Collision Problem

Physics
#1

I have an issue with my collision listener; it is failing to detect the collision at all. Can someone please tell me what is wrong and provide a solution? Thanks.

Here is the code: 

[lua]

local escapesensor = display.newImage(“PlanetCollisionSensor.png”)

escapesensor.x = display.contentWidth/2

escapesensor.y = -20

physics.addBody(escapesensor, “static”)

escapesensor.isSensor = true

escapesensor.alpha = 0.01

escapesensor.rotation = 90

local escapesensor2 = display.newImage(“PlanetCollisionSensor.png”)

escapesensor2.x = display.contentWidth/2 

escapesensor2.y = 340

physics.addBody(escapesensor2, “static”)

escapesensor2.isSensor = true

escapesensor2.alpha = 0.01

escapesensor2.rotation = 90

local function escapeSensorCollision(self,event)

if (event.phase == “began”) then

if (event.other.name == “bluespaceship” or “redspaceship” or “purplespaceship” or “greenspaceship” or “bluegreenspaceship” or “pinkpurplespaceship”) then

spaceshipLives = spaceshipLives - 1

print(“Collision Received!”)

display.remove(event.other)

event.other = nil

end

end

end

escapesensor.collision = escapeSensorCollision

escapesensor:addEventListener(“collision”, escapesensor)

escapesensor2.collision = escapeSensorCollision

escapesensor2:addEventListener(“collision”, escapesensor2)

[/lua]

Basically, I have two rectangular sensors that detect whether a spaceship has hit it or not, and they are failing to detect the collisions. 

#2

Physics bodies that are “static” can only collide with “dynamic” physics bodies. It looks like both of your objects are static.

Also, I don’t see how you are placing force on the objects, but remember that transition. to isn’t a physical movement, and may not cause a collision.

http://docs.coronalabs.com/api/type/Body/bodyType.html

#3

Thank you so much! I had set the spaceship type to kinematic because of some bug before, but now that I’ve fixed that bug and changed the type, it works fine. 

I have one more question. When my game ends, the error ‘attempt to index field ‘contentBounds’ (a nil value)’ appears. I know the reason why the error is happening because it’s trying to access a function that is nil at the end of the game. I just don’t know how to fix it. Could you provide a solution to me?

Here is the code where the error is happening:

[lua]

local function hasCollided( obj1, obj2 )

  if ( obj1 == nil ) then  --make sure the first object exists

return false

  end

  if ( obj2 == nil ) then  --make sure the other object exists

      return false

  end

  local left = obj1.contentBounds.xMin <= obj2.contentBounds.xMin and obj1.contentBounds.xMax >= obj2.contentBounds.xMin

  local right = obj1.contentBounds.xMin >= obj2.contentBounds.xMin and obj1.contentBounds.xMin <= obj2.contentBounds.xMax

   local up = obj1.contentBounds.yMin <= obj2.contentBounds.yMin and obj1.contentBounds.yMax >= obj2.contentBounds.yMin

     local down = obj1.contentBounds.yMin >= obj2.contentBounds.yMin and obj1.contentBounds.yMin <= obj2.contentBounds.yMax

     return (left or right) and (up or down)

end

[/lua]

It is trying to access the variables, left, right, up, and down, but I’m not sure how to prevent this from happening. Thanks so much.

#4

Remember, if you’re calling any Runtime functions, you need to stop them when you exit the scene or end the game. If you have a “hasCollided” function running on a constant Runtime loop, remember to remove it when the game ends.

#5

I know the function needs to be removed, but how do I remove a function? Just like removing a display object using ‘display.remove(object); object = nil;’, how do you do that for functions?

#6

Without seeing your code, the above comment was referencing the fact that I assumed you were calling the hasCollided function as a Runtime listener. To learn more about functions, how they work and adding/removing them, check the below guide, as it will explain it better than I ever could:

http://docs.coronalabs.com/guide/events/detectEvents/index.html

#7

My hasCollided function isn’t a Runtime listener. I just call it whenever it needs to be called, and the place where I need it to be called is in a Runtime listener. I remove that Runtime listener when the game ends, but the error still remains, so I’m not sure how to remove it. I’m sorry if I didn’t make this clear. 

#8

Throw up some of your code so we can take a peek at where the error might lie.

#9

I have one large Runtime listener with a few Runtime listeners in it. Since the code for the large Runtime listener is too long, I’ll just post the small Runtime listener that uses the hasCollided function.

[lua]

local function detectSwipability(event)

     local target = event.target 

     local phase = event.phase

   

        if (hasCollided(bluespaceship, quadrant5)) then

bluespaceship.swipable = true

local function glowBlueSpaceship()

local function glowAlpha()

bluespaceshipGlowing = transition.to(bluespaceship,{time=100, alpha=1, onComplete=glowBlueSpaceship})

end

bluespaceshipGlowing = transition.to(bluespaceship, {time=100, alpha=0.95, onComplete=glowAlpha})

end

bluespaceshipGlowing = transition.to(bluespaceship,{time=100, alpha=1, onComplete=glowBlueSpaceship})

elseif (hasCollided(bluespaceship, quadrant6)) then

bluespaceship.swipable = false

bluespaceship.alpha = 1

elseif(hasCollided(bluespaceship, quadrant7)) then

bluespaceship.swipable = false

bluespaceship.alpha = 1

elseif(hasCollided(bluespaceship, quadrant8)) then

bluespaceship.swipable = false

bluespaceship.alpha = 1

else

if (bluespaceship ~= nil) then

bluespaceship.swipable = false

end

end

end

Runtime:addEventListener(“enterFrame”, detectSwipability)

[/lua]

#10

The problem with your code is that whilst you do check for obj1/obj2 being nil, you do not check for obj1.contentBounds being present. 

