Rotating an image context (CGContextRotateCTM) causing it to become blank. Why?

Gurpartap Singh picture Gurpartap Singh · May 12, 2011 · Viewed 29.2k times · Source
#define radians(degrees) (degrees * M_PI/180)

UIImage *rotate(UIImage *image) {
  CGSize size = image.size;;

  UIGraphicsBeginImageContext(size);
  CGContextRef context = UIGraphicsGetCurrentContext();

  // If this is commented out, image is returned as it is.
  CGContextRotateCTM (context, radians(90));

  [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
  UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();

  return newImage;
}

Could something else be wrong? Out of ideas.

Answer

nielsbot picture nielsbot · May 12, 2011

The problem is your context is being rotated around (0,0) which is the top left corner. If you rotate 90 degrees around the top left corner, all your drawing will occur out of the bounds of the context. You need a 2-step transform to move the origin of the context to it's middle, and THEN rotate. Also you need to draw your image centered around the moved/rotated origin, like this:

CGContextTranslateCTM( context, 0.5f * size.width, 0.5f * size.height ) ;
CGContextRotateCTM( context, radians( 90 ) ) ;

[ image drawInRect:(CGRect){ { -size.width * 0.5f, -size.height * 0.5f }, size } ] ;

HTH