Sprite Kit: Anchor Point is Changing Sprite`s actual Position

Question

please help me sort this confusion out.

From Sprite Kit Programming Guide:

A sprite node’s anchorPoint property determines which point in the frame is positioned at the sprite’s position.

My understanding of this is that if I change the Anchor Point, the sprite`s position should stay unchanged and only the texture rendering should be moved accordingly.

But when I set the anchor point, my sprite`s position actually changes! Take a look at this snippet:

/* debug */
if (self.currentState == self.editState) {
    printf("B: relativeAnchorPoint = %.02f,%.02f  ", relativeAnchorPoint.x, relativeAnchorPoint.y);
    printf("position = %.02f,%.02f\n",self.position.x, self.position.y);
}

[self setAnchorPoint:relativeAnchorPoint];

/* debug */
if (self.currentState == self.editState) {
    printf("A: relativeAnchorPoint = %.02f,%.02f  ", relativeAnchorPoint.x, relativeAnchorPoint.y);
    printf("position = %.02f,%.02f\n",self.position.x, self.position.y);
}

Output:

A: relativeAnchorPoint = 0.65,0.48 position = 1532.00,384.00

B: relativeAnchorPoint = 0.65,0.48 position = 1583.00,384.00

What am I missing?

Thanks in advance

*edit: additional info: * it only happens when my sprite has xScale to -1 to invert image

Answer 1

I made a quick test to confirm your observation, and it is indeed correct.

As the xScale becomes negative the anchorPoint does actually affect the node's position.

I tend to think of this as a bug since there seems to be no correlation between the negative xScale and the increase in x position. And it can't be considered normal behavior.

Also this only happens when you change the anchorPoint after the xScale is already negative. You can set anchorPoint, then change xScale all you want and things will be fine, position will not change.

I confirmed this issue exists in both Xcode 5.1 (iOS 7) and Xcode 6 beta (iOS 8 beta).

If you run the following code in a newly created Sprite Kit project in place of its auto-created MyScene.m file you'll see that as anchorPoint changes randomly between 0.0 and 1.0 the position of the sprite always remains the same until the xScale property changes to a negative value. At that point position.x starts to increase significantly.

#import "MyScene.h"

@implementation MyScene
{
    SKSpriteNode *sprite;
}
-(id) initWithSize:(CGSize)size
{
    if (self = [super initWithSize:size])
    {
        self.backgroundColor = [SKColor colorWithRed:0 green:0 blue:0.2 alpha:1];

        sprite = [SKSpriteNode spriteNodeWithImageNamed:@"Spaceship"];
        sprite.position = CGPointMake(CGRectGetMidX(self.frame),
                                       CGRectGetMidY(self.frame));
        sprite.anchorPoint = CGPointMake(0.2, 0.7);
        [self addChild:sprite];

        SKAction *action = [SKAction scaleXTo:-1.0 duration:10];
        [sprite runAction:[SKAction repeatActionForever:action]];
    }
    return self;
}
-(void) update:(CFTimeInterval)currentTime
{
    sprite.anchorPoint = CGPointMake(arc4random_uniform(10000) / 10000.0,
                                     arc4random_uniform(10000) / 10000.0);

    NSLog(@"pos: {%.1f, %.1f}, xScale: %.3f, anchor: {%.2f, %.2f}",
          sprite.position.x, sprite.position.y, sprite.xScale,
          sprite.anchorPoint.x, sprite.anchorPoint.y);
}
@end

There is a workaround for this bug:

If xScale is already negative, invert it, then set the anchorPoint, then re-invert xScale. You may need to do the same with yScale if that too can become negative.

The following update method incorporates this workaround and I confirmed that this is working as intended:

-(void) update:(CFTimeInterval)currentTime
{
    BOOL didInvert = NO;
    if (sprite.xScale < 0.0)
    {
        didInvert = YES;
        sprite.xScale *= -1.0;
    }

    sprite.anchorPoint = CGPointMake(arc4random_uniform(10000) / 10000.0,
                                     arc4random_uniform(10000) / 10000.0);

    if (didInvert)
    {
        sprite.xScale *= -1.0;
    }

    NSLog(@"pos: {%.1f, %.1f}, xScale: %.3f, anchor: {%.2f, %.2f}",
          sprite.position.x, sprite.position.y, sprite.xScale, 
          sprite.anchorPoint.x, sprite.anchorPoint.y);
}

The sprite.position now remains the same throughout the entire scaleXTo action duration.