Rotate rectangle around center

Question

I am playing with Brad Larsen's adaption of the trackball app. I have two views at a 60 degree angle to each other and was wondering how I get the rotation to be in the center of this (non-closed) rectangle? In the images below I would have liked the rotation to take place all within the blue lines. Code (modified to only rotate around x axis): ``` #import "MyView.h" //===================================================== // Defines //===================================================== #define DEGREES_TO_RADIANS(degrees) \ (degrees * (M_PI / 180.0f)) //===================================================== // Public Interface //===================================================== @implementation MyView - (void)awakeFromNib { transformed = [CALayer layer]; transformed.anchorPoint = CGPointMake(0.5f, 0.5f); transformed.frame = self.bounds; [self.layer addSublayer:transformed]; CALayer *imageLayer = [CALayer layer]; imageLayer.frame = CGRectMake(10.0f, 4.0f, self.bounds.size.width / 2.…

Accepted Answer

You need to set the `imageLayer.zPosition` of each of the triangle sides to the distance from the center of the triangle (which is an equilateral triangle in your case).

If `sideHeight = self.bounds.size.height / 2.0f;` Then `distanceFromCenter = ((sideHeight / 2.0f) / sqrt(3));`

Also, when setting the rotation on the sides, you need to move them into their required positions (in your code they are hardcoded).

Updated Code

```
- (void)awakeFromNib
{
    transformed = [CALayer layer];
    transformed.anchorPoint = CGPointMake(0.5f, 0.5f);

transformed.frame = self.bounds;
    [self.layer addSublayer:transformed];

CGFloat sideHeight = self.bounds.size.height / 2.0f;
    CGFloat distanceFromCenter = ((sideHeight / 2.0f) / sqrt(3));

CGFloat sideC = sideHeight / 2.0f;
    CGFloat sideA = sideC / 2.0f;
    CGFloat sideB = (sqrt(3) * sideA);

CALayer *imageLayer = [CALayer layer];
    imageLayer.frame = CGRectMake(10.0f, (self.bounds.size.height / 2.0f) - (sideHeight / 2.0f), self.bounds.size.width / 2.0f, sideHeight);
    imageLayer.transform = CATransform3DConcat(CATransform3DMakeRotation(-DEGREES_TO_RADIANS(60.0f), 1.0f, 0.0f, 0.0f),
                                               CATransform3DMakeTranslation(0, -sideA, -sideB));
    imageLayer.contents = (id)[[UIImage imageNamed:@"IMG_0051.png"] CGImage];
    imageLayer.borderColor = [UIColor yellowColor].CGColor;
    imageLayer.borderWidth = 2.0f;
    imageLayer.zPosition = distanceFromCenter;
    [transformed addSublayer:imageLayer];

imageLayer = [CALayer layer];
    imageLayer.frame = CGRectMake(10.0f, (self.bounds.size.height / 2.0f) - (sideHeight / 2.0f), self.bounds.size.width / 2.0f, sideHeight);
    imageLayer.transform = CATransform3DConcat(CATransform3DMakeRotation(DEGREES_TO_RADIANS(60.0f), 1.0f, 0.0f, 0.0f),
                                               CATransform3DMakeTranslation(0, sideA, -sideB));
    imageLayer.contents = (id)[[UIImage imageNamed:@"IMG_0089.png"] CGImage];
    imageLayer.borderColor = [UIColor greenColor].CGColor;
    imageLayer.zPosition = distanceFromCenter;
    imageLayer.borderWidth = 2.0f;

transformed.borderColor = [UIColor whiteColor].CGColor;
    transformed.borderWidth = 2.0f;
    [transformed addSublayer:imageLayer];

UIView *line = [[UIView alloc] initWithFrame:CGRectMake(0, self.bounds.size.height / 2.0f, self.bounds.size.width, 2)];
    [line setBackgroundColor:[UIColor redColor]];
    [self addSubview:line];

line = [[UIView alloc] initWithFrame:CGRectMake(0, self.bounds.size.height * (1.0f / 4.0f), self.bounds.size.width, 2)];
    [line setBackgroundColor:[UIColor blueColor]];
    [self addSubview:line];

line = [[UIView alloc] initWithFrame:CGRectMake(0, self.bounds.size.height * (3.0f / 4.0f), self.bounds.size.width, 2)];
    [line setBackgroundColor:[UIColor blueColor]];
    [self addSubview:line];
}
```

Rotate rectangle around center

Solution

Problem