… but it’s a completely different thing to understand why something works.
For example, this
I would have never been able to pick apart&adapt certain portions of horacebury’s code to my own project if I didn’t first understand how&why certain things work.
Another example:
print( "began: " … event.object1.myName … " & " … event.object2.myName )
Yes, you might be able to recognize “this print the names of both objects in the terminal” but do you understand how it does that? And how&why that syntax does what it does?
My whole point is that it takes time to learn all this.
Unfortunately I think Corona’s motto “Anyone can code” gives some people a false sense of what is really involved in learning to program a mobile application.
Too many people (generally with no prior experience) approach mobile app development with this unrealistic mindset where they think they can have a fully functional, content rich, bug free application within a couple weeks. Then upon realizing that there is quite a bit more to it, they give up. – not directed at you, just some thoughts I’ve had for a while… that I decided to vent. 
-Saer
Edit: I think you’re getting this line confused:
print( "began: " .. event.object1.myName .. " & " .. event.object2.myName )
object1 does not mean you should put the name of your object there, it means (as the docs state)
“Reference to the first object in the collision.”
Since you passed ‘self’ then whatever object you have calling the function will automatically be designated as ‘self’. If you have ‘ball’ calling your collision function, ball will = self and the other objects will = event.other
So you could do it this way ‘event.self.myName’ and 'event.other.myName’
What that will do is print the target object, in this case ‘ball’ is calling the function so it gets to be the target/self, and it will also print the second object - event.other (event - the collision. .other - the other object. event.other.myName - finds the other object’s .myName and prints it)
I think you are right Saer.
I give up.
I guess I’m not ready to detect collision at this point.
I will be doing something else. until I learn.
And yes! you are right – not everyone can learn how to program an app.
Thank you very much for all your help.
I tough it was going to be easier (the collision detection thing)
but I guess it’s only for a few privileged ones.
Thanks
I’m not saying you should give up at all.
All the information you need is in the docs, and what I posted should be very helpful.
Basically, if you’re trying to check collisions for a specific object on the ‘ended’ phase then just specify which object you’re checking for.
You already have ‘ball’ as ‘self’ so now the only thing you need to check is if ‘rect14’ is the other object the ball is colliding with, and if so then do something.
(again I’m not that great at explaining things, but hopefully this will help)
If event.other.myName gets the .myName of the other object involved in the collision, then ask yourself "what is the other object I’m checking for? ‘rect14’ okay, now how would I reference that object? Well, I know I have a >> ** _.myName ** << _assigned to my object, and if event.other.myName is checking the .myName of the other object then I guess I should pass in the .myName of my object so it can check whether or not the other object’s name is equal to my object’s name! 
It’s quite simple if you break it down step by step and ask yourself “What should happen here?”
Anyone can code yes, said person just better be prepared for what is involved.
Like when trying to learn an instrument (I play jazz piano) it takes a while, and same goes for trying to learn programming.
“I guess it’s only for a few privileged ones.” Who’s “privileged”? Me? haha I’m just your average eighteen year old guy who, almost two years ago, decided that he was going to make a game."
I’m getting very close to finishing my first game. I could have released it a while ago, but I want it to be a polished/complete game that people will look at and say, hopefully, “wow, that’s a very well designed&built game.”
A job well done takes time my friend.
-Saer
Don’t give up. I Will write for you something in the while.
Gonna be longer
First, I would like to talk about few basic matters.
In Lua (language you write in Corona) there is something like tables. Tables are special objects which has properties. But from the begining.
You create table like this
local myObject = {}
This created table which I called “myObject”. As you see, to create table you just type “= {}”.
I called it myObject because I want you to think about tables as objects.
So if tables are like objects then it must have some properties.
Let’s assume that myObject is a cat If it’s cat then it has color, age, color of eyes, number of teeth and so on
In lua to access property of table you use “.nameOfProperty”. So if we want to know color of our cat we would write
print( myObject.color )
and for age
print( myObject.age )
But hey, we have “nil” twice. Why? If myObject is a cat then it must have color and age. However our cat is empty (we used ‘{}’ to create empty table). If we want it to have color and age we must give him this properties. To do so we write
myObject.color = "brown" myObject.age = 5
And now it will have color and age.
This is full code:
local myObject = {} print( myObject.age ) print( myObject.color ) myObject.color = "brown" myObject.age = 5 print( myObject.age ) print( myObject.color )Event listeners
You have encountered this.
Event listeners are functions which listen to specific events and perform some action if it happens.
Such events can be touch, tap or collision.
Why event’s listeners? Instead of checking each time if there was touch, tap or collision yourself, you just say Corona to check it for you - so if for exemple there will be touch, Corona will execute some sort of action you sepecified.
Events listeners can be for object or Runtime. What’s the difference? Let’s take touch for example. If you addEventListener for touch for object, then just this object will react on touch. If you create it for Runtime then all touches on screen will be detected.
Do you remember
local function onLocalCollision( self, event ) -- some code here end
This is function which you said corona to execute when “collision” event is detected. It has 2 parameters. The important one is “event”. This is table. As I wrote earlier table have properties. And each type like “touch”, “tap”, “collision” have this table with different parameters.
For example touch, tap and collision have event table with
event.phase
but only “collision” has
event.other
For what each table have you must see docs.
Some comments on your posts
local function sound ()
if ball.y > 500 then
audio.play(win)
end
end
Runtime:addEventListener(“enterFrame”, sound )
The problem is that once the ball reaches 500
the audio plays, and plays and plays and keeps playing forever
because of the Runtime
How do I solve that.
