Presentation from July 12th CocoaCoder.org meeting about integrating Chipmunk Physics into an iOS App.

1. Chipmunk Physics
Integrating A Physics Engine with UIKIT
Carl Brown
CocoaCoder.org
12 July 2012

2. What is a Physics
Engine?

3. What Physics
Engines are there?
‣Box2D
‣Chipmunk Physics

4. Why might one
choose Box2D?
‣Larger Community
‣More Examples
‣Continuous Collisions ("Bullets")
‣Good Enough for Angry Birds

5. Why Chipmunk?
‣ C w/Objective-C* instead of C++
‣ //I *HATE* C++
*Objective-C Wrapper requires $89 indie license, C is MIT licensed/Free

6. Why Chipmunk?
‣ C w/Objective-C* instead of C++
‣ //I *HATE* C++
‣ Global (Space) Origin in the same
place as UIView
‣(Seems different for Box2D)
*Objective-C Wrapper requires $89 indie license, C is MIT licensed/Free

7. Why Chipmunk?
‣ C w/Objective-C* instead of C++
‣ //I *HATE* C++
‣ Global (Space) Origin in the same
place as UIView
‣(Seems different for Box2D)
‣ Units in pixels instead of meters
*Objective-C Wrapper requires $89 indie license, C is MIT licensed/Free

8. Why Chipmunk?
‣ C w/Objective-C* instead of C++
‣ //I *HATE* C++
‣ Global (Space) Origin in the same
place as UIView
‣(Seems different for Box2D)
‣ Units in pixels instead of meters
‣ Box2D: body->DoThing(&shapeObject)
‣ //I find that annoying
*Objective-C Wrapper requires $89 indie license, C is MIT licensed/Free

9. Why Chipmunk?
‣ C w/Objective-C* instead of C++
‣ //I *HATE* C++
‣ Global (Space) Origin in the same
place as UIView
‣(Seems different for Box2D)
‣ Units in pixels instead of meters
‣ Box2D: body->DoThing(&shapeObject)
‣ //I find that annoying
*Objective-C Wrapper requires $89 indie license, C is MIT licensed/Free
See http://www.cocoanetics.com/2010/05/physics-101-uikit-app-with-box2d-for-gravity/ for
Box2D code

10. What about
Cocos2D?
‣ Cocos2D is a Gaming Framework
‣ Uses Sprites/Scenes (kinda like Views)
‣ Integrates Either Box2D or Chipmunk

11. What about
Cocos2D?
‣ Cocos2D is a Gaming Framework
‣ Uses Sprites/Scenes (kinda like Views)
‣ Integrates Either Box2D or Chipmunk
‣ Still need to deal with Physics Engine

12. What about
Cocos2D?
‣ Cocos2D is a Gaming Framework
‣ Uses Sprites/Scenes (kinda like Views)
‣ Integrates Either Box2D or Chipmunk
‣ Still need to deal with Physics Engine
‣ Might as well learn Physics w/UIKit

13. What about
Cocos2D?
‣ Cocos2D is a Gaming Framework
‣ Uses Sprites/Scenes (kinda like Views)
‣ Integrates Either Box2D or Chipmunk
‣ Still need to deal with Physics Engine
‣ Might as well learn Physics w/UIKit
‣ Cocos2D + Physics Way too long a talk

14. Integrating
Chipmunk with
UIKit
‣Create Space

15. Integrating
Chipmunk with
UIKit
‣Create Space
‣Create Body+shape for UIView

16. Integrating
Chipmunk with
UIKit
‣Create Space
‣Create Body+shape for UIView
‣Iterate Space

17. Integrating
Chipmunk with
UIKit
‣Create Space
‣Create Body+shape for UIView
‣Iterate Space
‣Reset Frame Coordinates
‣ Happens Third, but we'll talk about it Last

18. Chipmunk Spaces
!self.space = [[ChipmunkSpace alloc] init];
!
[self.space addBounds:self.view.bounds
thickness:10.0f
elasticity:1.0f friction:1.0f
layers:CP_ALL_LAYERS group:CP_NO_GROUP
collisionType:borderType];
!self.space.gravity = cpv(0.0f, 10.0f);

19. Space Parameters
‣ Bounds
‣ Walls around the Space that things bounce off of

20. Space Parameters
‣ Bounds
‣ Walls around the Space that things bounce off of
‣ Elasticity
‣ "Bounciness"

21. Space Parameters
‣ Bounds
‣ Walls around the Space that things bounce off of
‣ Elasticity
‣ "Bounciness"
‣ Friction

22. Space Parameters
‣ Bounds
‣ Walls around the Space that things bounce off of
‣ Elasticity
‣ "Bounciness"
‣ Friction
‣ Layers/Group/CollisionType
‣ Think "Collision Filters"
‣ Allow for "Friendly Fire" type scenarios

