[Box2D, Allegro] Problem z konwersją jednostek, rysowaniem

#include <Allegro.h>
#include <Box2D/Box2D.h>

volatile long speed = 0;
void increment_speed()
{
    speed++;
}
END_OF_FUNCTION( increment_speed );
LOCK_VARIABLE( speed );
LOCK_FUNCTION( increment_speed );

int main()
{
    allegro_init();
    install_keyboard();
    install_mouse();
    install_timer();
   
    install_int_ex( increment_speed, BPS_TO_TIMER( 60 ) );
   
    set_color_depth( 24 );
    set_gfx_mode( GFX_AUTODETECT_WINDOWED, 800, 600, 0, 0 );
    clear_to_color( screen, makecol( 0, 0, 0 ) );
   
    BITMAP * buffer = create_bitmap( 800, 600 );
    clear_to_color( buffer, makecol( 0, 0, 0 ) );
   
    BITMAP * klocek = create_bitmap( 50, 50 );
    clear_to_color( klocek, makecol( 255, 0, 0 ) );
   
    BITMAP * ziemia = create_bitmap( 5000, 1000 );
    clear_to_color( ziemia, makecol( 0, 255, 0 ) );
   
    b2World world( b2Vec2( 0.0f, 9.8f ) );
   
    //ziemia
    b2BodyDef groundDef;
    groundDef.position.Set( 0.0f, 20.0f );
    b2Body * ground = world.CreateBody( & groundDef );
   
    b2PolygonShape groundShape;
    groundShape.SetAsBox( 100.0f / 2, 20.0f / 2 );
    ground->CreateFixture( & groundShape, 1.0f );
   
    //klocek
    b2BodyDef boxDef;
    boxDef.type = b2_dynamicBody;
    boxDef.position.Set( 7.5f, 5.0f );
    b2Body * box = world.CreateBody( & boxDef );
   
    b2PolygonShape boxShape;
    boxShape.SetAsBox( 1.0f / 2, 1.0f / 2 );
    b2FixtureDef boxFixDef;
    boxFixDef.shape = & boxShape;
    boxFixDef.density = 1.0f;
    boxFixDef.friction = 0.3f;
    box->CreateFixture( & boxFixDef );
   
   
    while( !key[ KEY_ESC ] )
    {
        while( speed > 0 )
        {
            speed--;
           
            world.Step( 1.0f / 100.0f, 8, 3 );
            b2Vec2 pos_klocek = box->GetPosition();
            b2Vec2 pos_ground = ground->GetPosition();
           
            blit( ziemia, buffer, 0, 0,( pos_ground.x * 50 ) -( 5000 / 2 ),( pos_ground.y * 50 ) -( 1000 / 2 ), 5000, 1000 );
            blit( klocek, buffer, 0, 0,( pos_klocek.x * 50 ) -( 50 / 2 ),( pos_klocek.y * 50 ) -( 50 / 2 ), 50, 50 );
           
            blit( buffer, screen, 0, 0, 0, 0, 800, 600 );
            clear_to_color( buffer, makecol( 0, 0, 0 ) );
        }
    }
   
    destroy_bitmap( buffer );
    destroy_bitmap( klocek );
    allegro_exit();
    return 0;
} END_OF_MAIN();