On 25/01/2013, at 12:42 AM, Matt Neuburg <m...@tidbits.com> wrote:
> They are not equivalent because the first one starts with
> CGContextTranslateCTM but the second one starts with
> CGAffineTransformMakeTranslation. In other words, the first one begins by
> translating the current transform. The second one begins with a plain vanilla
> translation. The affine transform equivalent of CGContextTranslateCTM is
> CGAffineTransformTranslate. You might want to say this:
>
> CGAffineTransform tfm = CGAffineTransformTranslate( layer.transform, pos.x,
> pos.y );
Thanks for chipping in Matt, but unfortunately you're wrong ;-) Your reasoning
seems sound but that's how transforms trip you up - they are rarely intuitive
and seemingly are hard to tame.
The correct code is:
CGRect br = self.bounds;
CGPoint anch = self.anchorPoint;
CGAffineTransform tfm = CGAffineTransformMakeTranslation(
self.position.x, self.position.y );
tfm = CGAffineTransformConcat( self.transform, tfm ); //<----- take
care that tfm is the second parameter
return CGAffineTransformTranslate( tfm, -(br.origin.x + anch.x *
br.size.width), -(br.origin.y + anch.y * br.size.height));
On the face of it this even seems equivalent to your suggestion, but I can
tell you the result is not the same.
I have eventually stumbled upon the right result which correctly transforms
down through a series of nested layers and back up again. Don't ask me how it
works or I'll start to whimper.
--Graham
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