Hi David,
On Jun 18, 2009, at 12:42 AM, David Duncan wrote:
Your solution works because you've scaled the coordinate system, so
the translation needs to be scaled too (to get the change specified
by the translation).
Yes, that was the point I was trying to make on earlier posts, though
in a confusing way. My intuition was correct but I was too tired to
understand why. It's actually rather simple. A scaling is like
changing the size of your ruler or meter-stick (a smaller ruler
produces larger lengths, so the object you're measuring appears
bigger, and vice-versa). The problem is, though, that changing your
ruler also changes how far you travel when you apply a translation
(changing your ruler *is* changing the scale of your entire coordinate
system). Thus, if you scale up, you also travel more, and if you scale
down, you travel less, and always by the same factor. So, in order to
travel the correct distance *and* make your object have a different
size, you need to divide the translation offset by the scale of your
scaling. The translation offset, of course, has to be the correct one,
and that was a vector from the object's center to the center of the
screen.
I am a bit confused now by something you said, though.
The issue with a translation is that it starts to confuse your
expectations of the relationship between a view's center and the
view's frame, because the center isn't altered by the transform,
only the frame is. This is also why I recommend not using
translations as part of your transform if they are not necessary,
because then you look at the center of the view and you discover
that it hasn't actually moved, yet your view is clearly not where it
was before you set the transform.
Of note at this point is that the view's center is in its super
view's coordinate system, while the view's bounds is in its own
coordinate system (and thus the origin is usually, but need not be,
CGPointZero).
If the center is in the superview's coordinate system, how can it not
be changed by a translation? In the view's own coordinate system, yes,
I can see that that's true, because then the center is half-way the
bounds' width and height, and those don't change under a translation.
Wagner
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