How does CALayer convert point from and to its sublayers? - ios

Question

Lets assume we have a 2D space (to simplify situation), and layer S and layer C, where C is sublayer of S.
The conversion process must affect bounds, position of C, transform of C, sublayersTransform of S, anchorPoint of C. My guess was the next:
CGAffineTransform transformToChild(CALayer *S, CALayer *C) {
CGFloat txa = - C.bounds.origin.x - C.bounds.size.width * C.anchorPoint.x;
CGFloat tya = - C.bounds.origin.y - C.bounds.size.height * C.anchorPoint.y;
CGFloat txb = C.position.x;
CGFloat tyb = C.position.y;
CGAffineTransform sublayerTransform = CATransform3DGetAffineTransform(S.sublayerTransform);
CGAffineTransform fromS = CGAffineTransformTranslate(sublayerTransform, txb, tyb);
fromS = CGAffineTransformConcat(fromS, C.affineTransform);
fromS = CGAffineTransformTranslate(fromS, txa, tya);
return fromS;
}
But this is not working when transform of the child layer is not identity (e.g. in case of rotation to M_PI_2 angle).
Whole code with layers:
CALayer *l1 = [CALayer new];
l1.frame = CGRectMake(-40, -40, 80, 80);
l1.bounds = CGRectMake(40, 40, 80, 80);
CALayer *l2 = [CALayer new];
l2.frame = CGRectMake(50, 40, 20, 20);
l2.bounds = CGRectMake(40, 40, 20, 20);
CGAffineTransform t2 = CGAffineTransformMakeRotation(M_PI / 2);
l2.affineTransform = t2;
[l1 addSublayer:l2];
CGAffineTransform toL2 = transformToChild(l1, l2);
CGPoint p = CGPointApplyAffineTransform(CGPointMake(70, 50), toL2);
NSLog(#"Custom Point %#", [NSValue valueWithCGPoint:p]);
p = [l1 convertPoint:CGPointMake(70, 50) toLayer:l2];
NSLog(#"CoreAnimation Point %#", [NSValue valueWithCGPoint:p]);
Comparison to system results:
Custom Point NSPoint: {-50, 80}
CoreAnimation Point NSPoint: {50, 40}

Answer 1

There's an old mailing list thread with some details about this here:
http://lists.apple.com/archives/quartz-dev/2008/Mar/msg00086.html
http://lists.apple.com/archives/quartz-dev/2008/Mar/msg00087.html
those messages are quite old, so e.g. they don't include the effects of the geometryFlipped property, which was added more recently, but that would just add another term onto the merged matrix.

Answer 2

So, I found this way of point conversion to and from sublayer's coordinate space, which works correctly with sublayer's non-identity transformation. sublayersTransform of the super layer is not covered here, but I think it would be not hard to extend these functions to support it.
CGPoint pointToChild(CALayer *C, CGPoint p) {
CGFloat txa = - C.bounds.origin.x - C.bounds.size.width * C.anchorPoint.x;
CGFloat tya = - C.bounds.origin.y - C.bounds.size.height * C.anchorPoint.y;
CGFloat txb = C.position.x;
CGFloat tyb = C.position.y;
p.x -= txb;
p.y -= tyb;
p = CGPointApplyAffineTransform(p, CGAffineTransformInvert(C.affineTransform));
if (C.isGeometryFlipped) {
CGAffineTransform flip = CGAffineTransformMakeScale(1.0f, -1.0f);
flip = CGAffineTransformTranslate(flip, 0, C.bounds.size.height * (2.0f * C.anchorPoint.y - 1.0f));
p = CGPointApplyAffineTransform(p, CGAffineTransformInvert(flip));
}
p.x -= txa;
p.y -= tya;
return p;
}
CGPoint pointFromChild(CALayer *C, CGPoint p) {
CGFloat txb = - C.bounds.origin.x - C.bounds.size.width * C.anchorPoint.x;
CGFloat tyb = - C.bounds.origin.y - C.bounds.size.height * C.anchorPoint.y;
CGFloat txa = C.position.x;
CGFloat tya = C.position.y;
p.x += txb;
p.y += tyb;
if (C.isGeometryFlipped) {
CGAffineTransform flip = CGAffineTransformMakeScale(1.0f, -1.0f);
flip = CGAffineTransformTranslate(flip, 0, C.bounds.size.height * (2.0f * C.anchorPoint.y - 1.0f));
p = CGPointApplyAffineTransform(p, flip);
}
p = CGPointApplyAffineTransform(p, C.affineTransform);
p.x += txa;
p.y += tya;
return p;
}