Iphone 更改UIImageView顶部和底部的alpha值

我正在从相册中选取一张照片，但我希望能够使用它执行以下操作：

将顶部和底部零件的alpha值（例如，从顶部起20个pixles，从底部起20个pixles）更改为0.5

>P>然后只复制中间部分，并将其分配给另一个UIVIEVIEW

我有这个密码：

@synthesize imageView, croppedImageView, choosePhotoBtn, takePhotoBtn;

-(IBAction) getPhoto:(id) sender {
    UIImagePickerController * picker = [[UIImagePickerController alloc] init];
    picker.delegate = self;

    if((UIButton *) sender == choosePhotoBtn) {
        picker.sourceType = UIImagePickerControllerSourceTypeSavedPhotosAlbum;
    } else {
        picker.sourceType = UIImagePickerControllerSourceTypeCamera;
    }


    [self presentModalViewController:picker animated:YES];
}



-(void) imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {
    [picker dismissModalViewControllerAnimated:YES];
    [picker release];
    imageView.image = [info objectForKey:UIImagePickerControllerOriginalImage]; 

        /* then here I want to change the alpha value of the top and bottom parts */
        /* then copy the unchanged part then assign it to the image value of croppedImageView */

}

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我认为实现这一点的最佳方法是使用遮罩图像。你可以在这篇博文中找到一个很好的例子：

非常棒，它帮助我实现了我想要的图像。非常感谢。