SKNode convertPoint toNode & fromNode confusion?

Question

I am a little confused by how the SKNode methods convertPoint:fromNode: and convertPoint:ToNode: are working, I have looked at the docs but its not that obvious what they do. For example, this (see diagram below) is a small test I did using convertPoint:fromNode: The black area is the SKScene background, the blue area is an SKSpriteNode parented to the SKScene, the red area is another SKSpriteNode parented to the blue sprite. The anchor points for both sprites are shown by the small green dots. What I wanted to do was get the scene position for the red sprite, which I did using the following code:

CGPoint positionInScene = [self convertPoint:[self position]                                      fromNode:[self redSprite]]; 

The result from this is

positionInScene = [105, 205] 

Which is what I expected, as that would be the origin of the red square in scene space. What I am confused about is the arguments. From what I can guess:

[SKNode_A convertPoint: CGPoint_B toNode: SKScene_C] 

• SKNode_A = The node coordinate space to convert to ...
• CGPoint_B = The point to convert (not sure why its [self position] above)
• SKNode_C = The node coordinate space to convert from ...

My initial try at this was [self convertPoint:[redSprite position] fromNode:redSprite] because I was wanting to convert the red sprites origin to the scene. It just seems a bit clunky to get your head round, if anyone can cast a little light & logic on both this and its friend convertPoint:toNode: it would be very much appreciated.

Answer 1

- (CGPoint)convertPoint:(CGPoint)point fromNode:(SKNode *)node

This essentially is saying: convert a point expressed in another node's coordinate system into the caller's(self) coordinate system. The key is that the point has to be expressed in the node's coordinate system for this to work. If you are using a sprite's position as the point, you will need to pass the sprite's parent node as the last argument.

Example: To get the red square's position in the scene's coordinate system:

CGPoint positionInScene = [self convertPoint:[redSprite position]                                     fromNode:[self blueSprite]]; 

[5, 5] in blueSprite's coordinate system --> [105, 205] in the scene's coordinate system

Your Example:

CGPoint positionInScene = [self convertPoint:[self position]                                      fromNode:[self redSprite]]; 

[0, 0] in redSprite's coordinate system --> [105, 205] in the scene's coordinate system

You ended up with the same result from your code, only because [self position] was [0, 0] and you used redSprite's coordinate system. Your initial try was close, you just needed to provide the coordinate system that redSprite's position was expressed in, which is the parent sprite's coordinate system; in this case, that's blueSprite.

- (CGPoint)convertPoint:(CGPoint)point toNode:(SKNode *)node

This method is very much the opposite of the first method. It will convert a point in the caller's coordinate system into another node's coordinate system. The key for this method is that the point given has to be expressed in the caller's coordinate system.

Example: Let's say your scene received a touch at [125, 225], and you wanted to add a new sprite at that location, which happens to be within the bounding box of redSprite. So to add a new sprite as a child of redSprite, you need to get the touch's location in redSprite's coordinate system. To get that:

CGPoint positionInRedSprite = [self convertPoint:touchLocation                                            toNode:[self redSprite]]; 

[125, 225] in the scene's coordinate system --> [20, 20] in redSprite's coordinate system