废话不多说,我们直接上代码.

关于如何旋转图像,StackOverflow上给出了很好的答案，比如这个:

- (UIImage *)fixOrientation 
{
     // No-op if the orientation is already correct
     if (self.imageOrientation == UIImageOrientationUp) return self;
  
     // We need to calculate the proper transformation to make the image upright.
     // We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored.
     CGAffineTransform transform = CGAffineTransformIdentity;
  
     switch (self.imageOrientation) {
         case UIImageOrientationDown:
         case UIImageOrientationDownMirrored:
             transform = CGAffineTransformTranslate(transform, self.size.width, self.size.height);
             transform = CGAffineTransformRotate(transform, M_PI);
             break;
  
         case UIImageOrientationLeft:
         case UIImageOrientationLeftMirrored:
             transform = CGAffineTransformTranslate(transform, self.size.width, 0);
             transform = CGAffineTransformRotate(transform, M_PI_2);
             break;
  
         case UIImageOrientationRight:
         case UIImageOrientationRightMirrored:
             transform = CGAffineTransformTranslate(transform, 0, self.size.height);
             transform = CGAffineTransformRotate(transform, -M_PI_2);
             break;
         case UIImageOrientationUp:
         case UIImageOrientationUpMirrored:
             break;
     }
  
     switch (self.imageOrientation) {
         case UIImageOrientationUpMirrored:
         case UIImageOrientationDownMirrored:
             transform = CGAffineTransformTranslate(transform, self.size.width, 0);
             transform = CGAffineTransformScale(transform, -1, 1);
             break;
  
         case UIImageOrientationLeftMirrored:
         case UIImageOrientationRightMirrored:
             transform = CGAffineTransformTranslate(transform, self.size.height, 0);
             transform = CGAffineTransformScale(transform, -1, 1);
             break;
         case UIImageOrientationUp:
         case UIImageOrientationDown:
         case UIImageOrientationLeft:
         case UIImageOrientationRight:
             break;
     }
  
     // Now we draw the underlying CGImage into a new context, applying the transform
     // calculated above.
     CGContextRef ctx = CGBitmapContextCreate(NULL, self.size.width, self.size.height,
                                              CGImageGetBitsPerComponent(self.CGImage), 0,
                                              CGImageGetColorSpace(self.CGImage),
                                              CGImageGetBitmapInfo(self.CGImage));
     CGContextConcatCTM(ctx, transform);
     switch (self.imageOrientation) {
         case UIImageOrientationLeft:
         case UIImageOrientationLeftMirrored:
         case UIImageOrientationRight:
         case UIImageOrientationRightMirrored:
             // Grr...
             CGContextDrawImage(ctx, CGRectMake(0,0,self.size.height,self.size.width), self.CGImage);
             break;
  
         default:
             CGContextDrawImage(ctx, CGRectMake(0,0,self.size.width,self.size.height), self.CGImage);
             break;
     }
  
     // And now we just create a new UIImage from the drawing context
     CGImageRef cgimg = CGBitmapContextCreateImage(ctx);
     UIImage *img = [UIImage imageWithCGImage:cgimg];
     CGContextRelease(ctx);
     CGImageRelease(cgimg);

     return img;
 }

代码有些长,不过却非常直观.这里面涉及到图像矩阵变换的操作,理解起来可能稍稍有些困难,接下来,还有一个简单的方法:

 - (UIImage *)normalizedImage
 {
     if (self.imageOrientation == UIImageOrientationUp) return self; 
  
     UIGraphicsBeginImageContextWithOptions(self.size, NO, self.scale);
     [self drawInRect:(CGRect){0, 0, self.size}];
     UIImage *normalizedImage = UIGraphicsGetImageFromCurrentImageContext();
     UIGraphicsEndImageContext();

     return normalizedImage;
 }

这里是利用了 UIImage 中的 drawInRect 方法,它会将图像绘制到画布上,并且已经考虑好了图像的方向.