iphone - 更改 UIImageView 顶部和底部的 alpha 值

Question

我正在从相册中选取一张照片，但我希望能够用它执行以下操作:

1. 将顶部和底部部分的 Alpha 值(假设距顶部 20 个像素和距底部 20 个像素)更改为 0.5

2. 然后只复制中间不变的部分，并将其分配给另一个UIImageView

我有这个代码:

@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 */

}

Answer 1

我认为实现此目的的最佳方法是使用 mask 图像。您可以在此博客文章中找到一个很好的示例:http://iosdevelopertips.com/cocoa/how-to-mask-an-image.html