I just want the function to execute once.
You use Runtime listener for enterFrame event, it means that all the time sound() will be executed. So if your ball.y is 501 then sound() will be called. If ball.y = 502 too. It will happen all the time if ball.y > 500 (and probably 30 times per second).
I just want the function to execute once.
So think, how would you do it? If you want something to happen once then… you must remember it happened once! So if it will happen once again then you do not do it again.
For example
local hasSoundPlayed = false local function sound () if ball.y \> 500 then if hasSoundPlayed == false then audio.play(win) hasSoundPlayed = true end end end Runtime:addEventListener("enterFrame", sound )Thank you piotrz55, for explaining how “.” attributes work… much better at this than I am.
As far as collisions go, If you don’t want to create tables for your objects (not advisable) then just specify a unique name/class/type/identifier for your objects.
playerBall.Myname = “userBall”
opponentBall.myName = “badBall”
playerBall.class = “userBall”
opponentBall.class = “badBall”
playerBall.obj_Type = “userBall”
opponentBall.obj_Type = “badBall”
then in a collision function, when checking the phase (began, moved, ended) you can just check the (as piotrz55 says - “.nameOfProperty”) of the other object to see if it is indeed the object you’re checking for, and then code what you want to happen.
In your case you have a .myName attribute assigned to both the ball and rect14.
So in your collision function just check if the other object’s .myName “=” what you have specified for rect14.
-Saer
Now on collision.
First we must include physics and make it start
local physics = require( "physics" ) physics.start()
Then we need some variables
local hasSoundPlayed = false local mySound = audio.loadSound( "BOINK.wav" )
my sound is BOINK.wav, you must write name of your sound.
Then let’s create 2 objects and floor
local myBall = display.newCircle( 150, 50, 20 ) myBall:setFillColor( 255, 0, 0 ) local myCrate = display.newRect( 50, 150, 50, 50 ) local myFloor = display.newRect( 0, 400, 300, 50 ) myFloor:setFillColor( 0, 255, 0 )
If you run simulator then there wil be 3 objects.
Remember as we talked about properties? For objects created with display.* we can also as for tables add properties so
myCrate.name = "crate" myBall.name = "ball"
Now we must make our objects physical
physics.addBody( myBall, "dynamic", { density = 0.1, friction = 0.9, bounce = 0.5, radius = 20 } ) physics.addBody( myCrate, "dynamic", { density = 100.0, friction = 0.9, bounce = 0.0 } ) physics.addBody( myFloor, "static", { density = 1.0, friction = 0.9, bounce = 0.0 } )
This will make myBall and myCrate react to gravity (“dynamic”) and make myFloor being still in place (“static”).
Now we want to detect collision with myFloor.
Do you remember how I wrote that for each event there is table with different properties and that “collision” had
event.other
? We will use it. event.other is the object with myFloor is colliding. As I also said there is
event.phase
Which will tell us phase of collision. It can be “began” or “ended”. Why event.phase ? Because corona will call our function twice so we must somehow tell what happened - is collision begining or ending.
So if we have event.other as object with which myFloor collided then we can check if it has .name property and is one of our names (“ball”, “crate”).
So taking all into one we have
local physics = require( "physics" ) physics.start() local hasSoundPlayed = false local mySound = audio.loadSound( "BOINK.wav" ) local myBall = display.newCircle( 150, 50, 20 ) myBall:setFillColor( 255, 0, 0 ) local myCrate = display.newRect( 50, 150, 50, 50 ) local myFloor = display.newRect( 0, 400, 300, 50 ) myFloor:setFillColor( 0, 255, 0 ) myCrate.name = "crate" myBall.name = "ball" physics.addBody( myBall, "dynamic", { density = 0.1, friction = 0.9, bounce = 0.5, radius = 20 } ) physics.addBody( myCrate, "dynamic", { density = 100.0, friction = 0.9, bounce = 0.0 } ) physics.addBody( myFloor, "static", { density = 1.0, friction = 0.9, bounce = 0.0 } ) local function onCollisionListener( self, event ) print(event.other.name, event.phase) -- we will exexute it for both phases "began" and "ended" but you can give here other "if" statement to differ them. if event.other.name == "ball" then if hasSoundPlayed == false then audio.play( mySound ) hasSoundPlayed = true end end end myFloor.collision = onCollisionListener myFloor:addEventListener("collision", myFloor)Hi piotrz55.
Thank you very much. For as long as I live I will remember and will be grateful to you
that you were the one to teach me how to do the collision event.
it works!! great.
Now I don’t really think I would have been able to make it work on my own.
you add the variable – local hasSoundPlayed = false
you use it here – if hasSoundPlayed == false then
and here – hasSoundPlayed = true
before I had
– Runtime:addEventListener(“enterFrame”, sound )
and now it’s
– myFloor:addEventListener(“collision”, myFloor).
So I saw a few changes but big changes.
now I will try to actually use that in my real app
that will be the challange.
And also I did not see at all the
if phase == “began” or “ended”
I will try to make that work, now that I see more or less the logic.
I’m not going to say that I get it 100% but this will save my life
Thank you one more time.
your friend
Victor
I don’t think I would have think of that.
the other thing is the change in – Runtime and addEvent
@Victor: I would recommend to read one or two books about Corona SDK (and Lua).
Dr. Brian Burton for example has written a few books about Corona SDK´s basics and explains how everything works. http://www.burtonsmediagroup.com/books/mobile-app-development-with-corona-getting-started/
I highly recommend it to you When I started almost 2/3 years ago I had no experience in LUA or any other programming language.
Max / CineTek