Objective-c: How to rotate CGRect

Question

I've tried to rotate my rectangle, but it does not work properly.

Here is part of my code, I found it in another post:

#define DEGREES_TO_RADIANS(x) (M_PI * x / 180.0)
-(void)drawRect:(NSRect)rect
{
   CGContextRef cRef = [[NSGraphicsContext currentContext] graphicsPort];

   // points[] - this is a CGPoints array, length_one is a double value
   CGRect rect_one = CGRectMake(points[0].x, points[0].y, (CGFloat)length_one, 40);

   // I've print out the origin of old one and new one
   NSLog(@"old rect -- %f, %f", rect_one.origin.x, rect_one.origin.y);

  float centerX = rect_one.origin.x + (rect_one.size.width / 2.0);
  float centerY = rect_one.origin.y + (rect_one.size.height / 2.0);

  CGAffineTransform rotation = CGAffineTransformMakeRotation(DEGREES_TO_RADIANS(10));
  CGAffineTransform moveAnchor = CGAffineTransformMakeTranslation(centerX, centerY);

  CGAffineTransform centeredRotation = CGAffineTransformConcat(moveAnchor, rotation);

  CGRect rotatedRect = CGRectApplyAffineTransform(rect_one, centeredRotation);
  CGContextAddRect(cRef, rotatedRect);

  // new one
  NSLog(@"new rect -- %f, %f", rotatedRect.origin.x, rotatedRect.origin.y);
}

And the origin changed a lot even I can not find my new rectangle from the view. Old origin is (x = 263.3, y = 502.8) and the new origin is (x=506.1, y=1132.0) How does this system work especially how to assign the angle of rotation? If possible, could you guys help me brief explain it. Thanks a lot!!!!

Caesaria

What do you exactly intend to rotate - NSImage, NSImageView, Custom View? – Valeryvalerye

CGRectApplyAffineTransform(rect, transform) does not return a transformed rectangle. It returns the smallest rectangle that fits rect when transform is applied. – Wira

CGRect as NSRect cannot expression a rotation; they have an origin and size and this is assumed to be unrotated. – Poland

@TBlue not a image or view. I use CGRect to draw a rectangle. Is it possible to only rotate the rectangle that I drew instead of the whole view? – Caesaria

@Poland so in order to rotate I need to put that rectangle into a view and then rotate? – Caesaria

Answer 1

As others have said, an NSRect or CGRect is always an axis-aligned rectangle. CGRectApplyAffineTransform returns the axis-aligned bounding box of the translated rectangle:

Note that the new rectangle (the bounding box) is likely to have different dimensions than the original rectangle, if you applied a rotation. This means it's useless to apply a transform to a rectangle if you want to draw the transformed rectangle.

Instead, you need to transform the coordinate system of the graphics context. Think of it like this: you're drawing on a sheet of paper on a desk. You always draw a rectangle that is aligned to the edges of the desk. To draw a rotated rectangle, you rotate the paper, then draw a rectangle as usual. Changing the CTM is like rotating (or moving or shrinking or expanding) the sheet of paper.

-(void)drawRect:(NSRect)rect {
    CGContextRef gc = [[NSGraphicsContext currentContext] graphicsPort];

    // points[] - this is a CGPoints array, length_one is a double value
    CGRect rect = CGRectMake(points[0].x, points[0].y, (CGFloat)length_one, 40);

    CGFloat xMid = CGRectGetMidX(rect);
    CGFloat yMid = CGRectGetMidY(rect);

    CGContextSaveGState(gc); {
        // Translate the origin to the midpoint of the rectangle, because rotations
        // always happen around the origin.
        CGContextTranslateCTM(gc, xMid, yMid);

        // Rotate the coordinate system by 10 degrees.
        CGContextRotateCTM(gc, 10 * M_PI / 180);

        // Prepare a new rectangle, with the same size as the original rectangle
        // but centered on the origin.
        CGRect newRect = rect;
        newRect.origin.x = -newRect.size.width / 2;
        newRect.origin.y = -newRect.size.height / 2;

        // Add the rectangle to the context's path.
        CGContextAddRect(gc, newRect);

        // Draw the new rectangle.
        CGContextSetFillColorWithColor(gc, [NSColor lightGrayColor].CGColor);
        CGContextSetStrokeColorWithColor(gc, [NSColor blackColor].CGColor);
        CGContextSetLineWidth(gc, 2);
        CGContextSetLineJoin(gc, kCGLineJoinMiter);
        CGContextDrawPath(gc, kCGPathFillStroke);
    } CGContextRestoreGState(gc);

}

Garrett

Answer 2

override func drawRect(rect: CGRect) {
    super.drawRect(rect)
    let context : CGContextRef = UIGraphicsGetCurrentContext()
    var rect = CGRectMake(100, 100, 120, 180)
    CGContextSaveGState(context);
    var path :CGMutablePathRef  = CGPathCreateMutable();
    var midX : CGFloat = CGRectGetMidX(rect);
    var midY : CGFloat = CGRectGetMidY(rect);
    var transfrom: CGAffineTransform  =
    CGAffineTransformConcat(
        CGAffineTransformConcat(CGAffineTransformMakeTranslation(-midX, -midY),CGAffineTransformMakeRotation(0.3)),
        CGAffineTransformMakeTranslation(midX, midY));
    CGPathAddRect(path, &transfrom, rect);
    CGContextSetFillColorWithColor(context, UIColor.blueColor().CGColor);
    CGContextAddPath(context, path);
    CGContextFillPath(context);
    CGContextRestoreGState(context);
}

Grazia