Re: Obtaining rotation from CTM
Re: Obtaining rotation from CTM
- Subject: Re: Obtaining rotation from CTM
- From: Nathan Vander Wilt <email@hidden>
- Date: Thu, 21 Aug 2008 09:01:20 -0700
On Aug 21, 2008, at 6:56 AM, Tilman Bender wrote:
Hi there,
Is it possible to obtain the angle of rotation from a tranformation-
matrix created with CGAffineTransfrom?
If you haven't applied any other transforms to your CGAffineTransform,
you could determine the rotation (in radians) via:
double rotationInRadians = acos(theTransform.a); // if the transform
ONLY rotates around the origin
My linear algebra is fairly rusty, so I may be forgetting some things,
but I don't think it's practical to determine in the general case
when you've also scaled and translated back and forth besides just
rotating. A few cases will not affect the result above, but in most
cases used in drawing, the above will probably not work. But I'm not
sure why you'd need to do this in the first place, see below...
Background:
I am building a simple kitchen-timer app, that should work like the
ones in your mom's kitchen:
You set the time by touching a control and rotating it. Now I would
like to obtain the current
angle of my clock.
I know how to use and create CGAffineTransform stuff, but I am not
really familiar with the extraction of specific information from
such a matrix.
My question is: why do you need to get the rotation out of an affine
transform?
Somewhere in your code you've determined an angle from a move event, I
would assume. It would make a lot more sense to keep track of the
current angle, and only convert that to a CGAffineTransform in your
drawing code, than to for some reason store a CGAffineTransform and
try to extract the angle from that.
hope this helps,
-natevw
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