Hi all,

I'm writing an iPhone application where I'd like the user to be able to push a button and be able to rotate / stretch a rectangle with regards to the red point in the picture below:

As you can see I'd like to be able to rotate around the red dot (or half way down the y axis) and would also be able to stretch the rectangle with it still being anchored to the red point (doesn't move).

I should mention that in iPhone, that anchor point is initially set to the center of the object.

I struggle badly with transformations, but was able to figure out the rotation:

    self.rotateView.transform = CGAffineTransformTranslate(self.rotateView.transform,   self.rotateView.bounds.size.width / 2, 0);
    self.rotateView.transform = CGAffineTransformRotate(self.rotateView.transform, M_PI / 180);
    self.rotateView.transform = CGAffineTransformTranslate(self.rotateView.transform, self.rotateView.bounds.size.width  / 2, 0);
 

The solution is to translate the rotation pivot to the origin, rotate and then translate back (in short, you're missing a sign in your code).

For instance assume you have a unit square (width and height = 1) centered on the origin (eg its far corners are [-0.5, -0.5] and [+0.5, +0.5]) and you want to rotate it around its upper left corner ([-0.5, 0.5]): you would first translate the cube's origin to this pivot, eg subtract [-0.5, +0.5] from all vertex coordinates, then rotate them and finally undo the translation by adding [-0.5, +0.5] to all vertices.

As for scaling - consider the following order: scale, translate, rotate, translate back. Store your rectangle in a "rest position" and calculate its transformed vertices from scratch every time it's scaled or rotated.