Re: Coordinate Rotation at a Point

FROM : Ron Fleckner
DATE : Tue Nov 06 06:47:04 2007

On 06/11/2007, at 4:33 PM, Gordon Apple wrote:

>    Although I found references to rotating a Bezier path, 
> searching for
> rotating coordinates at a point wasn't fruitful.  In my case, I 
> wanted to
> rotate a shape in place about its center, including shading and/or 
> content.
> I believe the following is about as simple as you can get and can 
> be used to
> rotate about any point.
>
> @implementation  NSAffineTransform (RTPTransformAdditions)
>
> - (void)rotate:(float)angle atPoint:(NSPoint)point
> {
>    [self rotateByDegrees:angle];
>    NSAffineTransform* pointXfrm = [NSAffineTransform transform];
>    [pointXfrm rotateByDegrees:-angle];    //    Needed to get 
> original
> point in rotated coordinates.
>    NSPoint oldPoint = [pointXfrm transformPoint:point];
>    float x = oldPoint.x - point.x;
>    float y = oldPoint.y - point.y;
>    [self translateXBy:x yBy:y];    //    Done in the rotated 
> coordinates.
> }


Hey, I think I know how to do this the easy way, thanks to Eric Buck:

> To rotate about some point, translate to that point, rotate, and then
> translate back.  The approach is not Cocoa specific and is covered in
> any introductory graphics text book.


so,     [theTransform translateToPoint:(thePointYouWantToRotateAround)];
   [theTransform rotateByWhatever];
   [theTransform translateToPoint:(-thePointYouWantToRotateAround)];
   [theTransform concat];

I forget the NSTransformer method names, but I think the intent is 
clear.

Ron