CGAffineTransformMake - how to save the state of UIimageview to futher process it

Question

I am using CGAffineTransformMake to flip an UIImageView vertically. It works fine but it does not seem to save the new flipped position of UIImageview, because when I try to flip it 2nd time (execute the line code below) it just does not work.

shape.transform = CGAffineTransformMake(1, 0, 0, 1, 0, 0);

help please.

Thanks in advance.

Kedar

Answer 1

Hi Kedar,

If you just want to reverse the effects of a previous transformation, you may like to look into setting the shape.transform property to the value CGAffineTransformIdentity.

When you set a view's transform property you are replacing any existing transform it has, not adding to it. So if you assign a transform which causes a rotation, it will forget about any flip you had previously configured.

If you want to add an additional rotation or scaling operation to a view which you have previously transformed you should investigate the functions which allow you to specify an existing transform.

I.e. instead of using

shape.transform = CGAffineTransformMakeRotation(M_PI);

which replaces the existing transform with the specified rotation, you could use

shape.transform = CGAffineTransformRotate(shape.transform, M_PI);

this applies the rotation to the existing transform (what ever that may be) and then assigns it to the view. Take a look at Apple's documentation for CGAffineTransformRotate, it may clarify things a little.

Answer 2

Transforms are not automatically additive/accumulative as you would expect. Assigning a transform just transforms the target once.

Each transform is highly specific. If apply a rotation transform that rotates a view +45 degrees, you will see it rotate only once. Applying the same transform again does not rotate the view an additional +45 degrees. All subsequent applications of the same transforms produce no visible effect because the view is already rotated +45 degrees and that is all that transform will ever do.

To make transforms accumulative you have apply the new transform to the existing transform instead of just replacing it. So as mentioned previously for each subsequent rotation you use:

shape.transform = CGAffineTransformRotate(shape.transform, M_PI);

Which adds the new transform to the existing transform. If you add a +45 degree transform in this manner the view will rotate an additional +45 each time it is applied.