If it is not present the obj1.contentBounds will return nil, and when you try to index that - obj1.contentBounds.xMin it throws a wobbler because obj1.contentBounds is not a table.

You need to find in your code the reason why obj1 has been ‘cancelled’ so to speak, but the reference itself has not been nilled.

#11

I see now! When I put in conditions checking if obj.contentBounds and obj2.contentBounds are nil, the error goes away. Thanks so much!

#12

Physics bodies that are “static” can only collide with “dynamic” physics bodies. It looks like both of your objects are static.

Also, I don’t see how you are placing force on the objects, but remember that transition. to isn’t a physical movement, and may not cause a collision.

http://docs.coronalabs.com/api/type/Body/bodyType.html

#13

Thank you so much! I had set the spaceship type to kinematic because of some bug before, but now that I’ve fixed that bug and changed the type, it works fine. 

I have one more question. When my game ends, the error ‘attempt to index field ‘contentBounds’ (a nil value)’ appears. I know the reason why the error is happening because it’s trying to access a function that is nil at the end of the game. I just don’t know how to fix it. Could you provide a solution to me?

Here is the code where the error is happening:

[lua]

local function hasCollided( obj1, obj2 )

  if ( obj1 == nil ) then  --make sure the first object exists

return false

  end

  if ( obj2 == nil ) then  --make sure the other object exists

      return false

  end

  local left = obj1.contentBounds.xMin <= obj2.contentBounds.xMin and obj1.contentBounds.xMax >= obj2.contentBounds.xMin

  local right = obj1.contentBounds.xMin >= obj2.contentBounds.xMin and obj1.contentBounds.xMin <= obj2.contentBounds.xMax

   local up = obj1.contentBounds.yMin <= obj2.contentBounds.yMin and obj1.contentBounds.yMax >= obj2.contentBounds.yMin

     local down = obj1.contentBounds.yMin >= obj2.contentBounds.yMin and obj1.contentBounds.yMin <= obj2.contentBounds.yMax

     return (left or right) and (up or down)

end

[/lua]

It is trying to access the variables, left, right, up, and down, but I’m not sure how to prevent this from happening. Thanks so much.

#14

Remember, if you’re calling any Runtime functions, you need to stop them when you exit the scene or end the game. If you have a “hasCollided” function running on a constant Runtime loop, remember to remove it when the game ends.

#15

I know the function needs to be removed, but how do I remove a function? Just like removing a display object using ‘display.remove(object); object = nil;’, how do you do that for functions?

#16

Without seeing your code, the above comment was referencing the fact that I assumed you were calling the hasCollided function as a Runtime listener. To learn more about functions, how they work and adding/removing them, check the below guide, as it will explain it better than I ever could:

http://docs.coronalabs.com/guide/events/detectEvents/index.html

#17

My hasCollided function isn’t a Runtime listener. I just call it whenever it needs to be called, and the place where I need it to be called is in a Runtime listener. I remove that Runtime listener when the game ends, but the error still remains, so I’m not sure how to remove it. I’m sorry if I didn’t make this clear. 

#18

Throw up some of your code so we can take a peek at where the error might lie.

#19

I have one large Runtime listener with a few Runtime listeners in it. Since the code for the large Runtime listener is too long, I’ll just post the small Runtime listener that uses the hasCollided function.

[lua]

local function detectSwipability(event)

     local target = event.target 

     local phase = event.phase

   

        if (hasCollided(bluespaceship, quadrant5)) then

bluespaceship.swipable = true

local function glowBlueSpaceship()

local function glowAlpha()

bluespaceshipGlowing = transition.to(bluespaceship,{time=100, alpha=1, onComplete=glowBlueSpaceship})

end

bluespaceshipGlowing = transition.to(bluespaceship, {time=100, alpha=0.95, onComplete=glowAlpha})

end

bluespaceshipGlowing = transition.to(bluespaceship,{time=100, alpha=1, onComplete=glowBlueSpaceship})

elseif (hasCollided(bluespaceship, quadrant6)) then

bluespaceship.swipable = false

bluespaceship.alpha = 1

elseif(hasCollided(bluespaceship, quadrant7)) then

bluespaceship.swipable = false

bluespaceship.alpha = 1

elseif(hasCollided(bluespaceship, quadrant8)) then

bluespaceship.swipable = false

bluespaceship.alpha = 1

else

if (bluespaceship ~= nil) then

bluespaceship.swipable = false

end

end

end

Runtime:addEventListener(“enterFrame”, detectSwipability)

[/lua]

#20

The problem with your code is that whilst you do check for obj1/obj2 being nil, you do not check for obj1.contentBounds being present. 

If it is not present the obj1.contentBounds will return nil, and when you try to index that - obj1.contentBounds.xMin it throws a wobbler because obj1.contentBounds is not a table.

You need to find in your code the reason why obj1 has been ‘cancelled’ so to speak, but the reference itself has not been nilled.