AGK Tier 1 - Box2D Physics - GameCreators Forum

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Grook

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Joined: 22nd Jun 2012

Location: Lancashire, England

Posted: 6th May 2016 17:44

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Have their been any updates recently (last 12 months) on Box2D physics at all?

I thought I'd revisit an old project that I dumped because of a particular issue, tried again using latest AppGameKit and it's still the same. Problem in a nutshell is that objects colliding with other objects at shallow angles don't rebound at a shallow angle, they just slide off the object. Anybody know a way to work around this?

"It is said that your life flashes before your eyes just before you die. That is true, it's called Life.”

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BatVink

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Joined: 4th Apr 2003

Location: Gods own County, UK

Posted: 7th May 2016 07:39

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Have you got any code to show the configuration? It could be that you need to adjust friction, restitution, mass and gravity to get the effect you want. It is also very dependent on the physics scale you use.

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Grook

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Location: Lancashire, England

Posted: 7th May 2016 10:40

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Cheers BatVink, I have been playing with those variables but I can't seem to make it behave better (though I can make it behave worse!). I'll put something together...

"It is said that your life flashes before your eyes just before you die. That is true, it's called Life.”

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Grook

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Location: Lancashire, England

Posted: 7th May 2016 11:58

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Here's a small sample to show the "issue".

+ Code Snippet

SetDisplayAspect          ( 1.3333 )
SetPhysicsScale           ( 0.2 )
SetPhysicsDebugOn        ( )
SetPhysicsGravity         ( 0, 0 )

liBall = CreateSprite(0)
SetSpriteColor(liBall, 200, 0, 0, 255 )
SetSpriteSize(liBall, 2, -1 )
SetSpriteOffset(liBall, GetSpritewidth(liBall)/2.0, GetSpriteHeight(liBall)/2.0)
SetSpriteShape(liBall, 1)
SetSpritePhysicsOn(liBall, 2)
SetSpritePhysicsMass(liBall, 100.1)
SetSpritePhysicsRestitution( liBall, 1.0 )
SetSpritePhysicsAngularDamping( liBall, 0.0 )
SetSpritePhysicsCanRotate(liBall,1)
SetSpritePhysicsFriction(liBall,0.0)
SetSpritePhysicsDamping(liBall,0.0)
SetSpritePositionByOffset( liBall, 80, 50 )

// Change the 1 to something higher and it WILL rebound off the wall
SetSpritePhysicsImpulse( liBall, 80, 50, 1, 20 )

do
    

    Print( ScreenFPS() )
    Sync()
loop

"It is said that your life flashes before your eyes just before you die. That is true, it's called Life.”

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BatVink

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Joined: 4th Apr 2003

Location: Gods own County, UK

Posted: 7th May 2016 12:22

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I would be tempted to say there is some kind of bug or inaccuracy here. I can't get the ball to "unstick" from the wall once it hits it. I is as if it has made a rounding error somehow and can't reverse the direction.
I tried it hitting all 4 walls but they all experience the same problem. I changed all of the parameters without success.
The only odd parameter you have it the mass, which seems very heavy. But changing it doesn't solve the problem anyway.

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TutCity is being rebuilt

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Grook

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Joined: 22nd Jun 2012

Location: Lancashire, England

Posted: 7th May 2016 20:51

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Yeah, sorry, was playing around with the mass, forgot to change it back.

Thanks for looking at it anyway. Any chance Paul could take a look I wonder? Is there a procedure to follow for reporting issues?

"It is said that your life flashes before your eyes just before you die. That is true, it's called Life.”

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Paul Johnston

TGC Developer

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Joined: 16th Nov 2002

Location: United Kingdom

Posted: 14th May 2016 16:45

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It looks like Box2D has a minimum velocity for elastic collisions of 1 unit per second. Since your physics scale is 0.2 this means that if GetSpritePhysicsVelocityX returns below 5 then it will not bounce. You can work around this by adjusting your physics scale, the following seems to work

+ Code Snippet

SetDisplayAspect          ( 1.3333 )
SetPhysicsScale           ( 1 )
SetPhysicsDebugOn        ( )
SetPhysicsGravity         ( 0, 0 )
 
liBall = CreateSprite(0)
SetSpriteColor(liBall, 200, 0, 0, 255 )
SetSpriteSize(liBall, 2, -1 )
SetSpriteOffset(liBall, GetSpritewidth(liBall)/2.0, GetSpriteHeight(liBall)/2.0)
SetSpriteShape(liBall, 1)
SetSpritePhysicsOn(liBall, 2)
SetSpritePhysicsMass(liBall, 1.1)
SetSpritePhysicsRestitution( liBall, 1.0 )
SetSpritePhysicsAngularDamping( liBall, 0.0 )
SetSpritePhysicsCanRotate(liBall,1)
SetSpritePhysicsFriction(liBall,0.0)
SetSpritePhysicsDamping(liBall,0.0)
SetSpritePositionByOffset( liBall, 80, 50 )

// scale has increased by 5 times, but impulse needs to increase by about 25 times to reach the same velocity 
SetSpritePhysicsImpulse( liBall, 80, 50, 25, 500 )
 
do
     
	print(GetSpritePhysicsVelocityX(liBall))
    Print( ScreenFPS() )
    Sync()
loop

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Grook

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Joined: 22nd Jun 2012

Location: Lancashire, England

Posted: 16th May 2016 12:49

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I appreciate the response Paul, thank you. Useful information too.
I'm still struggling with it in my main project even though I've used the same config as the example but it could be down to me missing something. I'll work further on it.

"It is said that your life flashes before your eyes just before you die. That is true, it's called Life.”

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AppGameKit Classic Chat / AGK Tier 1 - Box2D Physics