Where is the difference between an "transform" and an "affine transform"?

Question

There is a function like:

CATransform3DGetAffineTransform

Returns the affine transform represented by 't'. If 't' can not be exactly represented as an affine transform the returned value is undefined.

I'm not so math-orientated, so a easy to understand description would be very nice. Wikipedia was no big help here.

Answer 1

CoreAnimation uses 3D transforms. Affine transforms are a 2D subset of those 3D transforms. They allow only translation, rotation and scaling in X,Y plan.

Answer 2

Affine transformations are transformations that do not "overly distort" your geometry in the sense that:

• points that lay on a line before still lie on a line after the transformation
• the relative distances between points stay the same (so a square might not be square anymore, but still has equal side lengths)

So rotation and scaling are affine - projection, for example, is not.