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

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

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.

Answer 2

I have the same problem with you and I found the solution! I want to rotate the UIImageView, because I will have the animation. To save the image I use this method:
void CGContextConcatCTM(CGContextRef c, CGAffineTransform transform)
the transform param is the transform of your UIImageView so anything you have done to the imageView will be the same with image! And I have write a category method of UIImage.
-(UIImage *)imageRotateByTransform:(CGAffineTransform)transform{
// calculate the size of the rotated view's containing box for our drawing space
UIView *rotatedViewBox = [[UIView alloc] initWithFrame:CGRectMake(0,0,self.size.width, self.size.height)];
rotatedViewBox.transform = transform;
CGSize rotatedSize = rotatedViewBox.frame.size;
[rotatedViewBox release];
// Create the bitmap context
UIGraphicsBeginImageContext(rotatedSize);
CGContextRef bitmap = UIGraphicsGetCurrentContext();
// Move the origin to the middle of the image so we will rotate and scale around the center.
CGContextTranslateCTM(bitmap, rotatedSize.width/2, rotatedSize.height/2);
//Rotate the image context using tranform
CGContextConcatCTM(bitmap, transform);
// Now, draw the rotated/scaled image into the context
CGContextScaleCTM(bitmap, 1.0, -1.0);
CGContextDrawImage(bitmap, CGRectMake(-self.size.width / 2, -self.size.height / 2, self.size.width, self.size.height), [self CGImage]);
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}
Hope this will help you.

Answer 3

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.
BTW, the documentation says: "If you don’t plan to reuse an affine transform, you may want to use CGContextScaleCTM, CGContextRotateCTM, CGContextTranslateCTM, or CGContextConcatCTM."