CGPoint rotation changes distance from origin

Question

I want to rotate a CGPoint(red rect) around another CGPoint(blue rect) but it changes distance from the origin(blue rect)...when i give 270 in angle it creates the point right above the origin but when i give 90 as angle value it comes down the origin BUT CHANGES THE DISTANCE ALSO almost three times more....I want to keep the distance same and want to rotate CGPoint around another. Please guide any approach for rotation of cgpoints...

distance = 100;
angle = 270*M_PI/180;    
rotatedPoint.x = initialPoint.x+distance*cos(angle);
rotatedPoint.y = initialPoint.y+distance*sin(angle);
    //rotatedPoint.x = initialPoint.x+tan(angle);

[test setCenter:rotatedPoint];   
[test setBackgroundColor:[UIColor redColor]];

Thanks

WendiKidd WendiKidd · Accepted Answer · 2012-05-19T00:46:06

CGAffineTransform is a handy tool when it comes to rotation, translation, and scaling. To make sure a point is rotated properly, you must translate it to the origin, rotate it, and then translate it back.

To complete your transformation, something like the following should do the trick:

CGPoint pointToRotate = CGPointMake(30, 30);
float angleInRadians = DEGREES_TO_RADIANS(90);
CGPoint distanceFromOrigin = CGPointMake(0 - pointToRotate.x, 0 - pointToRotate.y);

CGAffineTransform translateToOrigin = CGAffineTransformMakeTranslation(distanceFromOrigin.x, distanceFromOrigin.y);
CGAffineTransform rotationTransform = CGAffineTransformMakeRotation(angleInRadians);
CGAffineTransform translateBackFromOrigin = CGAffineTransformInvert(translateToOrigin);

CGAffineTransform totalTransform = CGAffineTransformConcat(translateToOrigin, rotationTransform);
totalTransform = CGAffineTransformConcat(totalTransform, translateBackFromOrigin);

pointToRotate = CGPointApplyAffineTransform(pointToRotate, totalTransform);

And here is the documentation on CGAffineTransform, if you'd like to review it further: http://developer.apple.com/library/mac/#documentation/GraphicsImaging/Reference/CGAffineTransform/Reference/reference.html

Please let me know if you need anything else if this doesn't solve your problem!

CGPoint rotation changes distance from origin

1 Answers