iOS：使用覆盖裁剪从 UIImagePickerController 相机抓取的静态图像

我是 iOS 新手，过去一周我一直在网上寻找教程，例如：处理 Exif 图像 http://niftybean.com/main/blog/16-selecting-regions-from-rotated-exif-images-on-iphone, 调整图像大小 https://stackoverflow.com/questions/1260249/resizing-uiimages-pulled-from-the-camera-also-rotates-the-uiimage，以及 StackOverflow 上的更多随机问题。从这些我想到>=iOS 4.0，从相机拍摄的所有图像都包含EXIF基于旋转的信息。

什么不起作用：在尝试了不同的图像裁剪技术之后，我最终得到的只是在一些随机的角落裁剪图像，而且，生成的图像似乎被放大了:(当我使用png来自互联网的图像（不包含 EXIF 数据），裁剪正常。我所说的随机角是指 - 图像最终会在右上角/左上角被裁剪并放大。

我正在努力实现的目标：
我正在尝试裁剪图像100 px从顶部和100 px从底部。本质上，我使用了两个覆盖条 - 1 个在顶部，1 个在底部，CGRect(0.0, 0.0, SCREEN_WIDTH, 100.0) [a 100.0 px顶部的高条]和另一个CGRect(0.0, SCREEN_HEIGHT - 100, SCREEN_WIDTH, 100.0)[其他 100 px底部高条]。我需要获取这两个条带之间的图像：我假设图像的高度是：SCREEN_HEIGHT - 200.0.

使用覆盖层显示相机的 UIImagePickerController：

    //SCREEN_HEIGHT = 480 and SCREEN_WIDTH = 320
    UIImagePickerController *imagePicker = [[UIImagePickerController alloc] init];

    if([UIImagePickerController isSourceTypeAvailable:UIImagePickerControllerSourceTypeCamera] == NO) 
    {
        NSLog(@"Camera not available");
        return;
    }

    imagePicker.sourceType = UIImagePickerControllerSourceTypeCamera;
    imagePicker.delegate = self;
    imagePicker.allowsEditing = NO;


    // Hide the controls
    imagePicker.showsCameraControls = NO;
    imagePicker.navigationBarHidden = YES;

    // Make camera view full screen
    imagePicker.wantsFullScreenLayout = YES;
    //imagePicker.cameraViewTransform = CGAffineTransformScale(imagePicker.cameraViewTransform, CAMERA_TRANSFORM_X, CAMERA_TRANSFORM_Y);


    //Overlay
    //OverlayView is a plain UIView with the CGRects mentioned in the question.
    OverlayView *overlay = [[OverlayView alloc] initWithFrame:CGRectMake(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT)]; 
    [overlay initOverlay:self];
    imagePicker.cameraOverlayView = overlay;
    [self presentModalViewController:imagePicker animated:YES];

基于EXIF旋转图像的代码imageOrientation属性并裁剪它

- (UIImage *) cropImage: (UIImage *) originalImage 
{

    CGRect cropRect = CGRectMake(0, 100.0, SCREEN_WIDTH, SCREEN_HEIGHT - 100);

    CGRect transformedRect = [self TransformCGRectForUIImageOrientation:cropRect :originalImage.imageOrientation :originalImage.size];

    CGImageRef resultImageRef = CGImageCreateWithImageInRect(originalImage.CGImage, transformedRect);

    UIImage *newImage = [[[UIImage alloc] initWithCGImage:resultImageRef scale:1.0 orientation:originalImage.imageOrientation] autorelease];

    return newImage;
}

- (CGRect) TransformCGRectForUIImageOrientation: (CGRect) source: (UIImageOrientation) orientation: (CGSize) imageSize {

    switch (orientation) {
        case UIImageOrientationLeft: { // EXIF #8
            CGAffineTransform txTranslate = CGAffineTransformMakeTranslation(
                                                                             imageSize.height, 0.0);
            CGAffineTransform txCompound = CGAffineTransformRotate(txTranslate,
                                                                   M_PI_2);
            return CGRectApplyAffineTransform(source, txCompound);
        }
        case UIImageOrientationDown: { // EXIF #3
            CGAffineTransform txTranslate = CGAffineTransformMakeTranslation(
                                                                             imageSize.width, imageSize.height);
            CGAffineTransform txCompound = CGAffineTransformRotate(txTranslate,
                                                                   M_PI);
            return CGRectApplyAffineTransform(source, txCompound);
        }
        case UIImageOrientationRight: { // EXIF #6
            CGAffineTransform txTranslate = CGAffineTransformMakeTranslation(
                                                                             0.0, imageSize.width);
            CGAffineTransform txCompound = CGAffineTransformRotate(txTranslate,
                                                                   M_PI + M_PI_2);
            return CGRectApplyAffineTransform(source, txCompound);
        }
        case UIImageOrientationUp: // EXIF #1 - do nothing
        default: // EXIF 2,4,5,7 - ignore
            return source;
    }

The cropImage该方法似乎适用于从互联网下载的图像（不包含任何方向信息）。 我已经没有选择了。有人可以帮我吗？

谢谢阅读！

如果可以的话，跳过使用 Core Graphics 绘制图像会更容易：

- (UIImage *)cropImage:(UIImage *)oldImage {
    CGSize imageSize = oldImage.size
    UIGraphicsBeginImageContextWithOptions( CGSizeMake( imageSize.width,
                                                        imageSize.height - 200),
                                            NO,
                                            0.);
    [oldImage drawAtPoint:CGPointMake( 0, -100)
                blendMode:kCGBlendModeCopy
                    alpha:1.];
    UIImage *croppedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return croppedImage;
}