简单的2d图形变换--仿设变换AffineTransform

.
                    | a,  b,  0 |
{x‘,y‘,1}={x,y,1} x | c,  d,  0 |
                    | tx, ty, 1 |

| 1,  0,  0 |
| 0,  1,  0 |
| 0,  0,  1 |

| 1,  0,  0 |
| 0,  1,  0 |
|tx, ty,  1 |

-(void)fireBullet
{
    // Don‘t fire bullets if the ship is destroyed.
    if(_ship == nil) return;
    
    // This is sort of a fancy math way to figure out where to fire the bullet from.
    // You could figure this out with more code, but I wanted to have fun with some maths.
    // This gets the transform of one of the "gunports" that I marked in the CCB file with a special node.
    CGAffineTransform transform = _ship.gunPortTransform;
    
    // An affine transform looks like this when written as a matrix:
    // | a, c, tx |
    // | b, d, ty |
    // The first column, (a, b), is the direction the new x-axis will point in.
    // The second column, (c, d), is the direction the new y-axis will point in.
    // The last column, (tx, ty), is the location of the origin of the new transform.
    
    // The position of the gunport is just the matrix‘s origin point (tx, ty).
    CGPoint position = ccp(transform.tx, transform.ty);
    
    // The original sprite pointed downwards on the y-axis.
    // So the transform‘s y-axis, (c, d), will point in the opposite direction of the gunport.
    // We just need to flip it around.
    CGPoint direction = ccp(-transform.c, -transform.d);
    
    // So by "fancy math" I really just meant knowing what the numbers in a CGAffineTransform are. ;)
    // When I make my own art, I like to align things on the positive x-axis to make the code "prettier".
    
    // Now we can create the bullet with the position and direction.
    Bullet *bullet = (Bullet *)[CCBReader load:@"Bullet"];
    bullet.position = position;
    //bullet.rotation = -CC_RADIANS_TO_DEGREES(ccpToAngle(direction));
    
    // Make the bullet move in the direction it‘s pointed.
    bullet.physicsBody.velocity = ccpMult(direction, bullet.speed);
    
    [_physics addChild:bullet];
    [_bullets addObject:bullet];
    
    // Give the bullet a finite lifetime.
    [bullet scheduleBlock:^(CCTimer *timer){
        [self destroyBullet:bullet];
    } delay:bullet.duration];
    
    // Make some noise. Add a little chromatically tuned pitch bending to make it more musical.
    int half_steps = (arc4random()%(2*4 + 1) - 4);
    float pitch = pow(2.0f, half_steps/12.0f);
    [[OALSimpleAudio sharedInstance] playEffect:@"Laser.wav" volume:1.0 pitch:pitch pan:0.0 loop:NO];
}