23. Space Parameters
‣ Bounds
‣ Walls around the Space that things bounce off of
‣ Elasticity
‣ "Bounciness"
‣ Friction
‣ Layers/Group/CollisionType
‣ Think "Collision Filters"
‣ Allow for "Friendly Fire" type scenarios
‣ Gravity

24. Chipmunk Body/
Shape
ChipmunkBody *body = [self.space add:[ChipmunkBody
bodyWithMass:mass andMoment:cpMomentForBox(mass,
view.bounds.size.width, view.bounds.size.height)]];
[body setData:view]; //Make retrievable
ChipmunkShape *shape = [self.space add:
[ChipmunkPolyShape boxWithBody:body
width:view.bounds.size.width
height:view.bounds.size.height]];
shape.friction = 0.8f;
shape.elasticity = 0.1f;

25. Chipmunk Body/
Shape
‣ Body
‣ Used for motion
‣ Acceleration/Mass/Momentum/Energy/Rotation/Etc

26. Chipmunk Body/
Shape
‣ Body
‣ Used for motion
‣ Acceleration/Mass/Momentum/Energy/Rotation/Etc
‣ Shape
‣ Used for Collisions and Intersections

27. Iterate Space
_displayLink = [CADisplayLink displayLinkWithTarget:self
selector:@selector(update:)];
![self.displayLink addToRunLoop:[NSRunLoop mainRunLoop]
forMode:NSRunLoopCommonModes];
-(void) update:(id) sender {
!cpFloat dt =
self.displayLink.duration*self.displayLink.frameInterval;
![self.space step:dt];
for (ChipmunkBody *body in self.space.bodies) {
UIView *view = (UIView *) [body data];
[view setTransform:body.affineTransform];
}
}
*DisplayLink requires the QuartzCore Framework

28. Display Link
‣ Calls a method on every screen refresh
‣ Used to update Motion/Position
‣ Can be set to every N frames

29. [Space Step]
‣ Tell the physics simulator to advance by
a given timestep
‣ Simple multiplication here
‣ Using an accumulator is more accurate
‣ See: http://gafferongames.com/game-physics/fix-
your-timestep/

30. setTransform
‣ Chipmunk creates an AffineTransform
for each body on each step
‣ Use that value to set the "transform"
property of the UIView

31. What the @#$% is
a Transform?
‣ Remember Matrix Math?
‣ Yeah, I didn't think so
‣ It's a Mathematical representation (matrix)
that's used to describe Position and
Rotation of an object
‣ You don't have to understand them to use
them
‣ Though it helps when debugging
‣ See http://iphonedevelopment.blogspot.com/2008/10/
demystifying-cgaffinetransform.html for more

32. Reset Frame
body.pos = cpv(view.center.x,view.center.y);
//Reset the frame to the origin and set the Transform
// to move the object back to where the frame had it
view.center=CGPointMake(0.0f, 0.0f);
view.transform=CGAffineTransformMakeTranslation(body.pos.x,
body.pos.y);

33. Why Reset Frame?
https://developer.apple.com/library/ios/#documentation/UIKit/Reference/
UIView_Class/UIView/UIView.html#//apple_ref/occ/instp/UIView/transform

34. Wait, What?
‣Think of it like This:
‣ You can EITHER use a Frame OR a Transform
to Position views
‣ NEVER BOTH
‣ IF you set both, they interact with each
other in weird ways

35. Wait, What?
‣Think of it like This:
‣ You can EITHER use a Frame OR a Transform
to Position views
‣ NEVER BOTH
‣ IF you set both, they interact with each
other in weird ways
‣ To Switch: set one to Origin and then set the
other to the same place

36. Wait, What?
‣Think of it like This:
‣ You can EITHER use a Frame OR a Transform
to Position views
‣ NEVER BOTH
‣ IF you set both, they interact with each
other in weird ways
‣ To Switch: set one to Origin and then set the
other to the same place
‣ Happens fast enough, hard to notice
‣ You can Hide or remove subview first if it's noticeable

37. Now we're set up
Not much of a game so far.
Now What?

38. Collision
Callbacks
‣ "Kill Pigs"
‣ Add Score
‣ Play Sounds
‣ [Space setDefaultCollisionHandler:...]

39. Joints &
Constraints
‣ "Build House to shoot Birds at"
‣ "Hook up Spring to Slingshot"
‣ [Space addConstraint:...]
‣ http://www.youtube.com/watch?v=ZgJJZTS0aMM

40. Apply Impulses
‣ "Shoot something out of a cannon"
‣ [Body applyImpulse:offset:...]

41. RayCasting
(ChipmunkSegmentQueryInfo)
‣ Does "Bullet" handling (Avoids
Tunneling)
‣ [Space segmentQueryFirstFrom:to:...]

42. Where Do I get it?
‣ Downloads:
‣ http://chipmunk-physics.net/downloads.php
‣ Documentation:
‣ http://chipmunk-physics.net/documentation.php