NSCoriolisBlog

Arbitrary rotation of a CGImage

September 4, 2009 – 11:13 Share on Twitter

For my current project, I have to rotate a CGImageRef by an arbitrary angle.

Here is the code:

- (CGImageRef)CGImageRotatedByAngle:(CGImageRef)imgRef angle:(CGFloat)angle
{
  CGFloat angleInRadians = angle * (M_PI / 180);
  CGFloat width = CGImageGetWidth(imgRef);
  CGFloat height = CGImageGetHeight(imgRef);

  CGRect imgRect = CGRectMake(0, 0, width, height);
  CGAffineTransform transform = CGAffineTransformMakeRotation(angleInRadians);
  CGRect rotatedRect = CGRectApplyAffineTransform(imgRect, transform);

  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
  CGContextRef bmContext = CGBitmapContextCreate(NULL,
                                                 rotatedRect.size.width,
                                                 rotatedRect.size.height,
                                                 8,
                                                 0,
                                                 colorSpace,
                                                 kCGImageAlphaPremultipliedFirst);
  CGContextSetAllowsAntialiasing(bmContext, FALSE);
  CGContextSetInterpolationQuality(bmContext, kCGInterpolationNone);
  CGColorSpaceRelease(colorSpace);
  CGContextTranslateCTM(bmContext,
                        +(rotatedRect.size.width/2),
                        +(rotatedRect.size.height/2));
  CGContextRotateCTM(bmContext, angleInRadians);
  CGContextTranslateCTM(bmContext,
                        -(rotatedRect.size.width/2),
                        -(rotatedRect.size.height/2));
  CGContextDrawImage(bmContext, CGRectMake(0, 0,
                                           rotatedRect.size.width,
                                           rotatedRect.size.height),
                     imgRef);

  CGImageRef rotatedImage = CGBitmapContextCreateImage(bmContext);
  CFRelease(bmContext);
  [(id)rotatedImage autorelease];

  return rotatedImage;
}