Question

Can anyone give me some idea on how to solve this problem for use on an iPhone app. Users can rotate and flip image thumbnails but I want to save these transformations so I can apply them to the full size image later. I want to be able to work out the simplest transformation from a string off all transformations applied. I’ve include some examples below.

flipH+flipV = r180

flipH+flipV+flipH = flipV

flipV+flipH+flipV = flipH

r90CW+flipH+flipV+flipH+r180CCW = flipH+r90CCW

Answer 1

I understand you are worried by the cost of applying each transformation in a sequence to your image; as opposed to this, you would like to store a composed transformation and apply it just once.

If this is correct, then you can use the methods that are listed under the “Modifying Affine Transformations ” section in CGAffineTransform Reference. I.e., you would start with a basic identity transform:

 CGAffineTransform transform = CGAffineTransformIdentity;

then you would apply all of your transform in sequence and build a composed transform:

transform = CGAffineTransformRotate(transform, rotation);
transform = CGAffineTransformTranslate(transform, dx, dy);
...

that you can apply to your image.

EDIT:

Composing 2 transformations means multiplying 2 matrices. If you just compose rotations, you will get a rotation matrix with different values (0s are kept in the same locations); if you compose translation and rotation, then you will get a new matrix with 2 more values (as compared to a rotation matrix). Have a look at this explanation, which is very visual.

Affine transforms are entirely ok for any iOS version (they are really a basic thing), including iOS 5.