CGImageCreateWithImageInRect图片裁剪问题

当我们直接使用CGImageCreateWithImageInRect获取图片时，会发现我们实际得到的截取图像，恰好是当rect旋转90°时，应该得到的图像：

- (UIImage *)cropImage:(UIImage*)image toRect:(CGRect)rect {
CGImageRef imageRef = CGImageCreateWithImageInRect(image.CGImage,rect);
UIImage *result = [UIImage imageWithCGImage:imageRef scale:image.scale orientation:image.imageOrientation];
return result;

这是因为CGImageCreateWithImageInRect中得rect是获取自UIImage中得rect，而不是UIImageView的；而在UIImage的坐标系中，(0,0)点位于左下角，因此在裁剪区域确定时，需要转换成对应坐标系中得区域：以下是转换代码：

- (UIImage *)cropImage:(UIImage*)image toRect:(CGRect)rect {
CGFloat (^rad)(CGFloat) = ^CGFloat(CGFloat deg) {
return deg / 180.0f * (CGFloat) M_PI;
};

// determine the orientation of the image and apply a transformation to the crop rectangle to shift it to the correct position
CGAffineTransform rectTransform;
switch (image.imageOrientation) {
case UIImageOrientationLeft:
rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(90)), 0, -image.size.height);
break;
case UIImageOrientationRight:
rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(-90)), -image.size.width, 0);
break;
case UIImageOrientationDown:
rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(-180)), -image.size.width, -image.size.height);
break;
default:
rectTransform = CGAffineTransformIdentity;
};

// adjust the transformation scale based on the image scale
rectTransform = CGAffineTransformScale(rectTransform, image.scale, image.scale);

// apply the transformation to the rect to create a new, shifted rect
CGRect transformedCropSquare = CGRectApplyAffineTransform(rect, rectTransform);
// use the rect to crop the image
CGImageRef imageRef = CGImageCreateWithImageInRect(image.CGImage, transformedCropSquare);
// create a new UIImage and set the scale and orientation appropriately
UIImage *result = [UIImage imageWithCGImage:imageRef scale:image.scale orientation:image.imageOrientation];
// memory cleanup
CGImageRelease(imageRef);

return result;
}

参考链接：CGImageCreateWithImageInRect not Cropping Correctly