transform rotation and translation issue - rotation

Question

i have the following situation , i'm trying to make a photo editor and i need to do scale,translate and rotate operations for an image with around center of view
the problem is that if i apply translate after 90 degree translate left is translate top and the anchor point is not the center of the view after translation
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view.
// ImageView
v = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 768, 500)];
[v setBackgroundColor:[UIColor redColor]];
// ImageView's Image
UIImage *img = [UIImage imageNamed:#"_my.jpg"];
[v setImage:img];
v.contentMode = UIViewContentModeScaleAspectFit;
v.layer.anchorPoint = CGPointMake(0.5, 0.5);
[self.view setBackgroundColor:[UIColor clearColor]];
// UiView containing ImageView
vc = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 768, 500)];
[vc addSubview:v];
[self.view addSubview:vc];
}
- (IBAction)Click:(id)sender {
// Translate
if ([sender tag] == 1) {
CGAffineTransform t0 = v.transform;
CGAffineTransform t1 = CGAffineTransformTranslate(t0, 10.0, 0.0);
v.transform = t1;
}
// Rotate
if ([sender tag] == 2) {
CGAffineTransform t0 = v.transform;
CGAffineTransform t1 = CGAffineTransformRotate(t0, DEGREES_TO_RADIANS(10));
v.transform = t1;
}
}
does anyone know how can i keep the center of view as the anchor point and translate after rotation to work properly ?
tx

Answer 1

Transformations with Origin
However, especially in 2D, you frequently need to add an origin displacement to your transformation. That's usually added to the beggining, and must be negated too, so:
// World Matrix with Origin
-OriginTranslation * Scale * Rotation * PositionTranslation
The same thing can also be done with a view matrix, but you add it to the end and this time you don't negate it:
// View Matrix with Origin
-PositionTranslation * -Rotation * Zoom * OriginTranslation
Transformations with Selective Origin
There are also some cases where you want to have an origin displacement, but you want it to affect only your scale and rotation, but not your translation. The way to handle that is to undo the origin transformation before reaching the translation component. For example, with the world matrix:
I don't remember, exactly, how transformation is applying for view, one from those should be work.
// World Matrix with Origin for Scale and Rotation
-OriginTranslation * Scale * Rotation * OriginTranslation * PositionTranslation
And the view matrix:
// View Matrix with Origin
-PositionTranslation * -OriginTranslation * -Rotation * Zoom * OriginTranslation
Example of transformations without origin, order: scale * rotation * translation