Introduction to Game Physics With Box2D by Ian Parberry

Author:Ian Parberry
Language: eng
Format: mobi, epub
Tags: Games, Computers, Programming, Video & Animation, Digital Media
ISBN: 9781466565760
Publisher: CRC Press
Published: 2013-02-06T00:00:00+00:00

5.4 • Our First Box2D App

127

Descending from the highest level of the code in MyGame.cpp to the

lowest level of the code for the lowly Game Object, we see in object.h

that CGameObject retains only its m_nObjectType member variable and

its constructor and destructor. Everything else has vanished. In return,

it gets a b2Body pointer m_pBody and a SetPhysicsBody function that

should be familiar from the description of function CreateBook a couple of

paragraphs ago. The latter function sets the former member variable to the

value of its parameter. Object.cpp should once again hold no surprises

for you, with the one obvious exception being that its destructor calls

g_b2dPhysicsWorld’s DestroyBody function to release the Physics body

pointed to by m_pBody.

C G a m e O b j e c t ::~ C G a m e O b j e c t (){

if ( m _ p B o d y )

g _ b 2 d P h y s i c s W o r l d . D e s t r o y B o d y ( m _ p B o d y );

} // d e s t r u c t o r

Moving up a level from the Game Object to the Object Manager, we

see in ObjectManager.cpp that CObjectManager’s draw function used to

have code to get the object’s position from the Game Object directly like

this:

if ( p ){

D 3 D X V E C T O R 2

v = p - > m _ v P o s i t i o n ;

g _ c R e n d e r W o r l d . draw ( p - > m _ n O b j e c t T y p e , v . x , v . y );

} // if

Now, it has to get the object’s position and orientation from the Physics

World instead, as follows:

if ( p ){

f l o a t a = p - > m_pBody - > G e t A n g l e ();

b 2 V e c 2 v = p - > m_pBody - > G e t P o s i t i o n ();

g _ c R e n d e r W o r l d . draw ( p - > m _ n O b j e c t T y p e ,

P W 2 R W ( v . x ) , P W 2 R W ( v . y ) , a );

} // if

The Object Manager’s move function is now only a single line of code.

Instead of doing any kind of physics calculations itself, it asks the Physics

World to perform a simulation step for it. What could be easier?

void

C O b j e c t M a n a g e r :: move (){

g _ b 2 d P h y s i c s W o r l d . Step (1.0 f / 6 0 . 0 f , 6 , 2);

} // move

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