Can't get the bullet image to spawn at right location - sprite-kit

Question

hi guys I have this Sprite Kit cannon game that it works perfectly but with one problem. if I point the cannon to a very top or very bottom as you can see in the image, the bullet since it is spawning from the center of the cannon's sprite, it shows clearly that the bullet is being fired not from the cannon's tunnel but from the center of the image, how can I fix this bullet position so that it would always spawn from the mouth of the tunnel.
here is my code so far.
- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
CGPoint location = [_Player position];
UITouch *touch = [touches anyObject];
CGPoint touchLocation = [touch locationInNode:self.scene];
bullet = [SKSpriteNode spriteNodeWithImageNamed:#"cannonbullet"];
bullet.xScale = 0.06;
bullet.yScale = 0.06;
bullet.position =
CGPointMake(location.x+_Player.zRotation,location.y+_Player.zRotation);
bullet.zPosition = 0;
CGPoint offset = rwSub(touchLocation, bullet.position);
if (offset.x <= 0) return;
[self addChild:bullet];
CGPoint direction = rwNormalize(offset);
CGPoint shootAmount = rwMult(direction, 400);
CGPoint realDest = rwAdd(shootAmount, bullet.position);
float velocity = 480.0/1.0;
float realMoveDuration = self.size.width / velocity;
SKAction * actionMove = [SKAction moveTo:realDest duration:realMoveDuration];
SKAction * actionMoveDone = [SKAction removeFromParent];
[bullet runAction:[SKAction sequence:#[actionMove, actionMoveDone]]];
[self animStarter];
}
as you can see in the image the bullet is spawning from a very off location if the cannon is not shooting straight.

Answer 1

Make sure that your cannon's barrel sprite image is lined up with the center X axis of the image (see the picture). You can drag out or copy the image if you want to duplicate the project.
Below is the code to rotate the cannon to the touch location and fire a cannonball once it has reached the the desired rotation angle. I think you are still using the zombie conga code so I used its functions for your convenience.
#import "MyScene.h"
static inline CGPoint CGPointSubtract(const CGPoint a,
const CGPoint b)
{
return CGPointMake(a.x - b.x, a.y - b.y);
}
static inline CGPoint CGPointMultiplyScalar(const CGPoint a,const CGFloat b)
{
return CGPointMake(a.x * b, a.y * b);
}
static inline CGFloat CGPointLength(const CGPoint a)
{
return sqrtf(a.x * a.x + a.y * a.y);
}
static inline CGPoint CGPointNormalize(const CGPoint a)
{
CGFloat length = CGPointLength(a);
return CGPointMake(a.x / length, a.y / length);
}
static inline CGFloat CGPointToAngle(const CGPoint a)
{
return atan2f(a.y, a.x);
}
static inline CGFloat ScalarSign(CGFloat a)
{
return a >= 0 ? 1 : -1;
}
static inline CGFloat ScalarShortestAngleBetween(const CGFloat a, const CGFloat b)
{
CGFloat difference = b - a;
CGFloat angle = fmodf(difference, M_PI * 2);
if (angle >= M_PI) {
angle -= M_PI * 2;
}
return angle;
}
static const float ROTATE_RADIANS_PER_SEC = 4 * M_PI;
static const float MOVE_POINTS_PER_SEC = 120.0;
#implementation MyScene
{
SKSpriteNode *cannon;
NSTimeInterval _lastUpdateTime;
NSTimeInterval _dt;
CGPoint _velocity;
CGPoint _lastTouchLocation;
BOOL fireCannon;
CGPoint destination;
}
-(id)initWithSize:(CGSize)size
{
if (self = [super initWithSize:size])
{
[self createCannon];
fireCannon = false;
}
return self;
}
-(void)createCannon
{
cannon = [SKSpriteNode spriteNodeWithImageNamed:#"cannon"];
cannon.position = CGPointMake(self.size.height/2, self.size.width/2);
[self addChild:cannon];
}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
UITouch *touch = [touches anyObject];
CGPoint touchLocation = [touch locationInNode:self.scene];
fireCannon = true;
destination = touchLocation;
_lastTouchLocation = touchLocation;
CGPoint offset = CGPointSubtract(touchLocation, cannon.position);
CGPoint direction = CGPointNormalize(offset);
_velocity = CGPointMultiplyScalar(direction, MOVE_POINTS_PER_SEC);
}
-(void)update:(CFTimeInterval)currentTime
{
if (_lastUpdateTime) {
_dt = currentTime - _lastUpdateTime;
} else {
_dt = 0;
}
_lastUpdateTime = currentTime;
[self rotateSprite:cannon toFace:_velocity rotateRadiansPerSec:ROTATE_RADIANS_PER_SEC];
}
- (void)rotateSprite:(SKSpriteNode *)sprite
toFace:(CGPoint)velocity
rotateRadiansPerSec:(CGFloat)rotateRadiansPerSec
{
float targetAngle = CGPointToAngle(velocity);
float shortest = ScalarShortestAngleBetween(cannon.zRotation, targetAngle);
float amtToRotate = rotateRadiansPerSec * _dt;
if (ABS(shortest) < amtToRotate)
{
amtToRotate = ABS(shortest);
}
sprite.zRotation += ScalarSign(shortest) * amtToRotate;
if ((ABS(shortest) == amtToRotate) && (fireCannon == true))
{
fireCannon = false;
[self fire:targetAngle];
}
}
-(void)fire:(float)targetAngle
{
SKSpriteNode *cannonBall = [SKSpriteNode spriteNodeWithColor:[SKColor redColor] size:CGSizeMake(5, 5)];
cannonBall.position = cannon.position;
[self addChild:cannonBall];
int x = cannon.position.x + 1000 * cos(targetAngle);
int y = cannon.position.y + 1000 * sin(targetAngle);
[cannonBall runAction:[SKAction moveTo:CGPointMake(x, y) duration:2]];
}
#end