On Jun 17, 2009, at 11:53 PM, Erik Buck wrote:
My third grader was taught about associative and commutative math
operations. Some matrix operations are associative and some are
commutative. That's why the order of operations matters.
Just in case: I didn't mean to imply that even third graders should
understand matrix math.
They do in Star Trek The Next Generation. Or maybe it was calculus.
You're absolutely correct in general, but - if I'm not mistaken -
CGAffineTransform rotations and scalings already do their thing with
respect to the center of the view they're being applied to. In fact,
I tested that before writing my solution to the OP's problem. And
that is indeed the source of the problem. Rotations and scalings, as
implemented by CGAffineTransform, are with respect to the view's
center but translations are with respect to the view's origin.
Therefore, if you scale while you translate, the view's origin
changes and the translation is no longer being applied with the
correct offset.
To be honest, I'm a bit surprised that CGAffineTransform rotations
and scalings are already defined with respect to the center of the
view. There's still a part of my brain that thinks that that isn't
true, but my tests indicated otherwise.
Wagner
OK. I will have to try this at home. If you create a
CGAffineTransform as I did and then use [myUIView
setTransform:myTransform], how can a UIView know that part of my
transform matrix applies one way and another part applies another
way ? I think it is mathematically impossible unless the view is
pulling components out of the transform matrix rather than just
concatenating the matrix I supply with whatever matrix it's
superview uses. If it pulls components out of the transform matrix,
it is not a proper affine transform (because shear and skew and some
degenerate cases will not work) and all of the functions are misnamed.
Your objections are precisely the reasons why some part of my brain is
screaming at the rest of it for publicly making a statement which it
believes to be incorrect. It may sound strange, but I'll be happy to
be proven wrong on this one.
Wagner
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