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core-animationcore-graphicsrotationquartz-graphicscgpath

How to rotate a cgpath before drawing it?

I can draw a path...and i am somewhat sure that i need to use affine transform.

what i don't get is how to rotate the path and keep the same center.

I apologize for the lack of code here...but all I have is a failed call to CGContextRotateCTM that seemed to halt all drawing.

Solution

  • To rotate around the center:

    CGContextTranslateCTM (ctx, center.x, center.y);
CGContextRotateCTM (ctx, angleInRadians);
CGContextTranslateCTM (ctx, -center.x, -center.y);
CGContextAddPath (ctx, path);

    I might have the translate signs backward (i.e. negative translation before rotate instead of after), although I believe the CTM methods pre-concat, so the mathematical representation of those calls is

    CTMnew = -T * R * T * CTMcurrent